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Sun in a red sky.

The sun is a star. It is the source of light and heat for all of the planets in our solar system. The sun releases energy that reaches all the way to earth.

It's fun to play in the sun, but did you know that too much sun can be bad for you? If you ever had sunburn, you have felt some of the bad effects of too much sun and ultraviolet (UV) radiation. Have you ever wondered how this happens? Well, it has a lot to do with the ozone layer.

What is the ozone layer?

The ozone (say: OH-zone) layer is gas in the sky, very high up, that acts as a screen to absorb most harmful UV radiation. The thickness of the ozone layer depends on the time of year and changing weather patterns. The ozone layer is very thin in some places because of some man-made chemicals released into the sky. These chemicals eat away at the ozone layer and make it thinner. When the ozone layer is too thin, UV rays can get through all the way to the earth. Too many UV rays can cause health problems for people. Learn more about the health problems caused by UV ray.

UV levels

The levels of UV radiation here on earth depend on many things:

  • How much ozone is in an area – If the ozone layer is thin, more UV rays can get through.
  • Time of day – At noon, the sun is at its highest in our sky. The sun’s rays have a shorter distance to travel to the earth at this time. That is why you are more likely to get sunburned in the middle of the day. In the early morning and late evening, UV radiation is less strong because the rays have farther to travel to get to the earth.
  • Time of year – The sun’s angle varies with the seasons, so the strength of UV rays change, too. UV rays are strongest in the summer months.
  • Latitude – The sun’s rays are strongest at the equator where the sun is most directly overhead. This is why in southern states, like Florida or Texas, you may be more likely to get sunburned than in northern states like Maine. (But you still need to wear sunscreen in Maine!)
  • Altitude – UV rays get stronger the closer you are to the sun. This is because there is less atmosphere to absorb the damaging rays. Be careful when you are in the mountains, or in a high-altitude city (like Denver, Colorado)! The UV rays are strong up there.
  • Weather conditions – Cloud cover lowers UV levels, but not all the way. Depending on the thickness of the clouds, it is possible to get sunburned on a cloudy day. Even if it doesn’t feel warm or seem bright, you could still get a sunburn!
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What is the UV Index?

The UV Index tells you how strong the sun’s rays will be each day. It is calculated by a computer based on different things like weather and time of year.

The UV Index predicts UV levels on a 1 to 11+ scale in the following way:

UV Index NumberExposure Level
2 or lessLow
3 to 5Moderate
6 to 7High
8 to 10Very High

The higher the UV Index forecast, the stronger the sun will be. The stronger the sun is, the greater the need for you to protect yourself from it with sunscreen or shade.

Some medications cause serious sun sensitivity, like medicines used to treat acne. This means you can get a sunburn very easily if you are taking medicine like this. The UV Index is not intended for use by people with serious sun-sensitivity. Talk to your doctor about extra steps you can take to protect yourself if you take a medicine that makes you sensitive to the sun.

For more information, visit the Environmental Protection Agency.

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How did the ozone layer get too thin in places?

Until the late 1990s, chloroflurocarbons (say: KLOR-oh-FLOOR-oh-car-bons), or CFCs, and other chemicals were used all around the world. CFCs move up into the air after they are used and break apart the ozone layer. This thinning of the ozone layer is one of the issues that people are concerned about when they talk about global warming. Countries around the world, including the United States, have noticed the problms caused by the thinning of the ozone layer and have agreed to stop using substances that break down the ozone layer.


Content last reviewed July 20, 2010
Page last updated October 31, 2013