Topical medications useful in protecting against sunlight contain either chemical compounds that absorb ultraviolet light, called sun-screens, or opaque materials such as titanium dioxide that reflect light, called sunshades. The three classes of chemical compounds most commonly used in sunscreens are p-aminobenzoic acid (PABA) and its esters, the benzophenones, and the dibenzoylmethanes.
Most sunscreen preparations are designed to absorb ultraviolet light in the ultraviolet B (UVB) wavelength range from 280 to 320 nm, which is the range responsible for most of the erythema and sunburn associated with sun exposure and tanning. Chronic expo-sure to light in this range induces aging of the skin and photocarcino-genesis. Para-aminobenzoic acid and its esters are the most effective available absorbers in the B region. Ultraviolet in the longer UVA range, 320–400 nm, is also associated with skin aging and cancer.
The benzophenones include oxybenzone, dioxybenzone, and sulisobenzone. These compounds provide a broader spectrum of absorption from 250 to 360 nm, but their effectiveness in the UVB erythema range is less than that of PABA. The dibenzoyl-methanes include Parasol and Eusolex. These compounds absorb wavelengths throughout the longer UVA range, with maximum absorption at 360 nm. Patients particularly sensitive to UVA wavelengths include individuals with polymorphous light erup-tion, cutaneous lupus erythematosus, and drug-induced photo-sensitivity. In these patients, dibenzoylmethane-containing sunscreen may provide improved photoprotection. Ecamsule (Mexoryl) appears to provide greater UVA protection than the dibenzoylmethanes and is less prone to photodegradation.The sun protection factor (SPF) of a given sunscreen is a mea-sure of its effectiveness in absorbing erythrogenic ultraviolet light. It is determined by measuring the minimal erythema dose with and without the sunscreen in a group of normal people. The ratio of the minimal erythema dose with sunscreen to the minimal erythema dose without sunscreen is the SPF.
Recently updated FDA regulations limit the claimed maximum SPF value on sunscreen labels to 50+ because there is not sufficient data to show that products with SPF values higher than 50 provide greater protection for users. These new FDA regulations require that sunscreens labeled as “broad spectrum” sunscreens will have to pass a standard test comparing the amount of UVA radiation pro-tection in relation to the amount of UVB radiation. Broad spec-trum sunscreens with SPF values of 15 or higher help protect against not only sunburn, but also skin cancer and early skin aging when used as directed. Sunscreens with an SPF value between 2 and 14 will only be allowed to claim to help prevent sunburn. In addition, products claiming to be water resistant must indicate whether they remain effective for 40 minutes or 80 minutes while swimming or sweating, based on standard testing.