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Sunscreen and Sunblock (cont.)

The Meaning of SPF

SPF stands for Sun Protection Factor. It is determined in the laboratory by exposing human skin to an ultraviolet light source producing a fixed amount of energy and determining the length of time it takes for the skin to turn red. This is then compared to the length of time it takes for the skin to turn red after sunscreen has been applied to it. For example, if it takes one minute for the skin to turn red without sunscreen and 50 minutes for the skin to turn red when coated with a specific amount of sunscreen, the SPF equals 50. Please note that this test does not measure the sunscreen's ability to prevent photo-aging or skin cancer. Conveniently, it turns out that the higher the SPF, the better the sunscreen is at preventing skin cancer and aging.


The spectrum of ultraviolet light produced by the sun has been separated into three divisions. Generally, the only ultraviolet light we need to worry about is light between the wavelengths of 290 nm to 400 nm. Wavelengths shorter than 290 nm never reach the earth's surface because they are absorbed by the atmosphere. Wavelengths longer than 400 nm are visible light and seem to have very little effect on the skin. Wavelengths between 290 nm-320 nm are designated as ultraviolet B rays (UVB) while wavelengths from 320 nm to 400 nm are ultraviolet A rays, or UVA. Although the quantity of UVB light varies and makes up only 5% or less of all UV radiation (depending on time of day and latitude), UVB is most responsible for the sunburn response and coincidently the most efficient at producing mutations in the DNA of skin cells as well as producing wrinkles by damaging collagen. That is not to say the UVA cannot produce mutations and skin cancers along with wrinkles. It is just not as effective.

Medically Reviewed by a Doctor on 11/17/2017
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