This summer, sunscreens with SPF 70 hit the market. The fair-skinned of the world might be tempted to take this as an open invitation to slather some on, jump onto a lounge chair and bake for 12 hours without getting burned. The higher the number, the better, right? Before you throw caution to the wind, though, you should know a few things about SPF. What is it, exactly? What do the numbers mean, and how high can they go? We use sunscreen to block ultraviolet light from damaging the skin. There are two categories of UV light -- UVA and UVB -- that we consider in terms of sunscreen. UVB causes sunburn, and UVA has more long-term damaging effects on the skin, like premature aging. SPF, or sun protection factor, numbers were introduced in 1962 to measure a sunscreen's effect against UVB rays.

To determine a sunscreen's SPF, testers round up 20 sun-sensitive people and measure the amount of UV rays it takes them to burn without sunscreen. Then they redo the test with sunscreen. The "with sunscreen" number is divided by the "without sunscreen" number, and the result is rounded down to the nearest five. This is the SPF.

SPF numbers start at 2 and have just recently reached 70. 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

For example, if you burn after 10 minutes of sun exposure, an SPF of 15 will allow you to be in the sun for up to 150 minutes without burning. But before you grab your calculator and head for the beach, you should know that this equation is not always accurate. People usually use far less sunscreen than the amount used in testing. In the real world, the average sun worshipper uses half the amount of sunscreen used in the laboratory, which could result in a sunburn in half the time.

（来源 howstuffworks.com 英语点津 Helen 编辑）

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