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Sponges or sea sponges are primarily marine invertebrates of the metazoan phylum Porifera^[4] (/pəˈrɪfərəˌ pɔː-/ pər-IF-ər-ə, por-; meaning ‘pore bearer’),^[5] a basal animal clade and a sister taxon of the diploblasts.^[6] They are sessile filter feeders that are bound to the seabed, and are one of the most ancient members of macrobenthos, with many historical species being important reef-building organisms.

Sponges are multicellular organisms consisting of jelly-like mesohyl sandwiched between two thin layers of cells, and usually have tube-like bodies full of pores and channels that allow water to circulate through them. They have unspecialized cells that can transform into other types and that often migrate between the main cell layers and the mesohyl in the process. They do not have complex nervous,^[7] digestive or circulatory systems. Instead, most rely on maintaining a constant water flow through their bodies to obtain food and oxygen and to remove wastes, usually via flagella movements of the so-called “collar cells“.

Sponges are believed to have been the first outgroup to branch off the evolutionary tree from the last common ancestor of all animals,^[6] with fossil evidence of primitive sponges such as Otavia from as early as the Tonian period (around 800 Mya). The branch of zoology that studies sponges is spongiology.^[8]

Etymology

The term sponge derives from the Ancient Greek word σπόγγος spóngos.^[9] The scientific name Porifera is a neuter plural of the Modern Latin term porifer, which comes from the roots porus meaning “pore, opening”, and -fer meaning “bearing or carrying”.

Overview

Sponges are similar to other animals in that they are multicellular, heterotrophic, lack cell walls and produce sperm cells. Unlike other animals, they lack true tissues^[10] and organs.^[11] Some of them are radially symmetrical, but most are asymmetrical. The shapes of their bodies are adapted for maximal efficiency of water flow through the central cavity, where the water deposits nutrients and then leaves through a hole called the osculum. The single-celled choanoflagellates resemble the choanocyte cells of sponges which are used to drive their water flow systems and capture most of their food. This along with phylogenetic studies of ribosomal molecules have been used as morphological evidence to suggest sponges are the sister group to the rest of animals.^[12] A great majority are marine (salt-water) species, ranging in habitat from tidal zones to depths exceeding 8,800 m (5.5 mi), though there are freshwater species. All adult sponges are sessile, meaning that they attach to an underwater surface and remain fixed in place (i.e., do not travel). While in their larval stage of life, they are motile.

Many sponges have internal skeletons of spicules (skeletal-like fragments of calcium carbonate or silicon dioxide), and/or spongin (a modified type of collagen protein).^[10] An internal gelatinous matrix called mesohyl functions as an endoskeleton, and it is the only skeleton in soft sponges that encrust such hard surfaces as rocks. More commonly, the mesohyl is stiffened by mineral spicules, by spongin fibers, or both. 90% of all known sponge species that have the widest range of habitats including all freshwater ones are demosponges that use spongin; many species have silica spicules, whereas some species have calcium carbonate exoskeletons. Calcareous sponges have calcium carbonate spicules and, in some species, calcium carbonate exoskeletons, are restricted to relatively shallow marine waters where production of calcium carbonate is easiest.^[13]: 179 The fragile glass sponges, with “scaffolding” of silica spicules, are restricted to polar regions and the ocean depths where predators are rare. Fossils of all of these types have been found in rocks dated from 580 million years ago. In addition Archaeocyathids, whose fossils are common in rocks from 530 to 490 million years ago, are now regarded as a type of sponge.

Although most of the approximately 5,000–10,000 known species of sponges feed on bacteria and other microscopic food in the water, some host photosynthesizing microorganisms as endosymbionts, and these alliances often produce more food and oxygen than they consume. A few species of sponges that live in food-poor environments have evolved as carnivores that prey mainly on small crustaceans.^[14]

Most sponges reproduce sexually, but they can also reproduce asexually. Sexually reproducing species release sperm cells into the water to fertilize ova released or retained by its mate or “mother”; the fertilized eggs develop into larvae which swim off in search of places to settle.^{[13]: 183–185} Sponges are known for regenerating from fragments that are broken off, although this only works if the fragments include the right types of cells. Some species reproduce by budding. When environmental conditions become less hospitable to the sponges, for example as temperatures drop, many freshwater species and a few marine ones produce gemmules, “survival pods” of unspecialized cells that remain dormant until conditions improve; they then either form completely new sponges or recolonize the skeletons of their parents.^{[13]: 120–127}

The few species of demosponge that have entirely soft fibrous skeletons with no hard elements have been used by humans over thousands of years for several purposes, including as padding and as cleaning tools. By the 1950s, though, these had been overfished so heavily that the industry almost collapsed, and most sponge-like materials are now synthetic. Sponges and their microscopic endosymbionts are now being researched as possible sources of medicines for treating a wide range of diseases. Dolphins have been observed using sponges as tools while foraging.^[16]

Distinguishing features

Further information: Cnidaria and Ctenophore

Sponges constitute the phylum Porifera, and have been defined as sessile metazoans (multicelled immobile animals) that have water intake and outlet openings connected by chambers lined with choanocytes, cells with whip-like flagella.^[13]: 29 However, a few carnivorous sponges have lost these water flow systems and the choanocytes.^{[13]: 39 [17]} All known living sponges can remold their bodies, as most types of their cells can move within their bodies and a few can change from one type to another.^[17][18]

Even if a few sponges are able to produce mucus – which acts as a microbial barrier in all other animals – no sponge with the ability to secrete a functional mucus layer has been recorded. Without such a mucus layer their living tissue is covered by a layer of microbial symbionts, which can contribute up to 40–50% of the sponge wet mass. This inability to prevent microbes from penetrating their porous tissue could be a major reason why they have never evolved a more complex anatomy.^[19]

Like cnidarians (jellyfish, etc.) and ctenophores (comb jellies), and unlike all other known metazoans, sponges’ bodies consist of a non-living jelly-like mass (mesohyl) sandwiched between two main layers of cells.^[20][21] Cnidarians and ctenophores have simple nervous systems, and their cell layers are bound by internal connections and by being mounted on a basement membrane (thin fibrous mat, also known as “basal lamina“).^[21] Sponges do not have a nervous system similar to that of vertebrates but may have one that is quite different.^[7] Their middle jelly-like layers have large and varied populations of cells, and some types of cells in their outer layers may move into the middle layer and change their functions.^[18]

	Sponges[18][20]	Cnidarians and ctenophores[21]
Nervous system	No/Yes	Yes, simple
Cells in each layer bound together	No, except that Homoscleromorpha have basement membranes.[22]	Yes: inter-cell connections; basement membranes
Number of cells in middle “jelly” layer	Many	Few
Cells in outer layers can move inwards and change functions	Yes	No

Basic structure

Cell types

    Mesohyl

    Pinacocyte

    Choanocyte

    Lophocyte

    Porocyte

    Oocyte

    Archeocyte

    Sclerocyte

    Spicule

    Water flow

Main cell types of Porifera^[23]

A sponge’s body is hollow and is held in shape by the mesohyl, a jelly-like substance made mainly of collagen and reinforced by a dense network of fibers also made of collagen. 18 distinct cell types have been identified.^[24] The inner surface is covered with choanocytes, cells with cylindrical or conical collars surrounding one flagellum per choanocyte. The wave-like motion of the whip-like flagella drives water through the sponge’s body. All sponges have ostia, channels leading to the interior through the mesohyl, and in most sponges these are controlled by tube-like porocytes that form closable inlet valves. Pinacocytes, plate-like cells, form a single-layered external skin over all other parts of the mesohyl that are not covered by choanocytes, and the pinacocytes also digest food particles that are too large to enter the ostia,^[18][20] while those at the base of the animal are responsible for anchoring it.^[20]

Other types of cells live and move within the mesohyl:^[18][20]

• Lophocytes are amoeba-like cells that move slowly through the mesohyl and secrete collagen fibres.
• Collencytes are another type of collagen-producing cell.
• Rhabdiferous cells secrete polysaccharides that also form part of the mesohyl.
• Oocytes and spermatocytes are reproductive cells.
• Sclerocytes secrete the mineralized spicules (“little spines”) that form the skeletons of many sponges and in some species provide some defense against predators.
• In addition to or instead of sclerocytes, demosponges have spongocytes that secrete a form of collagen that polymerizes into spongin, a thick fibrous material that stiffens the mesohyl.
• Myocytes (“muscle cells”) conduct signals and cause parts of the animal to contract.
• “Grey cells” act as sponges’ equivalent of an immune system.
• Archaeocytes (or amoebocytes) are amoeba-like cells that are totipotent, in other words, each is capable of transformation into any other type of cell. They also have important roles in feeding and in clearing debris that block the ostia.

Many larval sponges possess neuron-less eyes that are based on cryptochromes. They mediate phototaxic behavior.^[25]

Glass sponges present a distinctive variation on this basic plan. Their spicules, which are made of silica, form a scaffolding-like framework between whose rods the living tissue is suspended like a cobweb that contains most of the cell types.^[18] This tissue is a syncytium that in some ways behaves like many cells that share a single external membrane, and in others like a single cell with multiple nuclei.

Water flow and body structures

Asconoid

Syconoid

Leuconoid

    Pinacocytes

    Choanocytes

    Mesohyl

    Water flow

Porifera body structures^[26]

Most sponges work rather like chimneys: they take in water at the bottom and eject it from the osculum at the top. Since ambient currents are faster at the top, the suction effect that they produce by Bernoulli’s principle does some of the work for free. Sponges can control the water flow by various combinations of wholly or partially closing the osculum and ostia (the intake pores) and varying the beat of the flagella, and may shut it down if there is a lot of sand or silt in the water.^[18]

Although the layers of pinacocytes and choanocytes resemble the epithelia of more complex animals, they are not bound tightly by cell-to-cell connections or a basal lamina (thin fibrous sheet underneath). The flexibility of these layers and re-modeling of the mesohyl by lophocytes allow the animals to adjust their shapes throughout their lives to take maximum advantage of local water currents.^[18]: 83

The simplest body structure in sponges is a tube or vase shape known as “asconoid”, but this severely limits the size of the animal. The body structure is characterized by a stalk-like spongocoel surrounded by a single layer of choanocytes. If it is simply scaled up, the ratio of its volume to surface area increases, because surface increases as the square of length or width while volume increases proportionally to the cube. The amount of tissue that needs food and oxygen is determined by the volume, but the pumping capacity that supplies food and oxygen depends on the area covered by choanocytes. Asconoid sponges seldom exceed 1 mm (0.039 in) in diameter.^[18]

Some sponges overcome this limitation by adopting the “syconoid” structure, in which the body wall is pleated. The inner pockets of the pleats are lined with choanocytes, which connect to the outer pockets of the pleats by ostia. This increase in the number of choanocytes and hence in pumping capacity enables syconoid sponges to grow up to a few centimeters in diameter.

