SPF stands for “Sun Protection Factor” and indicates a sunscreen’s ability to protect skin from sunburn rays (UVB rays). UVB rays are responsible for the redness and tenderness of a sunburn (remember UVB for Burning). The higher the SPF, the longer the sunscreen will allow you to stay exposed to the sun before you burn. For example, an SPF of 15 will let you spend 15 times more minutes in the sun before you burn, but an SPF of 30 will allow you to spend 30 times more minutes exposed before burning. Said differently, if you were to sunburn after 30 minutes of noontime sun without wearing sunscreen, you could spend 45 minutes exposed wearing SPF 15 before you burned, or you could spend 90 minutes exposed wearing SPF 30 before you burned.
SPF 15 provides 93% protection against harmful UVB rays if applied in the FDA testing amount of 2mg/cm2 (a very thick white film). But studies show that most people apply sunscreen in a thinner film than how the sunscreens are tested. Usually, we apply 25% to 50% of the amount tested, and therefore we get 25% to 50% of the advertised SPF number. If you are like most people when you apply a sunscreen labeled SPF 30, you are probably getting only SPF 7.5 to SPF 15. Keeping all this in mind, sunscreens with SPF of 30 or higher are appropriate.
Importantly, UVB and UVA rays should be blocked for optimal skin protection from the sun. Therefore, be sure your sunscreen bottle clearly states UVA blockage, since SPF only address UVB coverage. UVA coverage is important to prevent Aging as well as skin cancer.
More Answers from Catherine Balestra, MD