However, scientists are finding UVA rays are about 1000 times stronger than UVB and may be a significant factor in skin changes associated with sunlight. According to the Centers for Disease Control and Prevention, only about half of all Americans take any steps to protect themselves from the sun. Most sunscreens only protect against UVB rays. Reapply the product about every two hours, or more often if you're coming out of water. UV rays penetrate the clouds, so sunscreens should be used even on cloudy days.
Sunlight can also reflect on some surfaces, such as the water, sand, or snow.
So even if you're sitting in the shade, you can still get a burn from reflected light. Don't let sunscreens give you a false sense of security you shouldn't stay out in the sun longer than you need to. The National Weather Service and the Environmental Protection Agency have developed the UV Index, a forecast based on the expected UV exposure over the next day. The higher the number, the greater protection needed against the sun's rays. The UV Index is localized for dozens of cities across the U.S. and published in many of the major newspapers.