The “leuconoid” pattern boosts pumping capacity further by filling the interior almost completely with mesohyl that contains a network of chambers lined with choanocytes and connected to each other and to the water intakes and outlet by tubes. Leuconid sponges grow to over 1 m (3.3 ft) in diameter, and the fact that growth in any direction increases the number of choanocyte chambers enables them to take a wider range of forms, for example, “encrusting” sponges whose shapes follow those of the surfaces to which they attach. All freshwater and most shallow-water marine sponges have leuconid bodies. The networks of water passages in glass sponges are similar to the leuconid structure.^[18]

In all three types of structure, the cross-section area of the choanocyte-lined regions is much greater than that of the intake and outlet channels. This makes the flow slower near the choanocytes and thus makes it easier for them to trap food particles.^[18] For example, in Leuconia, a small leuconoid sponge about 10 centimetres (3.9 in) tall and 1 centimetre (0.39 in) in diameter, water enters each of more than 80,000 intake canals at 6 cm per minute. However, because Leuconia has more than 2 million flagellated chambers whose combined diameter is much greater than that of the canals, water flow through chambers slows to 3.6 cm per hour, making it easy for choanocytes to capture food. All the water is expelled through a single osculum at about 8.5 cm per second, fast enough to carry waste products some distance away.^[27]

Skeleton

In zoology, a skeleton is any fairly rigid structure of an animal, irrespective of whether it has joints and irrespective of whether it is biomineralized. The mesohyl functions as an endoskeleton in most sponges, and is the only skeleton in soft sponges that encrust hard surfaces such as rocks. More commonly the mesohyl is stiffened by mineral spicules, by spongin fibers or both. Spicules, which are present in most but not all species,^[28] may be made of silica or calcium carbonate, and vary in shape from simple rods to three-dimensional “stars” with up to six rays. Spicules are produced by sclerocyte cells,^[18] and may be separate, connected by joints, or fused.^[17]

Some sponges also secrete exoskeletons that lie completely outside their organic components. For example, sclerosponges (“hard sponges”) have massive calcium carbonate exoskeletons over which the organic matter forms a thin layer with choanocyte chambers in pits in the mineral. These exoskeletons are secreted by the pinacocytes that form the animals’ skins.^[18]

Vital functions

Movement

Although adult sponges are fundamentally sessile animals, some marine and freshwater species can move across the sea bed at speeds of 1–4 mm (0.039–0.157 in) per day, as a result of amoeba-like movements of pinacocytes and other cells. A few species can contract their whole bodies, and many can close their oscula and ostia. Juveniles drift or swim freely, while adults are stationary.^[18]

Respiration, feeding and excretion

Sponges do not have distinct circulatory, respiratory, digestive, and excretory systems – instead, the water flow system supports all these functions. They filter food particles out of the water flowing through them. Particles larger than 50 micrometers cannot enter the ostia and pinacocytes consume them by phagocytosis (engulfing and intracellular digestion). Particles from 0.5 μm to 50 μm are trapped in the ostia, which taper from the outer to inner ends. These particles are consumed by pinacocytes or by archaeocytes which partially extrude themselves through the walls of the ostia. Bacteria-sized particles, below 0.5 micrometers, pass through the ostia and are caught and consumed by choanocytes.^[18] Since the smallest particles are by far the most common, choanocytes typically capture 80% of a sponge’s food supply.^[29] Archaeocytes transport food packaged in vesicles from cells that directly digest food to those that do not. At least one species of sponge has internal fibers that function as tracks for use by nutrient-carrying archaeocytes,^[18] and these tracks also move inert objects.^[20]

It used to be claimed that glass sponges could live on nutrients dissolved in sea water and were very averse to silt.^[30] However, a study in 2007 found no evidence of this and concluded that they extract bacteria and other micro-organisms from water very efficiently (about 79%) and process suspended sediment grains to extract such prey.^[31] Collar bodies digest food and distribute it wrapped in vesicles that are transported by dynein “motor” molecules along bundles of microtubules that run throughout the syncytium.^[18]

Sponges’ cells absorb oxygen by diffusion from water into cells as water flows through body, into which carbon dioxide and other soluble waste products such as ammonia also diffuse. Archeocytes remove mineral particles that threaten to block the ostia, transport them through the mesohyl and generally dump them into the outgoing water current, although some species incorporate them into their skeletons.^[18]

Carnivorous sponges

In waters where the supply of food particles is very poor, some species prey on crustaceans and other small animals. So far only 137 species have been discovered.^[33] Most belong to the family Cladorhizidae, but a few members of the Guitarridae and Esperiopsidae are also carnivores.^[34] In most cases, little is known about how they actually capture prey, although some species are thought to use either sticky threads or hooked spicules.^[34][35] Most carnivorous sponges live in deep waters, up to 8,840 m (5.49 mi),^[36] and the development of deep-ocean exploration techniques is expected to lead to the discovery of several more.^[18][34] However, one species has been found in Mediterranean caves at depths of 17–23 m (56–75 ft), alongside the more usual filter-feeding sponges. The cave-dwelling predators capture crustaceans under 1 mm (0.039 in) long by entangling them with fine threads, digest them by enveloping them with further threads over the course of a few days, and then return to their normal shape; there is no evidence that they use venom.^[36]

Most known carnivorous sponges have completely lost the water flow system and choanocytes. However, the genus Chondrocladia uses a highly modified water flow system to inflate balloon-like structures that are used for capturing prey.^[34][37]

Endosymbionts

Freshwater sponges often host green algae as endosymbionts within archaeocytes and other cells and benefit from nutrients produced by the algae. Many marine species host other photosynthesizing organisms, most commonly cyanobacteria but in some cases dinoflagellates. Symbiotic cyanobacteria may form a third of the total mass of living tissue in some sponges, and some sponges gain 48% to 80% of their energy supply from these micro-organisms.^[18] In 2008, a University of Stuttgart team reported that spicules made of silica conduct light into the mesohyl, where the photosynthesizing endosymbionts live.^[38] Sponges that host photosynthesizing organisms are most common in waters with relatively poor supplies of food particles and often have leafy shapes that maximize the amount of sunlight they collect.^[20]

A recently discovered carnivorous sponge that lives near hydrothermal vents hosts methane-eating bacteria and digests some of them.^[20]

“Immune” system

Sponges do not have the complex immune systems of most other animals. However, they reject grafts from other species but accept them from other members of their own species. In a few marine species, gray cells play the leading role in rejection of foreign material. When invaded, they produce a chemical that stops movement of other cells in the affected area, thus preventing the intruder from using the sponge’s internal transport systems. If the intrusion persists, the grey cells concentrate in the area and release toxins that kill all cells in the area. The “immune” system can stay in this activated state for up to three weeks.^[20]

Reproduction

Asexual

Sponges have three asexual methods of reproduction: after fragmentation, by budding, and by producing gemmules. Fragments of sponges may be detached by currents or waves. They use the mobility of their pinacocytes and choanocytes and reshaping of the mesohyl to re-attach themselves to a suitable surface and then rebuild themselves as small but functional sponges over the course of several days. The same capabilities enable sponges that have been squeezed through a fine cloth to regenerate.^[18]: 239 A sponge fragment can only regenerate if it contains both collencytes to produce mesohyl and archeocytes to produce all the other cell types.^[29] A very few species reproduce by budding.^{[18]: 90–94}

Gemmules are “survival pods” which a few marine sponges and many freshwater species produce by the thousands when dying and which some, mainly freshwater species, regularly produce in autumn. Spongocytes make gemmules by wrapping shells of spongin, often reinforced with spicules, round clusters of archeocytes that are full of nutrients.^{[18]: 87–88} Freshwater gemmules may also include photosynthesizing symbionts.^[39] The gemmules then become dormant, and in this state can survive cold, drying out, lack of oxygen and extreme variations in salinity.^[18] Freshwater gemmules often do not revive until the temperature drops, stays cold for a few months and then reaches a near-“normal” level.^[39] When a gemmule germinates, the archeocytes round the outside of the cluster transform into pinacocytes, a membrane over a pore in the shell bursts, the cluster of cells slowly emerges, and most of the remaining archeocytes transform into other cell types needed to make a functioning sponge. Gemmules from the same species but different individuals can join forces to form one sponge.^{[18]: 89–90} Some gemmules are retained within the parent sponge, and in spring it can be difficult to tell whether an old sponge has revived or been “recolonized” by its own gemmules.^[39]

Sexual

Most sponges are hermaphrodites (function as both sexes simultaneously), although sponges have no gonads (reproductive organs). Sperm are produced by choanocytes or entire choanocyte chambers that sink into the mesohyl and form spermatic cysts while eggs are formed by transformation of archeocytes, or of choanocytes in some species. Each egg generally acquires a yolk by consuming “nurse cells”. During spawning, sperm burst out of their cysts and are expelled via the osculum. If they contact another sponge of the same species, the water flow carries them to choanocytes that engulf them but, instead of digesting them, metamorphose to an ameboid form and carry the sperm through the mesohyl to eggs, which in most cases engulf the carrier and its cargo.^[18]: 77

A few species release fertilized eggs into the water, but most retain the eggs until they hatch. By retaining the eggs, the parents can transfer symbiotic microorganisms directly to their offspring through vertical transmission, while the species who release their eggs into the water has to acquire symbionts horizontally (a combination of both is probably most common, where larvae with vertically transmitted symbionts also acquire others horizontally).^[40][41] There are four types of larvae, but all are lecithotrophic (non-feeding) balls of cells with an outer layer of cells whose flagella or cilia enable the larvae to move. After swimming for a few days the larvae sink and crawl until they find a place to settle. Most of the cells transform into archeocytes and then into the types appropriate for their locations in a miniature adult sponge.^{[18]: 77 [42]}

Glass sponge embryos start by dividing into separate cells, but once 32 cells have formed they rapidly transform into larvae that externally are ovoid with a band of cilia round the middle that they use for movement, but internally have the typical glass sponge structure of spicules with a cobweb-like main syncitium draped around and between them and choanosyncytia with multiple collar bodies in the center. The larvae then leave their parents’ bodies.^[43]

Meiosis

The cytological progression of porifera oogenesis and spermatogenesis (gametogenesis) is very similar to that of other metazoa.^[44] Most of the genes from the classic set of meiotic genes, including genes for DNA recombination and double-strand break repair, that are conserved in eukaryotes are expressed in the sponges (e.g. Geodia hentscheli and Geodia phlegraei).^[44] Since porifera are considered to be the earliest divergent animals, these findings indicate that the basic toolkit of meiosis including capabilities for recombination and DNA repair were present early in eukaryote evolution.^[44]

