Much of life as we know it depends on the sun, but it also depends on the ozone, a protective layer of oxygen that’s about 10 to 25 miles above our heads. Ozone allows visible light and warm infrared rays through but absorbs most damaging ultraviolet B (UVB) radiation. It’s UVB that’s been proven to cause DNA damage that can lead to skin cancer, according to the American Academy of Dermatology.

“The UV absorption by the stratospheric ozone is so effective, even a small drop in ozone can increase the UVB reaching the Earth’s surface,” says Edward De Fabo, PhD, a research professor in the department of environmental and occupational health at George Washington University’s School of Public Health and Health Sciences in Washington, D.C.

Changes in the ozone do occur, based on seasonal changes including the weather. That’s why it’s important to know the UV Index for your area. Developed by the National Weather Service and the Environmental Protection Agency, the UV Index predicts the next day’s ultraviolet radiation levels. To find out the UV Index in your area, log on to www.epa.gov/sunwise/uvindex.html and enter your ZIP code or city and state. (The index can also be found in the weather section of many newspapers.) Your UV Index will appear, ranging from a 1 (low) to an 11-plus (extreme). The higher the UV Index, the greater the precautions you need to take to protect your skin.