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UV radiation is divided into shorter wavelenght, more energetic UVB (290-320 nanometers) and longer wavelength, deeper penetrating UVA (320-400 nanometers). UVB is associated more directly with skin cancer (via damage to DNA in our skin cells) and sunburn. UVA can also help cause skin cancer, but it is more linked to tanning and premature aged appearence of the skin. Protecting against both forms of UV rays is important..
Sunscreens were initally developed in the late 1920's, and by World War II, were in widespread use in the Pacific theater (red petrolatum). Patented in 1943, PABA (para-aminobenzoic acid) was the first commercially available, very effective UVB chemical sunscreen. The FDA regulates sunscreens as over the counter drugs to ensure efficacy and maintain safety standards.
The sun protection factor (SPF) used on sunscreen labels only refers to the amount of UVB protection offered, although sunscreens with higher SPF numbers (30 or above) do offer some UVA protection as well. Generally speaking, the SPF represents the ratio of how much UVB radiation is required to cause sunburn (minimal erythemal dose) in sunscreen protected skin versus unprotected skin. For example, applicaiton of an SPF 8 product provides sunburn protection 8 times longer than unprotected skin. A sunscreen with a rating of SPF 15 filters about 94% of UVB rays, and SPF 30 blocks about 97%. In recent years, specific UVA filters have been developed (see below). The FDA has developed a 4 star rating system for assessing a product's ability to absorb UVA rays and, will be implemented in December of 2012.
Traditionally, sunscreens have been divided into chemical absorbers and physical blockers. Chemical absorbers do just that; these organic compounds absorb UV radiation and convert it to heat. Most chemical absorbers proctect against UVB radiation only (PABA, padimate O, cinnamates, saliacylates, octocrylene, ensulizole). Some absorb both UVB and UVA (benzophenones), where as stll others are excellent UVA absorbers (Parsol 1789, Helioplex, and Mexoryl SX). Most commercially available sunscreens use a combination of these chemical absorbers to maximize protection and water resistance, and minimize problems (such as staining of the skin, degradation upon exposure to sunlight, interaction with each other, and irritaion or allergy).
Products that contain physical blockers work by reflecting or scattering UV radiation over a broad spectrum. These compounds are inorganic particulates, and by far the most commonly used agents are zinc oxide and titanium dioxide. Iron oxide is occasionally used as well, primarly because it's reddish hue can mask the white opacity of the former two blocker. Until recent years, these products have not been as widley used because of cosmetic unacceptiablilty (i.e. a white hue when applied). However, more recently, microsizing the particles has resulted in much more aesthetically pleasing formulations. These products are broad spectrum, don't degrade easily in sunlight, very rarely cause irritiation or allergy, and are often used in childrens' sunscreens.
Because a significant porportion of our vitamin D requirement is obtained via UV dependent cutaneous synthesis (the remainder being of dietary origin), there exists controversy over the possible roll of regular, correct sunscreen usage in the production of vitamin D deficiency. Most studies suggest, however, that in pratical terms sunscreen use has little effect on vitamin D status. Those at most risk of vitamin D deficiency are persons who are housebound, elderly, darker skin type, or live at northerly latitudes.
In December of 2012, the FDA's new requirements for sunscreen labeling will take effect. In order to claim "broad spectrum" protection, a product will have to demonstrate UVA protection (1 - 4 stars) and have an SPF of at least 15. Terms such as "waterproof," "sweatproof," and "all-day protection" will no longer be allowed. Sunscreens will be rated either water resistant 40 minutes or water resistant 80 minutes. Products that meet or exceed these criteria may assert that they protect against sunburn, skin cancer and premature aging. The FDA is also considering capping the SPF at 50+.
Proper application of sunscreen is important. Select a broad spectrum sunscreen with a vehicle appropriate for planned activities (e.g water resistant 80 rating for swimming or expected heavy perspiration). Apply liberally and uniformly approximately 15 to 30 minutes before heading outdoors. While wearing swimming attire, an average adult should apply about 1 ounce of sunscreen (a golf ball sized amount). Reapply every 90 minutes if swimming or perspiring. Adopt other sun protective behaviors, such as seeking shade, broad-brimmed hats, appropriate clothing and sunglasses.