Life cycle

Sponges in temperate regions live for at most a few years, but some tropical species and perhaps some deep-ocean ones may live for 200 years or more. Some calcified demosponges grow by only 0.2 mm (0.0079 in) per year and, if that rate is constant, specimens 1 m (3.3 ft) wide must be about 5,000 years old. Some sponges start sexual reproduction when only a few weeks old, while others wait until they are several years old.^[18]

Coordination of activities

Adult sponges lack neurons or any other kind of nervous tissue. However, most species have the ability to perform movements that are coordinated all over their bodies, mainly contractions of the pinacocytes, squeezing the water channels and thus expelling excess sediment and other substances that may cause blockages. Some species can contract the osculum independently of the rest of the body. Sponges may also contract in order to reduce the area that is vulnerable to attack by predators. In cases where two sponges are fused, for example if there is a large but still unseparated bud, these contraction waves slowly become coordinated in both of the “Siamese twins“. The coordinating mechanism is unknown, but may involve chemicals similar to neurotransmitters.^[45] However, glass sponges rapidly transmit electrical impulses through all parts of the syncytium, and use this to halt the motion of their flagella if the incoming water contains toxins or excessive sediment.^[18] Myocytes are thought to be responsible for closing the osculum and for transmitting signals between different parts of the body.^[20]

Sponges contain genes very similar to those that contain the “recipe” for the post-synaptic density, an important signal-receiving structure in the neurons of all other animals. However, in sponges these genes are only activated in “flask cells” that appear only in larvae and may provide some sensory capability while the larvae are swimming. This raises questions about whether flask cells represent the predecessors of true neurons or are evidence that sponges’ ancestors had true neurons but lost them as they adapted to a sessile lifestyle.^[46]

Ecology

Habitats

See also: Sponge ground and Sponge reef

Sponges are worldwide in their distribution, living in a wide range of ocean habitats, from the polar regions to the tropics.^[29] Most live in quiet, clear waters, because sediment stirred up by waves or currents would block their pores, making it difficult for them to feed and breathe.^[30] The greatest numbers of sponges are usually found on firm surfaces such as rocks, but some sponges can attach themselves to soft sediment by means of a root-like base.^[47]

Sponges are more abundant but less diverse in temperate waters than in tropical waters, possibly because organisms that prey on sponges are more abundant in tropical waters.^[48] Glass sponges are the most common in polar waters and in the depths of temperate and tropical seas, as their very porous construction enables them to extract food from these resource-poor waters with the minimum of effort. Demosponges and calcareous sponges are abundant and diverse in shallower non-polar waters.^[49]

The different classes of sponge live in different ranges of habitat:

Class	Water type[20]	Depth[20]	Type of surface[20]
Calcarea	Marine	less than 100 m (330 ft)	Hard
Glass sponges	Marine	Deep	Soft or firm sediment
Demosponges	Marine, brackish; and about 150 freshwater species[18]	Inter-tidal to abyssal;[20] a carnivorous demosponge has been found at 8,840 m (5.49 mi)[36]	Any

As primary producers

Sponges with photosynthesizing endosymbionts produce up to three times more oxygen than they consume, as well as more organic matter than they consume. Such contributions to their habitats’ resources are significant along Australia’s Great Barrier Reef but relatively minor in the Caribbean.^[29]

Defenses

Many sponges shed spicules, forming a dense carpet several meters deep that keeps away echinoderms which would otherwise prey on the sponges.^[29] They also produce toxins that prevent other sessile organisms such as bryozoans or sea squirts from growing on or near them, making sponges very effective competitors for living space. One of many examples includes ageliferin.

A few species, the Caribbean fire sponge Tedania ignis, cause a severe rash in humans who handle them.^[18] Turtles and some fish feed mainly on sponges. It is often said that sponges produce chemical defenses against such predators.^[18] However, experiments have been unable to establish a relationship between the toxicity of chemicals produced by sponges and how they taste to fish, which would diminish the usefulness of chemical defenses as deterrents. Predation by fish may even help to spread sponges by detaching fragments.^[20] However, some studies have shown fish showing a preference for non chemically defended sponges,^[50] and another study found that high levels of coral predation did predict the presence of chemically defended species.^[51]

Glass sponges produce no toxic chemicals, and live in very deep water where predators are rare.^[30]

Predation

Further information: Spongivore

Spongeflies, also known as spongillaflies (Neuroptera, Sisyridae), are specialist predators of freshwater sponges. The female lays her eggs on vegetation overhanging water. The larvae hatch and drop into the water where they seek out sponges to feed on. They use their elongated mouthparts to pierce the sponge and suck the fluids within. The larvae of some species cling to the surface of the sponge while others take refuge in the sponge’s internal cavities. The fully grown larvae leave the water and spin a cocoon in which to pupate.^[52]

Bioerosion

The Caribbean chicken-liver sponge Chondrilla nucula secretes toxins that kill coral polyps, allowing the sponges to grow over the coral skeletons.^[18] Others, especially in the family Clionaidae, use corrosive substances secreted by their archeocytes to tunnel into rocks, corals and the shells of dead mollusks.^[18] Sponges may remove up to 1 m (3.3 ft) per year from reefs, creating visible notches just below low-tide level.^[29]

Diseases

Caribbean sponges of the genus Aplysina suffer from Aplysina red band syndrome. This causes Aplysina to develop one or more rust-colored bands, sometimes with adjacent bands of necrotic tissue. These lesions may completely encircle branches of the sponge. The disease appears to be contagious and impacts approximately ten percent of A. cauliformis on Bahamian reefs.^[53] The rust-colored bands are caused by a cyanobacterium, but it is unknown whether this organism actually causes the disease.^[53][54]

Collaboration with other organisms

In addition to hosting photosynthesizing endosymbionts,^[18] sponges are noted for their wide range of collaborations with other organisms. The relatively large encrusting sponge Lissodendoryx colombiensis is most common on rocky surfaces, but has extended its range into seagrass meadows by letting itself be surrounded or overgrown by seagrass sponges, which are distasteful to the local starfish and therefore protect Lissodendoryx against them; in return, the seagrass sponges get higher positions away from the sea-floor sediment.^[55]

Shrimps of the genus Synalpheus form colonies in sponges, and each shrimp species inhabits a different sponge species, making Synalpheus one of the most diverse crustacean genera. Specifically, Synalpheus regalis utilizes the sponge not only as a food source, but also as a defense against other shrimp and predators.^[56] As many as 16,000 individuals inhabit a single loggerhead sponge, feeding off the larger particles that collect on the sponge as it filters the ocean to feed itself.^[57] Other crustaceans such as hermit crabs commonly have a specific species of sponge, Pseudospongosorites, grow on them as both the sponge and crab occupy gastropod shells until the crab and sponge outgrow the shell, eventually resulting in the crab using the sponge’s body as protection instead of the shell until the crab finds a suitable replacement shell.^[58]

Sponge loop

Most sponges are detritivores which filter organic debris particles and microscopic life forms from ocean water. In particular, sponges occupy an important role as detritivores in coral reef food webs by recycling detritus to higher trophic levels.^[61]

The hypothesis has been made that coral reef sponges facilitate the transfer of coral-derived organic matter to their associated detritivores via the production of sponge detritus, as shown in the diagram. Several sponge species are able to convert coral-derived DOM into sponge detritus,^[62][63] and transfer organic matter produced by corals further up the reef food web. Corals release organic matter as both dissolved and particulate mucus,^{[64][65][66][67]} as well as cellular material such as expelled Symbiodinium.^[68][69][61]

Organic matter could be transferred from corals to sponges by all these pathways, but DOM likely makes up the largest fraction, as the majority (56 to 80%) of coral mucus dissolves in the water column,^[65] and coral loss of fixed carbon due to expulsion of Symbiodinium is typically negligible (0.01%)^[68] compared with mucus release (up to ~40%).^[70][71] Coral-derived organic matter could also be indirectly transferred to sponges via bacteria, which can also consume coral mucus.^{[72][73][74][61]}

Sponge holobiont

Besides a one to one symbiotic relationship, it is possible for a host to become symbiotic with a microbial consortium, resulting in a diverse sponge microbiome. Sponges are able to host a wide range of microbial communities that can also be very specific. The microbial communities that form a symbiotic relationship with the sponge can amount to as much as 35% of the biomass of its host.^[77] The term for this specific symbiotic relationship, where a microbial consortia pairs with a host is called a holobiotic relationship. The sponge as well as the microbial community associated with it will produce a large range of secondary metabolites that help protect it against predators through mechanisms such as chemical defense.^[78]

Some of these relationships include endosymbionts within bacteriocyte cells, and cyanobacteria or microalgae found below the pinacoderm cell layer where they are able to receive the highest amount of light, used for phototrophy. They can host over 50 different microbial phyla and candidate phyla, including Alphaprotoebacteria, Actinomycetota, Chloroflexota, Nitrospirota, “Cyanobacteria“, the taxa Gamma-, the candidate phylum Poribacteria, and Thaumarchaea.^[78]

Systematics

Taxonomy

Carl Linnaeus, who classified most kinds of sessile animals as belonging to the order Zoophyta in the class Vermes, mistakenly identified the genus Spongia as plants in the order Algae.^{[79][further explanation needed]} For a long time thereafter, sponges were assigned to subkingdom Parazoa (“beside the animals”) separated from the Eumetazoa which formed the rest of the kingdom Animalia.^[80]

The phylum Porifera is further divided into classes mainly according to the composition of their skeletons:^[17][29]

• Hexactinellida (glass sponges) have silicate spicules, the largest of which have six rays and may be individual or fused.^[17] The main components of their bodies are syncytia in which large numbers of cell share a single external membrane.^[29]
• Calcarea have skeletons made of calcite, a form of calcium carbonate, which may form separate spicules or large masses. All the cells have a single nucleus and membrane.^[29]
• Most Demospongiae have silicate spicules or spongin fibers or both within their soft tissues. However, a few also have massive external skeletons made of aragonite, another form of calcium carbonate.^[17][29] All the cells have a single nucleus and membrane.^[29]
• Archeocyatha are known only as fossils from the Cambrian period.^[80]

In the 1970s, sponges with massive calcium carbonate skeletons were assigned to a separate class, Sclerospongiae, otherwise known as “coralline sponges”.^[81] However, in the 1980s, it was found that these were all members of either the Calcarea or the Demospongiae.^[82]

So far scientific publications have identified about 9,000 poriferan species,^[29] of which: about 400 are glass sponges; about 500 are calcareous species; and the rest are demosponges.^[18] However, some types of habitat, vertical rock and cave walls and galleries in rock and coral boulders, have been investigated very little, even in shallow seas.^[29]

