Sunscreen

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Sunscreen
Sunscreen on back under normal and UV light.jpg
Sunscreen on back under normal and UV light
Synonyms Sun screen, sunblock, suntan lotion, sunburn cream, sun cream, block out[1]

Sunscreen, also known as sunblock and suntan lotion, is a lotion, spray, gel or other topical product that absorbs or reflects some of the sun's ultraviolet (UV) radiation and thus helps protect against sunburn. Diligent use of sunscreen can also slow or temporarily prevent the development of wrinkles and sagging skin.

Depending on the mode of action, sunscreens can be classified into physical sunscreens (i.e., those that reflect the sunlight) or chemical sunscreens (i.e., those that absorb the UV light).

Medical organizations such as the American Cancer Society recommend the use of sunscreen because it aids in the prevention of squamous cell carcinomas.[2] Many sunscreens do not block UVA radiation, which does not primarily cause sunburn but can increase the rate of melanoma and photodermatitis.[3] The use of broad-spectrum (UVA/UVB) sunscreens can address this concern.

Sunscreens are commonly rated and labeled with a sun protection factor (SPF) that measures the fraction of sunburn-producing UV rays that reach the skin. For example, "SPF 15" means that 1/15th of the burning radiation reaches the skin through the recommended thickness of sunscreen. Other rating systems indicate the degree of protection from non-burning UVA radiation.

Health effects[edit]

Benefits[edit]

Sunscreen use can help prevent melanoma[4][5][6] and squamous cell carcinoma, two types of skin cancer.[7] There is little evidence that it is effective in preventing basal cell carcinoma.[8]

A 2013 study concluded that the diligent, everyday application of sunscreen can slow or temporarily prevent the development of wrinkles and sagging skin.[9] 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.[9]

Minimizing UV damage is especially important for children and fair-skinned individuals and those who have sun sensitivity for medical reasons.[10]

Potential risks[edit]

In 2009, the Therapeutic Goods Administration of Australia updated a review of sunscreen safety studies and concluded: "The potential for titanium dioxide (TiO2) and zinc oxide (ZnO) nanoparticles in sunscreens to cause adverse effects depend primarily upon the ability of the nanoparticles to reach viable skin cells. To date, the current weight of evidence suggests that TiO2 and ZnO nanoparticles do not reach viable skin cells."[11] Sunscreen ingredients typically undergo extensive review by government regulators in multiple countries, and ingredients that present significant safety concerns (such as PABA) tend to be withdrawn from the consumer market.[12]

Concerns have also been raised about potential vitamin D deficiency arising from prolonged use of sunscreen. Typical use of sunscreen does not usually result in vitamin D deficiency; however, extensive usage may.[13] Sunscreen prevents ultraviolet light from reaching the skin, and even moderate protection can substantially reduce vitamin D synthesis.[14][15] However, adequate amounts of vitamin D can be produced with moderate sun exposure to the face, arms and legs, averaging 5–30 minutes twice per week without sunscreen. (The darker the complexion, or the weaker the sunlight, the more minutes of exposure are needed, approximating 25% of the time for minimal sunburn. Vitamin D overdose is impossible from UV exposure thanks to an equilibrium the skin reaches in which vitamin D degrades as fast as it is created.[16][17][18]

Measurements of protection[edit]

Sunscreen helps prevent sunburn, such as this, which has blistered.

Sun protection factor and labeling [edit]

Two photographs showing the effect of applying sunscreens in visible light and in UVA. The photograph on the right was taken using ultraviolet photography shortly after application of sunscreen to half of the face.

