Swetter: Sun protection factor, or SPF, was originally designed to measure sun protection from ultraviolet B rays, the primary cause of sunburn and skin cancer. Only in recent years has research shown that exposure to ultraviolet A rays is equally damaging to the skin, and its harmful effects have been seen in people exposed to high amounts of UVA and UVB radiation in indoor tanning booths. Without the warning signs of sunburn, UVA radiation penetrates the skin more deeply than UVB rays. UVA radiation contributes to skin photoaging — discoloration, wrinkling and sagging of the skin. It also passes through the ozone layer, clouds and window glass. UVA rays are also more plentiful than UVB because they are strong throughout the day and the year. While SPF values are generally easy for consumers to understand, they are not a good measure of UVA protection. Of even more concern is that of 60 sunscreen products recently tested by Consumer Reports, 28 (43 percent) failed to meet even the UVB protection claims on their labels. Because most consumers don’t apply the recommended amount of sunscreen to achieve the advertised SPF rating, much of the sun protection a person thinks they’re getting isn’t really happening.