Classes

Sponges were traditionally distributed in three classes: calcareous sponges (Calcarea), glass sponges (Hexactinellida) and demosponges (Demospongiae). However, studies have now shown that the Homoscleromorpha, a group thought to belong to the Demospongiae, has a genetic relationship well separated from other sponge classes.^{[13]: 153–154} Therefore, they have recently been recognized as the fourth class of sponges.^[83][84]

Sponges are divided into classes mainly according to the composition of their skeletons:^[20] These are arranged in evolutionary order as shown below in ascending order of their evolution from top to bottom:

Class	Type of cells[20]	Spicules[20]	Spongin fibers[20]	Massive exoskeleton[29]	Body form[20]
Hexactinellida	Mostly syncytia in all species	Silica May be individual or fused	Never	Never	Leuconoid
Demospongiae	Single nucleus, single external membrane	Silica	In many species	In some species. Made of aragonite if present.[17][29]	Leuconoid
Calcarea	Single nucleus, single external membrane	Calcite May be individual or large masses	Never	Common. Made of calcite if present.	Asconoid, syconoid, leuconoid or solenoid[85]
Homoscleromorpha	Single nucleus, single external membrane	Silica	In many species	Never	Sylleibid or leuconoid

Phylogeny

The phylogeny of sponges has been debated heavily since the advent of phylogenetics. Originally thought to be the most basal animal phylum, there is now considerable evidence that Ctenophora may hold that title instead.^[86][87] Additionally, the monophyly of the phylum is now under question. Several studies have concluded that all other animals emerged from within the sponges, and usually recover that the calcareous sponges and Homoscleromorpha are closer to other animals than to demosponges.^[88][89] The internal relationships of Porifera have proven to be less uncertain. A close relationship of Homoscleromorpha and Calcarea has been recovered in nearly all studies, whether or not they support sponge or eumetazoan monophyly.^{[88][6][84][83]} The position of glass sponges is also fairly certain, with a majority of studies recovering them as the sister of the demosponges.^[83][6][88] Thus, the uncertainty at the base of the animal family tree is probably best represented by the below cladogram.

Animalia	Hexactinellida Demospongiae Calcarea Homoscleromorpha Ctenophora ParaHoxozoa

Evolutionary history

Fossil record

“Primitive sponge” redirects here. For other uses, see Primitive sponge (disambiguation).

1

2

3

4

5

6

7

1: Gap  2: Central cavity  3 Internal wall  4: Pore (all walls have pores)  5 Septum  6 Outer wall  7 Holdfast

Archaeocyathid structure

Although molecular clocks and biomarkers suggest sponges existed well before the Cambrian explosion of life, silica spicules like those of demosponges are absent from the fossil record until the Cambrian.^[90] An unsubstantiated 2002 report exists of spicules in rocks dated around 750 million years ago.^[91] Well-preserved fossil sponges from about 580 million years ago in the Ediacaran period have been found in the Doushantuo Formation.^[92] These fossils, which include: spicules; pinacocytes; porocytes; archeocytes; sclerocytes; and the internal cavity, have been classified as demosponges. The Ediacaran record of sponges also contains two other genera: the stem-hexactinellid Helicolocellus from the Dengying Formation^[93] and the possible stem-archaeocyathan Arimasia from the Nama Group.^[94] These genera are both from the “Nama assemblage” of Ediacaran biota, although whether this is due to a genuine lack beforehand or preservational bias is uncertain. Fossils of glass sponges have been found from around 540 million years ago in rocks in Australia, China, and Mongolia.^[95] Early Cambrian sponges from Mexico belonging to the genus Kiwetinokia show evidence of fusion of several smaller spicules to form a single large spicule.^[96] Calcium carbonate spicules of calcareous sponges have been found in Early Cambrian rocks from about 530 to 523 million years ago in Australia. Other probable demosponges have been found in the Early Cambrian Chengjiang fauna, from 525 to 520 million years ago.^[97] Fossils found in the Canadian Northwest Territories dating to 890 million years ago may be sponges; if this finding is confirmed, it suggests the first animals appeared before the Neoproterozoic oxygenation event.^[98]

Freshwater sponges appear to be much younger, as the earliest known fossils date from the Mid-Eocene period about 48 to 40 million years ago.^[95] Although about 90% of modern sponges are demosponges, fossilized remains of this type are less common than those of other types because their skeletons are composed of relatively soft spongin that does not fossilize well.^[99] The earliest sponge symbionts are known from the early Silurian.^[100]

A chemical tracer is 24-isopropyl cholestane, which is a stable derivative of 24-isopropyl cholesterol, which is said to be produced by demosponges but not by eumetazoans (“true animals”, i.e. cnidarians and bilaterians). Since choanoflagellates are thought to be animals’ closest single-celled relatives, a team of scientists examined the biochemistry and genes of one choanoflagellate species. They concluded that this species could not produce 24-isopropyl cholesterol but that investigation of a wider range of choanoflagellates would be necessary in order to prove that the fossil 24-isopropyl cholestane could only have been produced by demosponges.^[101] Although a previous publication reported traces of the chemical 24-isopropyl cholestane in ancient rocks dating to 1,800 million years ago,^[102] recent research using a much more accurately dated rock series has revealed that these biomarkers only appear before the end of the Marinoan glaciation approximately 635 million years ago,^[103] and that “Biomarker analysis has yet to reveal any convincing evidence for ancient sponges pre-dating the first globally extensive Neoproterozoic glacial episode (the Sturtian, ~713 million years ago in Oman)”. While it has been argued that this ‘sponge biomarker’ could have originated from marine algae, recent research suggests that the algae’s ability to produce this biomarker evolved only in the Carboniferous; as such, the biomarker remains strongly supportive of the presence of demosponges in the Cryogenian.^{[104][105][106]}

Archaeocyathids, which some classify as a type of coralline sponge, are very common fossils in rocks from the Early Cambrian about 530 to 520 million years ago, but apparently died out by the end of the Cambrian 490 million years ago.^[97] It has been suggested that they were produced by: sponges; cnidarians; algae; foraminiferans; a completely separate phylum of animals, Archaeocyatha; or even a completely separate kingdom of life, labeled Archaeata or Inferibionta. Since the 1990s, archaeocyathids have been regarded as a distinctive group of sponges.^[80]

= skin

= aragonite

= flesh

Halkieriid sclerite structure^[107]

It is difficult to fit chancelloriids into classifications of sponges or more complex animals. An analysis in 1996 concluded that they were closely related to sponges on the grounds that the detailed structure of chancellorid sclerites (“armor plates”) is similar to that of fibers of spongin, a collagen protein, in modern keratose (horny) demosponges such as Darwinella.^[108] However, another analysis in 2002 concluded that chancelloriids are not sponges and may be intermediate between sponges and more complex animals, among other reasons because their skins were thicker and more tightly connected than those of sponges.^[109] In 2008, a detailed analysis of chancelloriids’ sclerites concluded that they were very similar to those of halkieriids, mobile bilaterian animals that looked like slugs in chain mail and whose fossils are found in rocks from the very Early Cambrian to the Mid Cambrian. If this is correct, it would create a dilemma, as it is extremely unlikely that totally unrelated organisms could have developed such similar sclerites independently, but the huge difference in the structures of their bodies makes it hard to see how they could be closely related.^[107]

Relationships to other animal groups

Simplified family tree showing calcareous sponges as closest to more complex animals[110]

OpisthokontaFungiChoanoflagellatesMetazoaGlass spongesDemospongesCalcareous spongesEumetazoaComb jelliesPlacozoaCnidaria (jellyfish, etc.)other metazoans

Simplified family tree showing Homoscleromorpha as closest to more complex animals[111]

EukaryotesPlantsFungiMetazoaMost demospongesCalcareous spongesHomoscleromorphaEumetazoaCnidaria (jellyfish, etc.)other metazoans

In the 1990s, sponges were widely regarded as a monophyletic group, all of them having descended from a common ancestor that was itself a sponge, and as the “sister-group” to all other metazoans (multi-celled animals), which themselves form a monophyletic group. On the other hand, some 1990s analyses also revived the idea that animals’ nearest evolutionary relatives are choanoflagellates, single-celled organisms very similar to sponges’ choanocytes – which would imply that most Metazoa evolved from very sponge-like ancestors and therefore that sponges may not be monophyletic, as the same sponge-like ancestors may have given rise both to modern sponges and to non-sponge members of Metazoa.^[110]

Analyses since 2001 have concluded that Eumetazoa (more complex than sponges) are more closely related to particular groups of sponges than to other sponge groups. Such conclusions imply that sponges are not monophyletic, because the last common ancestor of all sponges would also be a direct ancestor of the Eumetazoa, which are not sponges. A study in 2001 based on comparisons of ribosome DNA concluded that the most fundamental division within sponges was between glass sponges and the rest, and that Eumetazoa are more closely related to calcareous sponges (those with calcium carbonate spicules) than to other types of sponge.^[110] In 2007, one analysis based on comparisons of RNA and another based mainly on comparison of spicules concluded that demosponges and glass sponges are more closely related to each other than either is to the calcareous sponges, which in turn are more closely related to Eumetazoa.^[95][112]

Other anatomical and biochemical evidence links the Eumetazoa with Homoscleromorpha, a sub-group of demosponges. A comparison in 2007 of nuclear DNA, excluding glass sponges and comb jellies, concluded that:

• Homoscleromorpha are most closely related to Eumetazoa;
• calcareous sponges are the next closest;
• the other demosponges are evolutionary “aunts” of these groups; and
• the chancelloriids, bag-like animals whose fossils are found in Cambrian rocks, may be sponges.^[111]

The sperm of Homoscleromorpha share features with the sperm of Eumetazoa, that sperm of other sponges lack. In both Homoscleromorpha and Eumetazoa layers of cells are bound together by attachment to a carpet-like basal membrane composed mainly of “typ IV” collagen, a form of collagen not found in other sponges – although the spongin fibers that reinforce the mesohyl of all demosponges is similar to “type IV” collagen.^[22]

The analyses described above concluded that sponges are closest to the ancestors of all Metazoa, of all multi-celled animals including both sponges and more complex groups. However, another comparison in 2008 of 150 genes in each of 21 genera, ranging from fungi to humans but including only two species of sponge, suggested that comb jellies (ctenophora) are the most basal lineage of the Metazoa included in the sample.^{[113][114][115][116]} If this is correct, either modern comb jellies developed their complex structures independently of other Metazoa, or sponges’ ancestors were more complex and all known sponges are drastically simplified forms. The study recommended further analyses using a wider range of sponges and other simple Metazoa such as Placozoa.^[113]

