Harvard Women's Health Watch

By the way, doctor: What's in sunscreen that protects against UVA rays?

Q. What UVA-blocking ingredients should I look for in a sunscreen? And does a higher SPF rating mean greater UVA protection?

A. The sun emits three types of ultraviolet rays, distinguished by wavelength ranges: ultra violet A (UVA) is the longest; ultraviolet B (UVB) radiation is medium-wavelength; and ultraviolet C (UVC) has the shortest wavelength range. UVC rays never touch our skin. UVB rays are the main cause of sunburn, and UVA rays penetrate the skin more deeply and contribute to skin aging; both types can cause skin cancer. An effective sunscreen should block both UVA and UVB.

Sun protection factor, or SPF, refers only to a product's ability to screen out UVB rays. SPF is calculated by comparing the time it takes to develop sunburn on sunscreen-protected skin to the time it takes on unprotected skin. If you usually turn red after 10 minutes in the sun without sunscreen, an SPF-15 sunscreen allows you to stay in the sun for 150 minutes (2.5 hours) before burning.

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