SPF is a laboratory measure of the efficacy of sunscreen and is defined as the amount of ultraviolet radiation needed to produce sunburn on protected skin relative to unprotected skin. It is a measurement of redness or “erythema” and is mainly a measure of UV-B radiation exposure not UV-A (the more damaging type of radiation to deep skin structure) exposure.
The SPF of a product is not related to the duration of UV radiation exposure. The relationship between SPF and UV-B radiation protection is not 1:1 or linear meaning that an SPF 30 does not protect you for twice as long as an SPF 15. For example, an SPF of 15 can filter 94% of the suns UV-B radiation while a SPF of 30 will filter 97%. UV radiation dosage depends on both how long you are out in the sun and how intense the UV radiation is.
The daily UV index is a measure of the level or intensity of UV radiation. It is presented on a scale of 1 (low) to 11+ (extremely high). The US National Weather Service and the US Environmental Protection Agency provide this data which is presented on most weather reports and published in newspapers and on line daily.
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Steven Reznick, M.D. - Boca Raton Concierge Doctor 