However, reanalysis of the data showed that the computer algorithms used for analysis were misled by the presence of specific ctenophore genes that were markedly different from those of other species, leaving sponges as either the sister group to all other animals, or an ancestral paraphyletic grade.^[117][118] ‘Family trees’ constructed using a combination of all available data – morphological, developmental and molecular – concluded that the sponges are in fact a monophyletic group, and with the cnidarians form the sister group to the bilaterians.^[119][120]

A very large and internally consistent alignment of 1,719 proteins at the metazoan scale, published in 2017, showed that (i) sponges – represented by Homoscleromorpha, Calcarea, Hexactinellida, and Demospongiae – are monophyletic, (ii) sponges are sister-group to all other multicellular animals, (iii) ctenophores emerge as the second-earliest branching animal lineage, and (iv) placozoans emerge as the third animal lineage, followed by cnidarians sister-group to bilaterians.^[121]

In March 2021, scientists from Dublin found additional evidence that sponges are the sister group to all other animals,^[122] while in May 2023, Schultz et al. found patterns of irreversible change in genome synteny that provide strong evidence that ctenophores are the sister group to all other animals instead.^[123]

Notable spongiologists

• Céline Allewaert
• Patricia Bergquist
• James Scott Bowerbank
• Maurice Burton
• Henry John Carter
• Max Walker de Laubenfels
• Arthur Dendy
• Édouard Placide Duchassaing de Fontbressin
• Randolph Kirkpatrick
• Robert J. Lendlmayer von Lendenfeld
• Edward Alfred Minchin
• Giovanni Domenico Nardo
• Eduard Oscar Schmidt
• Émile Topsent

Use

By dolphins

A report in 1997 described use of sponges as a tool by bottlenose dolphins in Shark Bay in Western Australia. A dolphin will attach a marine sponge to its rostrum, which is presumably then used to protect it when searching for food in the sandy sea bottom.^[124] The behavior, known as sponging, has only been observed in this bay and is almost exclusively shown by females. A study in 2005 concluded that mothers teach the behavior to their daughters and that all the sponge users are closely related, suggesting that it is a fairly recent innovation.^[16]

By humans

Main articles: Sea sponge aquaculture and Sponge diving

Skeleton

Main article: Sponge (material)

The calcium carbonate or silica spicules of most sponge genera make them too rough for most uses, but two genera, Hippospongia and Spongia, have soft, entirely fibrous skeletons.^[13]: 88 Early Europeans used soft sponges for many purposes, including padding for helmets, portable drinking utensils and municipal water filters. Until the invention of synthetic sponges, they were used as cleaning tools, applicators for paints and ceramic glazes and discreet contraceptives. However, by the mid-20th century, overfishing brought both the animals and the industry close to extinction.^[125]

Many objects with sponge-like textures are now made of substances not derived from poriferans. Synthetic sponges include personal and household cleaning tools, breast implants,^[126] and contraceptive sponges.^[127] Typical materials used are cellulose foam, polyurethane foam, and less frequently, silicone foam.

The luffa “sponge”, also spelled loofah, which is commonly sold for use in the kitchen or the shower, is not derived from an animal but mainly from the fibrous “skeleton” of the sponge gourd (Luffa aegyptiaca, Cucurbitaceae).^[128]

Antibiotic compounds

Sponges have medicinal potential due to the presence in sponges themselves or their microbial symbionts of chemicals that may be used to control viruses, bacteria, tumors and fungi.^[129][130]

Other biologically active compounds

Main article: Sponge isolates

Lacking any protective shell or means of escape, sponges have evolved to synthesize a variety of unusual compounds. One such class is the oxidized fatty acid derivatives called oxylipins. Members of this family have been found to have anti-cancer, anti-bacterial and anti-fungal properties. One example isolated from the Okinawan Plakortis sponges, plakoridine A, has shown potential as a cytotoxin to murine lymphoma cells.^[131][132]

Sunscreen, also known as sunblock,^[a] sun lotion or sun cream, is a photoprotective topical product for the skin that helps protect against sunburn and prevent skin cancer. Sunscreens come as lotions, sprays, gels, foams (such as an expanded foam lotion or whipped lotion^[4]), sticks, powders and other topical products. Sunscreens are common supplements to clothing, particularly sunglasses, sunhats and special sun protective clothing, and other forms of photoprotection (such as umbrellas).

Sunscreens may be classified according to the type of active ingredient(s) present in the formulation (inorganic compounds or organic molecules) as:

• Mineral sunscreens (also referred to as physical), which use only inorganic compounds (zinc oxide and/or titanium dioxide) as active ingredients. These ingredients primarily work by absorbing UV rays but also through reflection and refraction.^[5][6]
• Chemical sunscreens, which use organic molecules as active ingredients. These products are sometimes referred to as petrochemical sunscreens since the active organic molecules are synthesized starting from building blocks typically derived from petroleum.^[7] Chemical sunscreen ingredients also mainly work by absorbing the UV rays.^[8] Over the years, some organic UV absorbers have been heavily scrutinised to assess their toxicity^[9] and a few of them have been banned in places such as Hawaii^[10] and Thailand^[11] for their impact on aquatic life and the environment.
• Hybrid sunscreens, which contain a combination of organic and inorganic UV filters.

Medical organizations such as the American Cancer Society recommend the use of sunscreen because it aids in the prevention of squamous cell carcinomas.^[12] The routine use of sunscreens may also reduce the risk of melanoma.^[13] To effectively protect against all the potential damages of UV light, the use of broad-spectrum sunscreens (covering both UVA and UVB radiation) has been recommended.^[3]

History

[edit]

Early civilizations used a variety of plant products to help protect the skin from sun damage. For example, ancient Greeks used olive oil for this purpose, and ancient Egyptians used extracts of rice, jasmine, and lupine plants whose products are still used in skin care today.^[14] Zinc oxide paste has also been popular for skin protection for thousands of years.^[15] Among the nomadic sea-going Sama-Bajau people of the Philippines, Malaysia, and Indonesia, a common type of sun protection is a paste called borak or burak, which was made from water weeds, rice and spices. It is used most commonly by women to protect the face and exposed skin areas from the harsh tropical sun at sea.^[16] In Myanmar, thanaka, a yellow-white cosmetic paste made of ground bark, is traditionally used for sun protection. In Madagascar, a ground wood paste called masonjoany has been worn for sun protection, as well as decoration and insect repellent, since the 18th century, and is ubiquitous in the Northwest coastal regions of the island to this day.^[17][18]

The first ultraviolet B filters were produced in 1928.^[19] Followed by the first sunscreen, invented in Australia by chemist H.A. Milton Blake, in 1932^[20] formulating with the UV filter ‘salol’ (Phenyl salicylate) at a concentration of 10%.^[21] Its protection was verified by the University of Adelaide.^[22][23] In 1936, L’Oreal released its first sunscreen product, formulated by French chemist Eugène Schueller.^[20]

The US military was an early adopter of sunscreen. In 1944, as the hazards of sun overexposure became apparent to soldiers stationed in the Pacific tropics at the height of World War II,^{[24][20][25][26]} Benjamin Green, an airman and later a pharmacist produced Red Vet Pet (for red veterinary petrolatum) for the US military. Sales boomed when Coppertone improved and commercialized the substance under the Coppertone girl and Bain de Soleil branding in the early 1950s. In 1946, Austrian chemist Franz Greiter introduced a product, called Gletscher Crème (Glacier Cream), subsequently became the basis for the company Piz Buin, named in honor of the mountain where Greiter allegedly received the sunburn.^[27][28][29]

In 1974, Greiter adapted earlier calculations from Friedrich Ellinger and Rudolf Schulze and introduced the “sun protection factor” (SPF), which has become the global standard for measuring UVB protection.^[24][30] It has been estimated that Gletscher Crème had an SPF of 2.

Water-resistant sunscreens were introduced in 1977,^[20] and recent development efforts have focused on overcoming later concerns by making sunscreen protection both longer-lasting and broader-spectrum (protection from both UVA & UVB rays), more environmentally friendly,^[31] more appealing to use^[24] and addressing the safety concerns of petrochemical sunscreens, i.e. FDA studies showing their systematic absorption into the bloodstream.^[32]

Health effects

[edit]

See also: Health effects of sunlight exposure

Benefits

[edit]

Sunscreen use can help prevent melanoma^[33][34][35] and squamous cell carcinoma, two types of skin cancer.^[36] There is little evidence that it is effective in preventing basal cell carcinoma.^[37]

A 2013 study concluded that the diligent, everyday application of sunscreen could slow or temporarily prevent the development of wrinkles and sagging skin.^[38] The study involved 900 white people in Australia and required some of them to apply a broad-spectrum sunscreen every day for four and a half years. It found that people who did so had noticeably more resilient and smoother skin than those assigned to continue their usual practices.^[38] A study on 32 subjects showed that daily use of sunscreen (SPF 30) reversed photoaging of the skin within 12 weeks and the amelioration continued until the end of the investigation period of one year.^[39] Sunscreen is inherently anti-ageing as the sun is the number one cause of premature ageing; it therefore may slow or temporarily prevent the development of wrinkles, dark spots, and sagging skin.

Minimizing UV damage is especially important for children and fair-skinned individuals and those who have sun sensitivity for medical reasons.^[40]

Risks

[edit]

In February 2019, the US Food and Drug Administration (FDA) started classifying already approved UV filter molecules into three categories: those which are generally recognized as safe and effective (GRASE), those which are non-GRASE due to safety issues, and those requiring further evaluation.^[41] As of 2021, only zinc oxide and titanium dioxide are recognized as GRASE.^[42] Two previously approved UV filters, para-aminobenzoic acid (PABA) and trolamine salicylate, were banned in 2021 due to safety concerns. The remaining FDA approved active ingredients were put in the third category as their manufacturers have yet to produce sufficient safety data — despite the fact that some of the chemicals have sold in sunscreen products for more than 40 years.^[7] Some researchers argue that the risk of sun-induced skin cancer outweighs concerns about toxicity and mutagenicity,^[43][44] although environmentalists say this ignores “ample safer alternatives available on the market containing the active ingredient minerals zinc oxide or titanium dioxide”, which are also safer for the environment.^[45]

Regulators can investigate and ban UV filters over safety concerns (such as PABA), which can result in withdrawal of products from the consumer market.^[24][46] Regulators, such as the TGA and the FDA, have also been concerned with recent reports of contamination in sunscreen products with known possible human carcinogens such as benzene and benzophenone.^[47] Independent laboratory testing carried out by Valisure found benzene contamination in 27% of the sunscreens they tested, with some batches having up to triple the FDA’s conditionally restricted limit of 2 parts per million (ppm).^[48] This resulted in a voluntary recall by some major sunscreen brands that were implicated in the testing, as such, regulators also help publicise and coordinate these voluntary recalls.^[49] VOC’s (Volatile Organic Compounds) such as benzene, are particularly harmful in sunscreen formulations as many active and inactive ingredients can increase permeation across the skin.^[50] Butane, which is used as a propellant in spray sunscreens, has been found to have benzene impurities from the refinement process.^[51]

