Sunscreen
Sunscreen or sunblock is a lotion that blocks ultraviolet radiation from the sun, and which reduces sunburn and other damage to the skin, leading to a lower risk of skin cancer.
The best sunscreens block both UVB (ultraviolet radiation with wavelength between 315 and 280 nm), which can cause sunburn, and UVA (380–315 nm), which damages the skin without causing sunburn. Most sunscreens work by containing either an organic chemical compound that absorbs ultraviolet light (such as oxybenzone) or an opaque material that reflects light (such as titanium dioxide, zinc oxide), or a combination of an organic chemical compound and an opaque compound.
Many people apply sunscreen when participating in outdoor activities during the summer. However, some experts suggest wearing sunscreen throughout the year to prevent cumulative damage, and to lower the risk of skin cancer. It is recommended that sunscreen be applied 30 minutes before exposure to the sun.
Dosing for sunscreen can be calculated using the formula for body surface area and subsequently subtracting the area covered by clothing. The dose used in FDA sunscreen testing is 2 mg/cm2. From a sample calculation in a FDA monograph, if one assumes an "average" adult build of height 5'4" (163 cm) and weight 150 lb (68 kg) with a 32" (82 cm) waist, that adult wearing a bathing suit covering the groin area should apply 29 grams (approximately 1 fluid ounce) evenly to the uncovered body area.
Sunscreen inhibits the production of Vitamin D. Though excessive sun exposure has been conclusively linked to some forms of skin cancer, Vitamin D is known to prevent other more dangerous forms of cancer. Season, geographic latitude, time of day, cloud cover, smog, skin type, and sunscreen all have an effect on Vitamin D production in the skin, but fifteen minutes per day of direct exposure to the sun is a generally accepted guideline to follow for optimum Vitamin D production.
History
The ancient Greeks used olive oil as a type of sunscreen. However, this was not very effective. Throughout the early twentieth century, H.A. Milton Blake, a South Australian chemist, as well as several other inventors attempted to create an effective sunscreen but failed.
It was not until 1944 that the first effective sunscreen was invented. At that time, World War II was in full swing and many soldiers were getting serious sunburn. A pharmacist named Benjamin Greene decided to create something that would save the soldiers from the sun’s harmful rays. In his wife’s oven, he created a sticky, red substance which he called "red vet pet". Greene tested it on his own bald head. It didn’t work nearly as well as modern sunscreens, but it was a start.
Greene then created a more user friendly sunscreen which he began selling to customers in and around Miami. He founded the Coppertone company and his sales boomed. The little protection his product offered had customers enthusiatic. People were no longer afraid of getting sunburn. In their eyes, the sun's harmful rays were conquered.
Sunscreen has come a long way since its initial conception. Modern products have much higher protection factors than Greene's sunscreen, and modern products can also be water- and sweat-proof. But there are also negative effects. Some people rely too much on the product and do not understand the limitations of the sun protection factor; they assume that buying anything over SPF 30 will automatically prevent them getting burnt no matter how long they can stay in the sun. Too much sunbathing is one of the major causes of skin cancer across the world.
Mechanism of Action
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. 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. However, the FDA concluded that "it is not aware of any safety or effectiveness problems associated with the photostability of avobenzone". Further details regarding the mechanism of action of sunscreen products can be found here.
Sun protection factor
The SPF (sun protection factor) of a sunscreen is a laboratory measure of the effectiveness of sunscreen. The higher the SPF, the more protection a sunscreen offers against UVB (the ultraviolet radiation that causes sunburn).
SPF is the inverse of the proportion of UVB that penetrates the sunscreen. Thus a sunscreen with an SPF of p blocks a proportion (p−1)/p of UVB. For example:
| Sun Protection Factor | Proportion of UVB blocked |
|---|---|
| SPF 10 | 90.0% |
| SPF 20 | 95.0% |
| SPF 30 | 96.7% |
| SPF 60 | 98.3% |
In theory the SPF is a multiplier that can be applied to the time taken to burn. For example, someone who would burn after 30 minutes in the sun would expect to burn after 5 hours (300 minutes) 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.
- Activites in which one engages (for example, swimming leads to a loss of sunscreen from the skin).
- Time of day and season.
- Percentage of UV reflected or scattered by the environment, for example snow or sand.
- 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. Normal sunscreen does not block UVA as effectively as UVB, and an SPF rating of 30+ may translate to significantly lower level of UVA protection according to a study from 2003 by researchers funded by the RAFT trust. Some broad spectrum sunscreens do provide significant UVA protection. According to a 2004 study, UVA also causes DNA damage to cells deep within the skin, increasing the risk of malignant melanomas.
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 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 making unrealistic claims about the level of protection offered e.g "all day protection", and because an SPF over 30 does not provide significantly better protection.
The following are the FDA allowable active ingredients in sunblocks:
- p-Aminobenzoic acid (PABA) up to 15 percent.
- Avobenzone up to 3 percent.
- Cinoxate up to 3 percent.
- Dioxybenzone up to 3 percent.
- Homosalate up to 15 percent.
- Menthyl anthranilate up to 5 percent.
- Octocrylene up to 10 percent.
- Octyl methoxycinnamate (Octinoxate) up to 7.5 percent.
- Octyl salicylate up to 5 percent.
- Oxybenzone up to 6 percent.
- Padimate O up to 8 percent.
- Phenylbenzimidazole sulfonic acid up to 4 percent.
- Sulisobenzone up to 10 percent.
- Titanium dioxide up to 25 percent.
- Trolamine salicylate up to 12 percent.
- Zinc oxide up to 25 percent.
Others include:
- Tinosorb - meant to be applied to clothing via treatment/detergent
- Mexoryl - not approved in the US, but available in Europe/Asia
Melanin
The hormone alpha-melanocyte stimulating hormone is made when the body is exposed to sunlight and is responsible for the development of the pigment melanin. Research is being done to create stable artificial forms of the hormone. A promising candidate, melanotan, might be useful in the prevention of skin cancer, by causing tanning without exposure to sunlight.
External links
- FDA monograph on sunscreen
- FDA monograph on dosing, mechanism of action, and photodegradation of sunscreen (PDF file)