AskDefine | Define sunscreen

Dictionary Definition

sunscreen n : a cream spread on the skin; contains a chemical (as PABA) to filter out ultraviolet light and so protect from sunburn [syn: sunblock, sun blocker]

User Contributed Dictionary

English

Etymology

sun + screen

Noun

sunscreen
  1. a form of sunblock, often having a lower sun protection factor

Translations

a form of sunblock

Extensive Definition

Sunscreen (also known as sunblock) is a lotion, spray or other topical product that absorbs or reflects the sun's ultraviolet (UV) radiation, and protects the skin. Medical organizations such as the American Cancer Society recommend the use of sunscreen, despite many epidemiological studies indicating an increased melanoma risk for the sunscreen user. (see sunscreen controversy for a discussion of possible reasons)
Sunscreens contain one or more UV filters of which there are three main types :

Dosing

The dose used in FDA sunscreen testing is 2 mg/cm² of exposed skin. Provided one assumes an "average" adult build of height 5 ft 4 in (163 cm) and weight 150 lb (68 kg) with a 32 in (82 cm) waist, that adult wearing a bathing suit covering the groin area should apply 29 g (approximately 1 oz) evenly to the uncovered body area. Considering only the face, this translates to about 1/4 to 1/3 of a teaspoon for the average adult face.
Contrary to the common advice that sunscreen should be reapplied every 2–3 hours, some research has shown that the best protection is achieved by application 15–30 minutes before exposure, followed by one reapplication 15–30 minutes after the sun exposure begins. Further reapplication is only necessary after activities such as swimming, sweating, and rubbing.
However, more recent research at the University of California, Riverside indicates that sunscreen needs to be reapplied within 2 hours in order to remain effective. Not reapplying could even cause more cell damage than not using sunscreen at all, due to the release of extra free radicals from those sunscreen chemicals which were absorbed into the skin.

History

The first effective sunscreen may have been developed by chemist Franz Greiter in 1938. The product, called Gletscher Crème (Glacier Cream), subsequently became the basis for the company Piz Buin (named in honor of the place Greiter allegedly obtained the sunburn that inspired his concoction), which today is a well-known marketer of sunscreen products. Some internet articles suggest that Gletscher Crème had a sun protection factor of 2, although a research citation is not readily available online.
The first widely used sunscreen was produced by Benjamin Greene, an airman and later a pharmacist, in 1944. The product, 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. This product was developed during the height of World War II, when it was likely that the hazards of sun overexposure were becoming apparent to soldiers in the Pacific and to their families at home.
Franz Greiter is credited with introducing the concept of Sun Protection Factor (SPF) in 1962, which has become a worldwide standard for measuring the effectiveness of sunscreen when applied at an even rate of 2 milligrams per square centimeter (mg/cm2). Some controversy exists over the usefulness of SPF measurements, especially whether the 2 mg/cm2 application rate is an accurate reflection of people’s actual use.
Newer sunscreens have been developed with the ability to withstand contact with water and sweat.

Measurements of sunscreen protection

Sunburn protection factor

The SPF of a sunscreen is a laboratory measure of the effectiveness of sunscreen; the higher the SPF, the more protection a sunscreen offers against UV-B (the ultraviolet radiation that causes sunburn). The SPF indicates the time a person can be exposed to sunlight before getting sunburn with a sunscreen applied relative to the time they can be exposed without sunscreen. For example, someone who would burn after 12 minutes in the sun would expect to burn after 2 (or more) hours (120 min) if protected by a sunscreen with SPF 10. In practice, the protection from a particular sunscreen depends on factors such as:
  • The skin type of the user.
  • The amount applied and frequency of re-application.
  • Activities in which one engages (for example, swimming leads to a loss of sunscreen from the skin).
  • Amount of sunscreen the skin has absorbed.
The SPF is an imperfect measure of skin damage because invisible damage and skin aging is also caused by the very common ultraviolet type A, which does not cause reddening or pain. Conventional sunscreen does not block UVA as effectively as it does UVB, and an SPF rating of 30+ may translate to significantly lower levels of UVA protection according to a 2003 study. According to a 2004 study, UVA also causes DNA damage to cells deep within the skin, increasing the risk of malignant melanomas. Even some products labeled "broad-spectrum UVA/UVB protection" do not provide good protection against UVA rays. The best UVA protection is provided by products that contain zinc oxide, avobenzone, and ecamsule. Titanium dioxide probably gives good protection, but does not completely cover the entire UV-A spectrum.
Due to consumer confusion over the real degree and duration of protection offered, labeling restrictions are in force in several countries. In the United States in 1999, the Food and Drug Administration (FDA) decided to institute the labelling of SPF 30+ for sunscreens offering more protection, and a similar restriction applies in Australia. This was done to discourage companies from making unrealistic claims about the level of protection offered (such as "all day protection"),. In the EU sunscreens are limited to SPF 50+, indicating a SPF of 60 or higher.
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 the UV-B range (290–350 nm), along with a table of how effective various wavelengths are in causing sunburn (the erythemal action spectrum) and the actual intensity spectrum of sunlight (see the figure). Such in vitro measurements agree very well with in vivo measurements.
Mathematically, the SPF is calculated from measured data as
\mathrm = \frac,
where E(\lambda) is the solar irradiance spectrum, A(\lambda) the erythemal action spectrum, and \mathrm(\lambda) the monochromatic protection factor, all functions of the wavelength \lambda. 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 UV-B region. If that were true, then applying two layers of SPF 5 sunscreen would be equivalent to SPF 25 (5 times 5). The actual combined SPF is always lower than the square of the single-layer SPF.

Measurements of UVA protection

Persistent Pigment Darkening (PPD), Immediate Pigment Darkening (IPD), Boots Star System, Japanese PA system
The Persistent Pigment Darkening (PPD) method is a method of measuring UVA protection, similar to the SPF method of measuring UVB light protection. Originally developed in Japan, and is the preferred method used by manufacturers such as L'Oreal.
Instead of measuring erythema or reddening of the skin, the PPD method uses UVA radiation to cause a permanent darkening or tanning of the skin.Theoretically, a sunscreen with a PPD rating of 10 should allow you to endure 10 times as much UVA as you would without protection. The PPD method is an in vivo test like SPF. In addition Colipa has introduced a method which is claimed can measure this in vitro and provide parity with the PPD method
As part of revised guidelines for sunscreens in the EU, a requirement to provide the consumer with a minimum level of UVA protection in relation to the SPF. This should a UVA PF of at least 1/3 of the SPF to carry the UVA seal. The implementation of this seal is in its phase-in period, so a sunscreen without may already offer this protection.

Star rating system

In the UK and Ireland, the Boots star rating system is a proprietary method used to describe the ratio of UVA to UVB protection offered by sun screen creams and sprays. Invented by Dr Diffey of the Boots Company in Nottingham UK, it has been adopted by most companies marketing these products in the UK. The logo and methodology of the test are freely licenced to any manufacturer or brand of sunscreens that wishes to place product on the UK market. It should not be confused with SPF which is measured with reference to burning and UVB. One star products provide the least ratio of UVA protection, five star products are the best. The method has recently been revised in light of the Colipa UVA PF test, and with the new EU recommendations regarding UVA PF. The method still uses a spectrophotometer to measure absorption of UVA vs 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 of photostability when the product is used

Active ingredients

The principal 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 photo-degradation.

FDA allowable ingredients

The following are the FDA allowable active ingredients in sunscreens:
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