There is a risk of an allergic reaction to sunscreen for some individuals, as “Typical allergic contact dermatitis may occur in individuals allergic to any of the ingredients that are found in sunscreen products or cosmetic preparations that have a sunscreen component. The rash can occur anywhere on the body where the substance has been applied and sometimes may spread to unexpected sites.”^[52]

Vitamin D production

[edit]

There are some concerns about potential vitamin D deficiency arising from prolonged use of sunscreen.^[53][54] The typical use of sunscreen does not usually result in vitamin D deficiency; however, extensive usage may.^[55] Sunscreen prevents ultraviolet light from reaching the skin, and even moderate protection can substantially reduce vitamin D synthesis.^[56][57] However, adequate amounts of vitamin D can be obtained via diet or supplements.^[58] Vitamin D overdose is impossible from UV exposure due to an equilibrium the skin reaches in which vitamin D degrades as quickly as it is created.^[59][60][61]

High-SPF sunscreens filter out most UVB radiation, which triggers vitamin D production in the skin. However, clinical studies show that regular sunscreen use does not lead to vitamin D deficiency. Even high-SPF sunscreens allow a small amount of UVB to reach the skin, sufficient for vitamin D synthesis. Additionally, brief, unprotected sun exposure can produce ample vitamin D, but this exposure also risks significant DNA damage and skin cancer. To avoid these risks, vitamin D can be obtained safely through diet and supplements. Foods like fatty fish, fortified milk, and orange juice, along with supplements, provide necessary vitamin D without harmful sun exposure. ^[62]

Studies have shown that sunscreen with a high UVA protection factor enabled significantly higher vitamin D synthesis than a low UVA protection factor sunscreen, likely because it allows more UVB transmission.^[63][64]

Measurements of protection

[edit]

Sun protection factor and labeling 

[edit]

The sun protection factor (SPF rating, introduced in 1974) is a measure of the fraction of sunburn-producing UV rays that reach the skin. For example, “SPF 15” means that ¹⁄₁₅ of the burning radiation will reach the skin, assuming sunscreen is applied evenly at a thick dosage of 2 milligrams per square centimeter^[65] (mg/cm²). It is important to note that sunscreens with higher SPF do not last or remain effective on the skin any longer than lower SPF and must be continually reapplied as directed, usually every two hours.^[66]

The SPF is an imperfect measure of skin damage because invisible damage and skin malignant melanomas are also caused by ultraviolet A (UVA, wavelengths 315–400 or 320–400 nm), which does not primarily cause reddening or pain. Conventional sunscreen blocks very little UVA radiation relative to the nominal SPF; broad-spectrum sunscreens are designed to protect against both UVB and UVA.^[67][68][69] According to a 2004 study, UVA also causes DNA damage to cells deep within the skin, increasing the risk of malignant melanomas.^[70] Even some products labeled “broad-spectrum UVA/UVB protection” have not always provided good protection against UVA rays.^[71] Titanium dioxide probably gives good protection but does not completely cover the UVA spectrum, with early 2000s research suggesting that zinc oxide is superior to titanium dioxide at wavelengths 340–380 nm.^[72]

Owing to consumer confusion over the real degree and duration of protection offered, labelling restrictions are enforced in several countries. In the EU, sunscreen labels can only go up to SPF 50+ (initially listed as 30 but soon revised to 50).^[73] Australia‘s Therapeutic Goods Administration increased the upper limit to 50+ in 2012.^[74][75] In its 2007 and 2011 draft rules, the US Food and Drug Administration (FDA) proposed a maximum SPF label of 50, to limit unrealistic claims.^[76][3][77] (As of August 2019, the FDA has not adopted the SPF 50 limit.^[78]) Others have proposed restricting the active ingredients to an SPF of no more than 50, due to lack of evidence that higher dosages provide more meaningful protection,^[79] despite a common misconception that protection directly scales with SPF; doubling when SPF is doubled.^[80][81] Different sunscreen ingredients have different effectiveness against UVA and UVB.^[82]

The SPF can be measured by applying sunscreen to the skin of a volunteer and measuring how long it takes before sunburn occurs when exposed to an artificial sunlight source. In the US, such an in vivo test is required by the FDA. It can also be measured in vitro with the help of a specially designed spectrometer. In this case, the actual transmittance of the sunscreen is measured, along with the degradation of the product due to being exposed to sunlight. In this case, the transmittance of the sunscreen must be measured over all wavelengths in sunlight’s UVB–UVA range (290–400 nm), along with a table of how effective various wavelengths are in causing sunburn (the erythemal action spectrum) and the standard intensity spectrum of sunlight (see the figure). Such in vitro measurements agree very well with in vivo measurements.^{[attribution needed]}

Numerous methods have been devised for evaluation of UVA and UVB protection. The most-reliable spectrophotochemical methods eliminate the subjective nature of grading erythema.^[83]

The ultraviolet protection factor (UPF) is a similar scale developed for rating fabrics for sun protective clothing. According to recent testing by Consumer Reports, UPF ~30+ is typical for protective fabrics, while UPF ~20 is typical for standard summer fabrics.^[84]

Mathematically, the SPF (or the UPF) is calculated from measured data as:^{[citation needed]}

SPF=∫A(λ)E(λ)dλ∫A(λ)E(λ)/MPF(λ)dλ,

where E(λ) is the solar irradiance spectrum, A(λ) the erythemal action spectrum, and MPF(λ) the monochromatic protection factor, all functions of the wavelength λ. The MPF is roughly the inverse of the transmittance at a given wavelength.^{[citation needed]}

The combined SPF of two layers of sunscreen may be lower than the square of the single-layer SPF.^[85]

UVA protection

[edit]

Persistent pigment darkening

[edit]

The persistent pigment darkening (PPD) method is a method of measuring UVA protection, similar to the SPF method of measuring sunburn protection. Originally developed in Japan, it is the preferred method used by manufacturers such as L’Oréal.

Instead of measuring erythema, the PPD method uses UVA radiation to cause a persistent darkening or tanning of the skin. Theoretically, a sunscreen with a PPD rating of 10 should allow a person 10 times as much UVA exposure as would be without protection. The PPD method is an in vivo test like SPF. In addition, the European Cosmetic and Perfumery Association (Colipa) has introduced a method that, it is claimed, can measure this in vitro and provide parity with the PPD method.^[86]

SPF equivalence

[edit]

As part of revised guidelines for sunscreens in the EU, there is a requirement to provide the consumer with a minimum level of UVA protection in relation to the SPF. This should be a UVA protection factor of at least 1/3 of the SPF to carry the UVA seal.^[87] The 1/3 threshold derives from the European Commission recommendation 2006/647/EC.^[88] This Commission recommendation specifies that the UVA protection factor should be measured using the PPD method as modified by the French health agency AFSSAPS (now ANSM) “or an equivalent degree of protection obtained with any in vitro method”.^[89]

A set of final US FDA rules effective from summer 2012 defines the phrase “broad spectrum” as providing UVA protection proportional to the UVB protection, using a standardized testing method.^[3]

Star rating system

[edit]

In the UK and Ireland, the Boots star rating system is a proprietary in vitro method used to describe the ratio of UVA to UVB protection offered by sunscreen creams and sprays. Based on original work by Brian Diffey at Newcastle University, the Boots Company in Nottingham, UK, developed a method that has been widely adopted by companies marketing these products in the UK.

One-star products provide the lowest ratio of UVA protection, five-star products the highest. The method was revised in light of the Colipa UVA PF test and the revised EU recommendations regarding UVA PF. The method still uses a spectrophotometer to measure absorption of UVA versus UVB; the difference stems from a requirement to pre-irradiate samples (where this was not previously required) to give a better indication of UVA protection and photostability when the product is used. With the current methodology, the lowest rating is three stars, the highest being five stars.

In August 2007, the FDA put out for consultation the proposal that a version of this protocol be used to inform users of American product of the protection that it gives against UVA;^[76] but this was not adopted, for fear it would be too confusing.^[79]

PA system

[edit]

Asian brands, particularly Japanese ones, tend to use The Protection Grade of UVA (PA) system to measure the UVA protection that a sunscreen provides. The PA system is based on the PPD reaction and is now widely adopted on the labels of sunscreens. According to the Japan Cosmetic Industry Association, PA+ corresponds to a UVA protection factor between two and four, PA++ between four and eight, and PA+++ more than eight. This system was revised in 2013 to include PA++++ which corresponds to a PPD rating of sixteen or above.

Expiration date

[edit]

Some sunscreens include an expiration date—a date indicating when they may become less effective.^[90]

Active ingredients

[edit]

Sunscreen formulations contain UV absorbing compounds (the active ingredients) dissolved or dispersed in a mixture of other ingredients, such as water, oils, moisturizers, and antioxidants. The UV filters can be either:

• Organic compounds which absorb ultraviolet light.^[91] Some organic compounds (bisoctrizole and phenylene bis-diphenyltriazine) also partially reflect incident light.^[92] These are also referred to as “chemical” UV filters.
• Inorganic compounds (zinc oxide and titanium dioxide), which reflect, scatter, and absorb UV light.^[93] These are also referred to as “mineral” filters.

The organic compounds used as UV filter are often aromatic molecules conjugated with carbonyl groups. This general structure allows the molecule to absorb high-energy ultraviolet rays and release the energy as lower-energy rays, thereby preventing the skin-damaging ultraviolet rays from reaching the skin. So, upon exposure to UV light, most of the ingredients (with the notable exception of avobenzone) do not undergo significant chemical change, allowing these ingredients to retain the UV-absorbing potency without significant photodegradation.^[94] A chemical stabilizer is included in some sunscreens containing avobenzone to slow its breakdown. The stability of avobenzone can also be improved by bemotrizinol,^[95] octocrylene^[96] and various other photostabilisers. Most organic compounds in sunscreens slowly degrade and become less effective over the course of several years even if stored properly, resulting in the expiration dates calculated for the product.^[97]

Sunscreening agents are used in some hair care products such as shampoos, conditioners and styling agents to protect against protein degradation and color loss. Currently, benzophenone-4 and ethylhexyl methoxycinnamate are the two sunscreens most commonly used in hair products. The common sunscreens used on skin are rarely used for hair products due to their texture and weight effects.

UV filters need usually to be approved by local agencies (such as the FDA in the United States) to be used in sunscreen formulations. As of 2023, 29 compounds are approved in the European Union and 17 in the USA.^[92] No UV filters have been approved by the FDA for use in cosmetics since 1999.

