According to the Associated Press, the SPF number stands for the amount of sun exposure needed to cause sunburn on sunscreen-protected skin compared with unprotected skin. For instance, an SPF rating of 30 means it would take a slathered sunbather 30 times longer to burn than a sunscreen abstainer.
To my surprise and displeasure, my new SPF 100+ sunscreen isn't twice as effective as SPF 50. In reality, a SPF 50 product might protect against 97% of sunburn-causing rays, while SPF 100 might block 98.5% of those rays.
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"The high SPF numbers are just a gimmick," says Marianne Berwick, a professor of epidemiology at the University of New Mexico. She adds, "Most people really don't need more than an SPF 30 and they should reapply it every couple of hours."
FDA acknowledged in 2011 that "labeling a product with a specific SPF value higher than 50 would be misleading to the consumer." At that time, the agency proposed capping all SPF values at 50 because "there is not sufficient data to show that products with SPF values higher than 50 provide greater protection."
However, sunscreen manufacturers have continued to push back against proposed limits, arguing that super-high SPFs provide a measurable benefit. As a result, FDA is reviewing studies and comments submitted by outside parties, but there is no deadline for the agency to set an SPF cap.