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 1/15th of the burning radiation will reach the skin, assuming sunscreen is applied evenly at a thick dosage of 2 milligrams per square centimeter (mg/cm2). A user can determine the effectiveness of a sunscreen "by multiplying the SPF factor by the length of time it takes for him or her to suffer a burn without sunscreen."[19] Thus, if a person develops a sunburn in 10 minutes when not wearing a sunscreen, the same person in the same intensity of sunlight will avoid sunburn for 150 minutes if wearing a sunscreen with an SPF of 15.[19] 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.[20]

The SPF is an imperfect measure of skin damage because invisible damage and skin aging are also caused by ultraviolet type 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.[21][22][23] According to a 2004 study, UVA also causes DNA damage to cells deep within the skin, increasing the risk of malignant melanomas.[24] Even some products labeled "broad-spectrum UVA/UVB protection" have not always provided good protection against UVA rays.[25] Titanium dioxide probably gives good protection, but does not completely cover the UVA spectrum, as early 2000s research suggests that zinc oxide is superior to titanium dioxide at wavelengths 340–380 nm.[26]

Owing to consumer confusion over the real degree and duration of protection offered, labeling 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).[27] Australia's Therapeutic Goods Administration increased the upper limit to 50+ in 2012.[28][29] 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.[30][31][32] (As of February 2017, the FDA has not adopted the SPF 50 limit.[33]) 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.[34]

UV sunlight spectrum (on a summer day in the Netherlands), along with the CIE Erythemal action spectrum. The effective spectrum is the product of the former two.

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.[35]

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 ~6 is typical for standard summer fabrics.[36]

Mathematically, the SPF (or the UPF) is calculated from measured data as

where is the solar irradiance spectrum, the erythemal action spectrum, and the monochromatic protection factor, all functions of the wavelength . The MPF is roughly the inverse of the transmittance at a given wavelength.

The above means that the SPF is not simply the inverse of the transmittance in the UVB region. If that were true, then applying two layers of SPF 5 sunscreen would always be equivalent to SPF 25 (5 times 5). The actual combined SPF may be lower than the square of the single-layer SPF.[37]

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 or reddening of the skin, 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, Colipa has introduced a method that, it is claimed, can measure this in vitro and provide parity with the PPD method.[38]

SPF equivalence[edit]

The UVA seal used in the EU
A tube of SPF 15 sun lotion

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 PF" of at least 1/3 of the SPF to carry the UVA seal.[39]

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.[31]

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 recently 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;[30] but this was not adopted, for fear it would be too confusing.[34]

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.

Sunblock[edit]

Sunblock typically refers to opaque sunscreen that is effective at blocking both UVA and UVB rays and uses a heavy carrier oil to resist being washed off. Titanium dioxide and zinc oxide are two minerals that are used in sunblock.[40]

The use of the word "sunblock" in the marketing of sunscreens is controversial. Since 2013, the FDA has banned such use because it can lead consumers to overestimate the effectiveness of products so labeled.[31] Nonetheless, many consumers use the words sunblock and sunscreen synonymously.

For total protection against damage from the sun, the skin needs to be protected from UVA, UVB, and also IRA (infrared-A light).[41] Roughly 35% of solar energy is IRA.[citation needed]. However, note that there is continuing debate within the dermatology community over the impact of sun-sourced IRA: Some sources indicate that early morning IRA exposure may be protective against further sun exposure by increasing cell proliferation and initiating anti-inflammatory cascades; these effects are not observed for artificial sources of intense IRA[42].

Active ingredients[edit]

In addition to moisturizers and other inactive ingredients, sunscreens contain one or more of the following active ingredients, which are either chemical or mineral in nature:

  • Organic chemical compounds that absorb ultraviolet light.
  • Inorganic particulates that reflect, scatter, and absorb UV light (such as titanium dioxide, zinc oxide, or a combination of both).[40]
  • Organic particulates that mostly absorb UV light like organic chemical compounds, but contain multiple chromophores that reflect and scatter a fraction of light like inorganic particulates. An example is Tinosorb M. The mode of action is about 90% by absorption and 10% by scattering.

The principal active ingredients in sunscreens are usually 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.[43] A chemical stabilizer is included in some sunscreens containing avobenzone to slow its breakdown; examples include formulations containing Helioplex[44] and AvoTriplex.[45] The stability of avobenzone can also be improved by bemotrizinol,[46] octocrylene[47] and various other photostabilisers. Most organic compounds in sunscreens slowly degrade and become less effective over the course of several years if stored properly, resulting in the expiration dates calculated for the product.[48]

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.