The following are the FDA allowable active ingredients in sunscreens:

UV-filter	Other names	Maximum concentration	Known permitting jurisdictions	Results of safety testing	UVA	UVB
p-Aminobenzoic acid	PABA	15% (USA), (EU: banned from sale to consumers from 8 October 2009)	USA			X
Padimate O	OD-PABA, octyldimethyl-PABA, σ-PABA	8% (USA, AUS) 10% (JP)(Not currently supported in EU and may be delisted)	EU, USA, AUS, JP			X
Phenylbenzimidazole sulfonic acid	Ensulizole, PBSA	4% (USA, AUS) 8% (EU) 3% (JP)	EU, USA, AUS, JP			X
Cinoxate	2-Ethoxyethyl p-methoxycinnamate	3% (USA) 6% (AUS)	USA, AUS		X	X
Dioxybenzone	Benzophenone-8	3% (USA)	USA, AUS		X	X
Oxybenzone	Benzophenone-3	6% (USA), 2.2% (body) / 6% (face) EU,[98] 10% AUS,	EU, USA, AUS	Banned in Hawaii since 2018[10] – “harmful to coral reefs, fish, and other ocean life”[99]	X	X
Homosalate	Homomethyl salicylate	7.34% (EU) 15% (USA, AUS)	EU, USA, AUS			X
Menthyl anthranilate	Meradimate	5% (USA)	USA, AUS		X
Octocrylene	Eusolex OCR, Parsol 340, 2-Cyano-3,3-diphenyl acrylic acid, 2-ethylhexylester	10% (USA)	EU, USA, AUS		X	X
Octinoxate	Octyl-methoxycinnamate, Ethylhexyl methoxycinnamate, 2-Ethylhexyl-paramethoxycinnamate	7.5% (USA) 10% (EU, AUS) 20% (JP)	EU, USA, AUS, JP	Banned in Hawaii since 2021 – harmful to coral[100]		X
Octyl salicylate	Octisalate, 2-Ethylhexyl salicylate	5% (EU, USA, AUS) 10% (JP)	EU, USA, AUS, JP			X
Sulisobenzone	2-Hydroxy-4-Methoxybenzophenone-5-sulfonic acid, 3-Benzoyl-4-hydroxy-6-methoxybenzenesulfonic acid, Benzophenone-4	5% (EU) 10% (USA, AUS, JP)	EU, USA, AUS, JP		X	X
Avobenzone	1-(4-methoxyphenyl)-3-(4-tert-butyl phenyl)propane-1,3-dione, Butyl methoxy dibenzoylmethane,	3% (USA) 5% (EU, AUS)	EU, USA, AUS		X
Ecamsule	Terephthalylidene Dicamphor Sulfonic Acid	10%	EU, AUS (US: approved in certain formulations up to 3% via New Drug Application (NDA) Route)		X
Titanium dioxide	CI77891, TiO₂	25% (US) No limit (JP)	EU, USA, AUS, JP	Generally recognized as safe and effective by the FDA[101]		X
Zinc oxide	CI77947, ZnO	25% (US) No limit (AUS, JP)	EU, USA, AUS, JP	Generally recognized as safe and effective by the FDA.[101] Protects against skin tumors in mice[102]	X	X

Zinc oxide was approved as a UV filter by the EU in 2016.^[103]

Other ingredients approved within the EU^[104] and other parts of the world,^[105] that have not been included in the current FDA Monograph:

UV-filter	Other names	Maximum concentration	Permitted in	Results of safety testing	UVA	UVB
4-Methylbenzylidene camphor	Enzacamene, MBC	4%*	EU, AUS			X
Bisoctrizole	Methylene Bis-Benzotriazolyl Tetramethylbutylphenol, MBBT	10%*	EU, AUS, JP		X	X
Bemotrizinol	Bis-ethylhexyloxyphenol methoxyphenol triazine, BEMT, anisotriazine	10% (EU, AUS) 3% (JP)*	EU, AUS, JP		X	X
Tris-biphenyl triazine		10%	EU, AUS		X	X
Trolamine salicylate	Triethanolamine salicylate	12%	AUS			X
Drometrizole trisiloxane		15%	EU, AUS		X	X
Benzophenone-9	CAS 3121-60-6, Sodium Dihydroxy Dimethoxy Disulfobenzophenone [106]	10%	JP
Ethylhexyl triazone	octyl triazone, EHT	5% (EU, AUS) 3% (JP)*	EU, AUS			X
Diethylamino hydroxybenzoyl hexyl benzoate		10%	EU, AUS, JP		X
Iscotrizinol	diethylhexyl butamido triazone, DBT	10% (EU) 5% (JP)*	EU, JP			X
Polysilicone-15	Dimethico-diethylbenzalmalonate	10%	EU, AUS, JP			X
Amiloxate	Isopentyl-4-methoxycinnamate, Isoamyl p-Methoxycinnamate, IMC	10%*	EU, AUS			X
Methoxypropylamino cyclohexenylidene ethoxyethylcyanoacetate		3%	EU		X

^* Time and Extent Application (TEA), Proposed Rule on FDA approval originally expected 2009, now expected 2015.^{[needs update]}

Many of the ingredients awaiting approval by the FDA are relatively new, and developed to absorb UVA.^[107] The 2014 Sunscreen Innovation Act was passed to accelerate the FDA approval process.^[108][109]

Inactive ingredients

[edit]

It is known that SPF is affected by not only the choice of active ingredients and the percentage of active ingredients but also the formulation of the vehicle/base. Final SPF is also impacted by the distribution of active ingredients in the sunscreen, how evenly the sunscreen applies on the skin, how well it dries down on the skin and the pH value of the product among other factors. Changing any inactive ingredient may potentially alter a sunscreen’s SPF.^[110][111]

When combined with UV filters, added antioxidants can work synergistically to affect the overall SPF value positively. Furthermore, adding antioxidants to sunscreen can amplify its ability to reduce markers of extrinsic photoaging, grant better protection from UV induced pigment formation, mitigate skin lipid peroxidation, improve the photostability of the active ingredients, neutralize reactive oxygen species formed by irradiated photocatalysts (e.g., uncoated TiO₂) and aid in DNA repair post-UVB damage, thus enhancing the efficiency and safety of sunscreens.^{[112][113][114][115]} Compared with sunscreen alone, it has been shown that the addition of antioxidants has the potential to suppress ROS formation by an additional 1.7-fold for SPF 4 sunscreens and 2.4-fold for SPF 15-to-SPF 50 sunscreens, but the efficacy depends on how well the sunscreen in question has been formulated.^[116] Sometimes osmolytes are also incorporated into commercially available sunscreens in addition to antioxidants since they also aid in protecting the skin from the detrimental effects of UVR.^[117] Examples include the osmolyte taurine, which has demonstrated the ability to protects against UVB-radiation induced immunosuppression^[118] and the osmolyte ectoine, which aids in counteracting cellular accelerated aging & UVA-radiation induced premature photoaging.^[119]

Other inactive ingredients can also assist in photostabilizing unstable UV filters. Cyclodextrins have demonstrated the ability to reduce photodecomposition, protect antioxidants and limit skin penetration past the uppermost skin layers, allowing them to longer maintain the protection factor of sunscreens with UV filters that are highly unstable and/or easily permeate to the lower layers of the skin.^{[120][121][113]} Similarly, film-forming polymers like polyester-8 and polycryleneS1 have the ability to protect the efficacy of older petrochemical UV filters by preventing them from destabilizing due to extended light exposure. These kinds of ingredients also increase the water resistance of sunscreen formulations.^[122][123]

In the 2010s and 2020s, there has been increasing interest in sunscreens that protect the wearer from the sun’s high-energy visible light and infrared light as well as ultraviolet light. This is due to newer research revealing blue & violet visible light and certain wavelengths of infrared light (e.g., NIR, IR-A) work synergistically with UV light in contributing to oxidative stress, free radical generation, dermal cellular damage, suppressed skin healing, decreased immunity, erythema, inflammation, dryness, and several aesthetic concerns, such as: wrinkle formation, loss of skin elasticity and dyspigmentation.^{[124][125][126][127][128][129][130]} Increasingly, a number of commercial sunscreens are being produced that have manufacturer claims regarding skin protection from blue light, infrared light and even air pollution.^[130] However, as of 2021 there are no regulatory guidelines or mandatory testing protocols that govern these claims.^[116] Historically, the American FDA has only recognized protection from sunburn (via UVB protection) and protection from skin cancer (via SPF 15+ with some UVA protection) as drug/medicinal sunscreen claims, so they do not have regulatory authority over sunscreen claims regarding protecting the skin from damage from these other environmental stressors.^[131] Since sunscreen claims not related to protection from ultraviolet light are treated as cosmeceutical claims rather than drug/medicinal claims, the innovative technologies and additive ingredients used to allegedly reduce the damage from these other environmental stressors may vary widely from brand to brand.

Some studies show that mineral sunscreens primarily made with substantially large particles (i.e., neither nano nor micronized) may help protect from visible and infrared light to some degree,^{[130][116][132]} but these sunscreens are often unacceptable to consumers due to leaving an obligatory opaque white cast on the skin. Further research has shown that sunscreens with added iron oxide pigments and/or pigmentary titanium dioxide can provide the wearer with a substantial amount of HEVL protection.^{[116][133][134][135]} Cosmetic chemists have found that other cosmetic-grade pigments can be functional filler ingredients. Mica was discovered to have significant synergistic effects with UVR filters when formulated in sunscreens, in that it can notably increase the formula’s ability to protect the wearer from HEVL.^[128]

There is a growing amount of research demonstrating that adding various vitamer antioxidants (eg; retinol, alpha tocopherol, gamma tocopherol, tocopheryl acetate, ascorbic acid, ascorbyl tetraisopalmitate, ascorbyl palmitate, sodium ascorbyl phosphate, ubiquinone) and/or a mixture of certain botanical antioxidants (eg; epigallocatechin-3-gallate, b-carotene, vitis vinifera, silymarin, spirulina extract, chamomile extract and possibly others) to sunscreens efficaciously aids in reducing damage from the free radicals produced by exposure to solar ultraviolet radiation, visible light, near infrared radiation and infrared-a radiation.^{[112][136][126][116][137][114][117]} Since sunscreen’s active ingredients work preventatively by creating a shielding film on the skin that absorbs, scatters, and reflects light before it can reach the skin, UV filters have been deemed an ideal “first line of defense” against sun damage when exposure can’t be avoided. Antioxidants have been deemed a good “second line of defense” since they work responsively by decreasing the overall burden of free radicals that do reach the skin.^[128] The degree of the free radical protection from the entire solar spectral range that a sunscreen can offer has been termed the “radical protection factor” (RPF) by some researchers.

