What the UV Index Means for Your Summer Routine

Wearing SPF in the summer is a no-brainer, but when the UV index rises, you may need to layer on extra protection to block the sun's UVA and UVB rays. Though these rays have different wavelengths and don't affect your skin in exactly the same way, they share similar damaging long-term effects, which include everything from sunburn and premature aging to skin cancer. "I think it's important to realize that there's no 'healthy' amount of UV exposure and that we're being exposed to UV light all of the time," says Dr. Anne Chapas, director of Union Square Dermatology in Manhattan. "When the index is higher, we need to take even more precautions than normal." Ahead, Dr. Chapas explains what this index actually measures and how to cover up appropriately when UV radiation rises.

The UV index measures the amount of UVA and UVB radiation at any given time.

"The UV index is a way to report the total of amount of UV radiation, including UVA and UVB, for a specific day at a specific elevation," notes Dr. Chapas. The index numbers are calculated by the National Oceanic and Atmospheric Association (NOOA) and the United States Environmental Protection Agency (EPA) using several factors, including the sun's position in the sky, the day's cloud cover, and the level of ozone in the stratosphere; the levels change from day to day throughout the country. An index measurement of zero to two indicates that you can be outside with little to no sun protection (level zero is nighttime); three to seven is considered "moderate to high," with the recommendation for sunscreen and protective clothing; and eight or above, labeled "very high to extreme," might mean staying inside during the hottest part of the day.

Whatever the index number, wear sunscreen.

While the index numbers climb as high as 15, the key element of your sun protection plan is essential in just about any weather. "Just because the UV index is low, people may think that they don't need to wear sunscreen," Dr. Chapas says. "They do! Even on cloudy days, 80 percent of UV radiation travels to earth and can cause significant sunburns. The American Academy of Dermatology recommends wearing sunscreen every day, regardless of what the UV index is."

Layer your sun protection for good measure.

If you have already had skin cancer, have fair skin, or burn easily, says Dr. Chapas, you should be especially careful in the sun no matter how low the UV index is—and even if none of those factors apply to you, you should always make sun protection part of your daily routine. "Everyone should wear a daily SPF 30 sunscreen on their face and apply SPF 50 to all sun-exposed areas if they are going to be outside for prolonged sun exposure," explains Dr. Chapas, who recommends reapplying your sunscreen every 80 minutes and after sweating or swimming. When the UV index is especially high, you should move your midday outdoor plans—anything occurring between 11 a.m. and 2 p.m.—to an inside location, but even in the early and later parts of the day, Dr. Chapas suggests layering your UV protection. "Everyone should use multiple types—sunscreen, hats, sun protection clothing, and shade-seeking behavior," she says. "There is no healthy amount of UV radiation."