After 33 years, the Food and Drug Administration today released a new set of rules regulating sunscreen in the United States. Issued just in time for the summer beach season, the new rules dictate that in order to earn a "broad spectrum" designation, sunscreens must protect from both UVB rays, which cause burning, and UVA rays, which cause wrinkles. Both types of ultraviolet rays cause cancer, The New York Times reported. The rules also do away with "waterproof" and "sweatproof" labels on sunscreens, because such claims are impossible. "Instead, they will be allowed to claim that the products are water resistant for either 40 minutes or 80 minutes, depending upon test results, but nothing more."

The rules also say only sunscreens with a sun protection factor higher than 15 can claim to reduce the risk of skin cancer, aging, and sunburn, and that those rated SPF 2 to 14 must be clearly labeled as not doing so. The administration didn't officially weigh in on what The Times called "an SPF arms race," with sunscreen manufacturers rating their products SPF 70 or even 100. In the past it had proposed limiting the SPF rating claim to 50 at the highest.

That brings up the question: what is SPF, exactly? Over at How Stuff Works, they give us the commonly held understanding, that the number relates to the amount of time a sunscreen allows you to extend your exposure: 

 To figure out how long you can stay in the sun with a given SPF, use this equation:

Minutes to burn without sunscreen x SPF number = maximum sun exposure time

But the FDA calls that a "common misconception," and clarifies on its own site that the number relates not simply to time of exposure, but to the amount of radiation picked up during that exposure. 

Generally, it takes less time to be exposed to the same amount of solar energy at midday compared to early morning or late evening because the sun is more intense at midday relative to the other times. Solar intensity is also related to geographic location, with greater solar intensity occurring at lower latitudes. Because clouds absorb solar energy, solar intensity is generally greater on clear days than cloudy days.

You've also got to consider your skin type, and the amount of sunscreen you're applying when figuring out what kind of SPF factor you need, the administration reminds us. "Fair-skinned consumers are likely to absorb more solar energy than dark-skinned consumers under the same conditions." But overall, the SPF refers not just to time, but to energy, and should be seen as a relative measure, the FDA counsels. "It allows consumers to compare the level of sunburn protection provided by different sunscreens. For example, consumers know that SPF 30 sunscreens provide more sunburn protection than SPF 8 sunscreens." The takeaway here: Put on more than you think you should, wear a hat and don't get wet.