Application

[edit]

SPF 30 or above must be used to effectively prevent UV rays from damaging skin cells. This is the amount that is recommended to prevent against skin cancer. Sunscreen must also be applied thoroughly and re-applied during the day, especially after being in the water. Special attention should be paid to areas like the ears and nose, which are common spots of skin cancer. Dermatologists may be able to advise about what sunscreen is best to use for specific skin types.^[138]

The dose used in FDA sunscreen testing is 2 mg/cm² of exposed skin.^[94] If one assumes an “average” adult build of height 5 ft 4 in (163 cm) and weight 150 lb (68 kg) with a 32-inch (82-cm) waist, that adult wearing a bathing suit covering the groin area should apply approximately 30 g (or 30 ml, approximately 1 oz) evenly to the uncovered body area. This can be more easily thought of as a “golf ball” size amount of product per body, or at least six teaspoonfuls. Larger or smaller individuals should scale these quantities accordingly.^[139] Considering only the face, this translates to about 1/4 to 1/3 of a teaspoon for the average adult face.

Some studies have shown that people commonly apply only 1/4 to 1/2 of the amount recommended for achieving the rated sun protection factor (SPF), and in consequence the effective SPF should be downgraded to a 4th root or a square root of the advertised value, respectively.^[85] A later study found a significant exponential relation between SPF and the amount of sunscreen applied, and the results are closer to linearity than expected by theory.^[140]

Claims that substances in pill form can act as sunscreen are false and disallowed in the United States.^[141]

Regulation

[edit]

This section needs expansion. You can help by adding to it. (January 2018)

Palau

On 1 January 2020, Palau banned the manufacturing and selling of sun cream products containing any of the following ingredients: benzophenone-3, octyl methoxycinnamate, octocrylene, 4-methyl-benzylidene camphor, triclosan, methylparaben, ethylparaben, butylparaben, benzyl paraben, and phenoxyethanol.^[142] The decision was taken to protect the local coral reef and sea life.^[143] Those compounds are known or suspected to be harmful to coral or other sea life.^[143]United States

Sunscreen labeling standards have been evolving in the United States since the FDA first adopted the SPF calculation in 1978.^[144] The FDA issued a comprehensive set of rules in June 2011, taking effect in 2012–2013, designed to help consumers identify and select suitable sunscreen products offering protection from sunburn, early skin aging, and skin cancer.^{[145][146][147]} However, unlike other countries, the United States classifies sunscreen as an over-the-counter drug rather than a cosmetic product. As FDA approval of a new drug is typically far slower than for a cosmetic, the result is fewer ingredients available for sunscreen formulations in the US compared with many other countries.^[148][149]

In 2019, the FDA proposed tighter regulations on sun protection and general safety, including the requirement that sunscreen products with SPF greater than 15 must be broad spectrum, and imposing a prohibition on products with SPF greater than 60.^[150]

• To be classified as “broad spectrum”, sunscreen products must provide protection against both UVA and UVB, with specific tests required for both.
• Claims of products being “waterproof” or “sweatproof” are prohibited, while the terms “sunblock” and “instant protection” and “protection for more than 2 hours” are all prohibited without specific FDA approval.
• “Water resistance” claims on the front label must indicate how long the sunscreen remains effective and specify whether this applies to swimming or sweating, based on standard testing.
• Sunscreens must include standardized “Drug Facts” information on the container. However, there is no regulation that deems it necessary to mention whether the contents contain nanoparticles of mineral ingredients. Furthermore, US products do not require the expiration date of products to be displayed on the label.^[151]

In 2021, the FDA introduced an additional administrative order regarding the safety classification of cosmetic UV filters, to categorize a given ingredient as either:

• Generally recognized as safe and effective (GRASE)
• Not GRASE due to safety issues
• Not GRASE because additional safety data are needed.^[92][152]

To be considered a GRASE active ingredient, the FDA requires it to have undergone both non-clinical animal studies as well as human clinical studies. The animal studies evaluate the potential for inducing carcinogenesis, genetic or reproductive harm, and any toxic effects of the ingredient once absorbed and distributed in the body. The human trials expand upon the animal trials, providing additional information on safety in the pediatric population, protection against UVA and UVB, and the potential for skin reactions after application. Two previously approved UV filters, para-aminobenzoic acid (PABA) and trolamine salicylate, were reclassified as not GRASE due to safety concerns and have consequently been removed from the market.^[92]

Europe

In Europe, sunscreens are considered a cosmetic product rather than an over-the-counter drug. These products are regulated by the Cosmetic Regulation (EC) No 1223/2009, which was created in July 2013.^[151] The recommendations for formulating sunscreen products are guided by the Scientific Community on Consumer Safety (SCCS).^[153] The regulation of cosmetic products in Europe requires the producer to follow six domains when formulating their product:

I. Cosmetic safety report must be conducted by a qualified personnel

II. The product must not contain substances banned for cosmetic products

III. The product must not contain substances restricted for cosmetic products

IV. The product must adhere to the approved list of colourants for cosmetic products.

V. The product must adhere to the approved list of preservatives for cosmetic products.

VI. The product must contain UV filters approved in Europe.^[153]

According to the EC, sunscreens at a minimum must exhibit:

1. A SPF of 6
2. UVA/UVB ratio ≥ 1/3
3. The critical wavelength is at least 370 nanometers (indicating that it is “broad-spectrum”).
4. Instructions for using and precautions.
5. Evidence the sunscreen meets UVA and SPF requirements.^[153]
6. Labels of European sunscreens must disclose the use of nanoparticles in addition to the shelf life of the product.^[151]

Canada

Regulation of sunscreen is dependent on the ingredient used; It is then classified and follows the regulations for either natural health products or drug product. Companies must complete a product licensing application prior to introducing their sunscreen on the market.^[153]

ASEAN (Brunei, Cambodia, Indonesia, Laos, Malaysia, Myanmar, the Philippines, Singapore, Thailand, Vietnam)

The regulation of sunscreen for ASEAN countries closely follows European regulations. However, products are regulated by the ASEAN scientific community rather than the SCCS. Additionally, there are minor differences in the allowed phrasing printed on sunscreen packages.^[153]

Japan

Sunscreen is considered a cosmetic product, and is regulated under the Japan Cosmetic Industry Association (JCIA). Products are regulated mostly for the type of UV filter and SPF. SPF may range from 2 to 50.^[153]

China

Sunscreen is regulated as cosmetic product under the State Food and Drug Administration (SFDA). The list of approved filters is the same as it is in Europe. However, sunscreen in China requires safety testing in animal studies prior to approval.^[153]

Australia

Sunscreens are divided into therapeutic and cosmetic sunscreens. Therapeutic sunscreens are classified into primary sunscreens (SPF ≥ 4) and secondary sunscreens (SPF < 4). Therapeutic sunscreens are regulated by the Therapeutic Goods Administration (TGA). Cosmetic sunscreens are products that contain a sunscreen ingredient, but do not protect from the sun. These products are regulated by the National Industrial Chemicals Notification and Assessment Scheme (NICNAS).^[153]

New Zealand

Sunscreen is classified as a cosmetic product, and closely follows EU regulations. However, New Zealand has a more extensive list of approved UV filters than Europe.^[153]

Mercosur

Mercosur is an international group consisting of Argentina, Brazil, Paraguay, and Uruguay. Regulation of sunscreen as a cosmetic product began in 2012, and is similar in structure to the European regulations. Sunscreens must meet specific standards including water resistance, sun protection factor, and a UVA/UVB ratio of 1/3. The list of approved sunscreen ingredients is greater than in Europe or the US.^[153]

Environmental effects

[edit]

Some sunscreen active ingredients have been shown to cause toxicity towards marine life and coral, resulting in bans in different states, countries and ecological areas.^[154][155] Coral reefs, comprising organisms in delicate ecological balances, are vulnerable to even minor environmental disturbances. Factors like temperature changes, invasive species, pollution, and detrimental fishing practices have previously been highlighted as threats to coral health.^[156][157]

In 2018, Hawaii passed legislation that prohibits the sale of sunscreens containing oxybenzone and octinoxate. In sufficient concentrations, oxybenzone and octinoxate can damage coral DNA, induce deformities in coral larvae,^[155] heighten the risk of viral infections, and make corals more vulnerable to bleaching. Such threats are even more concerning given that coral ecosystems are already compromised by climate change, pollution, and other environmental stressors. While there is ongoing debate regarding the real-world concentrations of these chemicals versus laboratory settings,^{[158][159][160][161]} an assessment in Kahaluu Bay in Hawaii showed oxybenzone concentrations to be 262 times higher than what the U.S. Environmental Protection Agency designates as high-risk. Another study in Hanauma Bay found levels of the chemical ranging from 30 ng/L to 27,880 ng/L, noting that concentrations beyond 63 ng/L could induce toxicity in corals.^[162]

Echoing Hawaii’s initiative, other regions including Key West, Florida,^[163] the U.S. Virgin Islands,^[164] Bonaire, and Palau^[165] have also instituted bans on sunscreens containing oxybenzone and octinoxate.

The environmental implications of sunscreen usage on marine ecosystems are multi-faceted and vary in severity. In a 2015 study, titanium dioxide nanoparticles, when introduced to water and subjected to ultraviolet light, were shown to amplify the production of hydrogen peroxide, a compound known to damage phytoplankton.^[166] In 2002, research indicated that sunscreens might escalate virus abundance in seawater, compromising the marine environment in a manner akin to other pollutants.^[167] Further probing the matter, a 2008 investigation examining a variety of sunscreen brands, protective factors, and concentrations revealed unanimous bleaching effects on hard corals. Alarmingly, the degree of bleaching magnified with increasing sunscreen quantities. When assessing individual compounds prevalent in sunscreens, substances such as butylparaben, ethylhexylmethoxycinnamate, benzophenone-3, and 4-methylbenzylidene camphor induced complete coral bleaching at even minimal concentrations.^[168]

A 2020 study from the journal Current Dermatology Report summarized the situation as the US FDA currently approving only zinc oxide (ZnO) and titanium dioxide (TiO₂) as safe ultraviolet filters, and for those concerned with coral bleaching, they should use non-nano ZnO or TiO₂ since they have the most consistent safety data.^[169]

Research and Development

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New products are in development such as sunscreens based on bioadhesive nanoparticles. These function by encapsulating commercially used UV filters, while being not only adherent to the skin but also non-penetrant. This strategy inhibits primary UV-induced damage as well as secondary free radicals.^[170] UV filters based on sinapate esters are also under study.^[171] Sunscreens with natural and sustainable connotations are increasingly being developed, as a result of increased environmental concern.^[