Explaining Sunscreen

One of my favorite online columns is slate.com's Explainer, which takes recent events from the news and does a run-through on the finer points, like if you can survive in space without a spacesuit, how they power an iPhone, or even how to determine if a bridge is structurally sound. It's the sort of clever column that appeals to your inner geek.

Today, for example, in light of a SPF 70 sunscreen being released this year, they addressed the important question of how scientists determine sunscreen's SPF (sun protection factor) rating in the first place. Here's how they figure it out-

A product's SPF refers to its ability to deflect ultraviolet rays. To calculate this figure, scientists gather 20 human volunteers who are especially susceptible to sunburn. According to FDA guidelines, volunteers must have a skin type of I, II, or III on the Fitzpatrick phototyping scale. (The categories correspond to the amount of pigment present in the skin: Very fair blonds or redheads are Type I, while those with dark brown or black skin are Type VI.) Using a device called a "solar simulator," experimenters irradiate a small patch of skin on each subject and then record the UV dose required to produce mild redness (in scientific parlance, the "minimal erythematic dose"). After applying a thick layer of sunscreen, the experimenters repeat the test. Then they divide the MED needed to redden the protected skin by the MED needed to redden bare skin. The result, rounded down to the nearest five, is the SPF.

Nifty, don't you think? Granted, the article goes on to say that the scientists apparently use a whole lot of sunscreen while doing their tests, ie a lot more than you probably use, so don't bet on being able to stay out in the sun 70 times longer without burning unless you douse yourself in the stuff. Just something to think about during the last days of summer.