The following are the FDA allowable active ingredients in sunscreens:

UV-filter Other names Maximum concentration Permitted in these countries Results of safety testing UVA UVB
p-Aminobenzoic acid PABA 15% (EU: banned from sale to consumers from 8 October 2009) USA, AUS Protects against skin tumors in mice.[49][50][51] Shown to increase DNA defects, however, and is now less commonly used. X
Padimate O OD-PABA, octyldimethyl-PABA, σ-PABA 8% (EU, USA, AUS) 10% (JP)

(Not currently supported in EU and may be delisted)

EU, USA, AUS, JP X
Phenylbenzimidazole sulfonic acid Ensulizole, Eusolex 232, PBSA, Parsol HS 4% (US, AUS) 8% (EU) 3% (JP) EU, USA, AUS, JP Genotoxic in bacteria[52] X
Cinoxate 2-Ethoxyethyl p-methoxycinnamate 3% (US) 6% (AUS) USA, AUS X X
Dioxybenzone Benzophenone-8 3% USA, AUS X X
Oxybenzone Benzophenone-3, Eusolex 4360, Escalol 567 6% (US) 10% (AUS, EU) 5% (JP) EU, USA, AUS, JP X X
Homosalate Homomethyl salicylate, HMS 10% (EU, JP) 15% (US, AUS) EU, USA, AUS, JP X
Menthyl anthranilate Meradimate 5% USA, AUS X
Octocrylene Eusolex OCR, 2-Cyano-3,3-diphenyl acrylic acid, 2-ethylhexylester 10% EU, USA, AUS, JP Increases ROS[53] X X
Octyl methoxycinnamate Octinoxate, EMC, OMC, Ethylhexyl methoxycinnamate, Escalol 557, 2-Ethylhexyl-paramethoxycinnamate, Parsol MCX 7.5% (US) 10% (EU, AUS) 20% (JP) EU, USA, AUS, JP X
Octyl salicylate Octisalate, 2-Ethylhexyl salicylate, Escalol 587, 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, Escalol 577 5% (EU) 10% (US, AUS, JP) EU, USA, AUS, JP X X
Trolamine salicylate Triethanolamine salicylate 12% USA, AUS X
Avobenzone 1-(4-methoxyphenyl)-3-(4-tert-butyl
phenyl)propane-1,3-dione, Butyl methoxy dibenzoylmethane, BMDBM, Parsol 1789, Eusolex 9020
3% (US) 5% (EU, AUS) 10% (JP) EU, USA, AUS, JP Not available[54] X
Ecamsule Mexoryl SX, Terephthalylidene Dicamphor Sulfonic Acid 10% EU, AUS (US: approved in certain formulations up to 3% via New Drug Application (NDA) Route) Protects against skin tumors in mice[55][56][57] X
Titanium dioxide CI77891 25% (US) No limit (JP) EU, USA, AUS, JP X X
Zinc oxide 25% (US) No limit (AUS, JP) EU, USA, AUS, JP Protects against skin tumors in mice[55] X X

Zinc oxide was approved as a UV filter by the EU in 2016.[58]

Other ingredients approved within the EU[59] and other parts of the world,[60] that have not been included in the current FDA Monograph:

UV-filter Other names Maximum concentration Permitted in
4-Methylbenzylidene camphor Enzacamene, Parsol 5000, Eusolex 6300, MBC 4%* EU, AUS
Tinosorb M Bisoctrizole, Methylene Bis-Benzotriazolyl Tetramethylbutylphenol, MBBT 10%* EU, AUS, JP
Tinosorb S Bis-ethylhexyloxyphenol methoxyphenol triazine, Bemotrizinol, BEMT, anisotriazine 10% (EU, AUS) 3% (JP)* EU, AUS, JP
Tinosorb A2B Tris-Biphenyl Triazine 10% EU
Neo Heliopan AP Bisdisulizole Disodium, Disodium phenyl dibenzimidazole tetrasulfonate, bisimidazylate, DPDT 10% EU, AUS
Mexoryl XL Drometrizole Trisiloxane 15% EU, AUS
Benzophenone-9 Uvinul DS 49, CAS 3121-60-6, Sodium Dihydroxy Dimethoxy Disulfobenzophenone [61] 10% JP
Uvinul T 150 Octyl triazone, ethylhexyl triazone, EHT 5% (EU, AUS) 3% (JP)* EU, AUS
Uvinul A Plus Diethylamino Hydroxybenzoyl Hexyl Benzoate 10% (EU, JP) EU, JP
Uvasorb HEB Iscotrizinol, Diethylhexyl butamido triazone, DBT 10% (EU) 5% (JP)* EU, JP
Parsol SLX Dimethico-diethylbenzalmalonate, Polysilicone-15 10% EU, AUS, JP
Amiloxate Isopentyl-4-methoxycinnamate, Isoamyl p-Methoxycinnamate, IMC, Neo Heliopan E1000 10%* EU, AUS

* Time and Extent Application (TEA), Proposed Rule on FDA approval originally expected 2009, now expected 2015.

Many of the ingredients awaiting approval by the FDA were relatively new, and developed to absorb UVA.[62] The 2014 Sunscreen Innovation Act was passed to accelerate the FDA approval process.[63][64]

Application[edit]

The dose used in FDA sunscreen testing is 2 mg/cm2 of exposed skin.[43] 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.[65] 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 square root or 4th root of the advertised value.[37] 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.[66]

Regulation[edit]

United States

Sunscreen labeling standards have been evolving in the United States since the FDA first adopted the SPF calculation in 1978.[67] 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:[31][68][69]

  • 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 "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. (The EU has stricter regulation against the use of nanoparticles, and in 2009 introduced labeling requirements for nanoparticle ingredients in certain sunscreens and cosmetics.)[70]

Environmental effects[edit]

Certain sunscreens in water under ultraviolet light can increase the production of hydrogen peroxide, which damages phytoplankton.[71] Oxybenzone damages coral.[72] Nanoparticles of titanium dioxide, an ingredient in some sunscreens, can accumulate in coastal waters and be ingested by marine animals.[71]

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.[73] Zinc oxide paste has also been popular for skin protection for thousands of years.[74]

Early synthetic sunscreens were first used in 1928.[73] The first major commercial product was brought to market in 1936, introduced by the founder of L'Oreal, French chemist Eugène Schueller.[75]

Among widely used modern sunscreens, one of the earliest was produced in 1944 for the US military by Benjamin Green, an airman and later a pharmacist, as the hazards of sun overexposure became apparent to soldiers in the Pacific tropics at the height of World War II.[12][75][76][77] The product, named Red Vet Pet (for red veterinary petrolatum), had limited effectiveness, working as a physical blocker of ultraviolet radiation. It was a disagreeable red, sticky substance similar to petroleum jelly. Sales boomed when Coppertone improved and commercialized the substance under the Coppertone girl and Bain de Soleil branding in the early 1950s.

In 1946, Swiss chemist Franz Greiter introduced what may have been the first effective modern sunscreen. The product, called Gletscher Crème (Glacier Cream), subsequently became the basis for the company Piz Buin, which is still today a marketer of sunscreen products, named in honor of the mountain where Greiter allegedly received the sunburn that inspired his concoction.[78][79][80] In 1974, Greiter adapted earlier calculations from Friedrich Ellinger and Rudolf Schulze and introduced the "sun protection factor" (SPF), which has become a worldwide standard for measuring the effectiveness of sunscreen.[12][81] It has been estimated that Gletscher Crème had an SPF of 2.

Water-resistant sunscreens were introduced in 1977,[75] and recent development efforts have focused on overcoming later concerns by making sunscreen protection both longer-lasting and broader-spectrum, as well as more appealing to use.[12]

Research[edit]

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.[82]

Notes[edit]

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