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How to use the UV Index to protect yourself from the sun's harmful rays

By Chaffin Mitchell, AccuWeather staff writer

While some sunlight is a beneficial resource for the human body, an overabundance of sunlight can be harmful due to the negative effects of ultraviolet (UV) radiation.

These invisible rays of sunlight are able to cause skin damage, and they are a proven human carcinogen according to the National Toxicology Program of the U.S. Department of Health and Human Services. Therefore, it's important to know when the sun's rays are most potent.

According to the United States Environmental Protection Agency (EPA), an easy way to tell how much UV exposure you are getting is to look for your shadow:

If your shadow is taller than you are, in the early morning and late afternoon, your UV exposure is likely to be lower.

If your shadow is shorter than you are, around midday, you are being exposed to higher levels of UV radiation.

Young woman playing tennis, elevated view

(Ryan McVay/Getty Images)

Since the EPA's method is an estimate and the naked eye cannot see certain wavelengths of sunlight, we rely on spacecraft to tell us the exact Ultraviolet (UV) Index level.

According to NASA, the Earth's atmosphere absorbs a lot of high-energy ultraviolet radiation, so scientists use data from satellites to sense UV radiation coming from the sun and other astronomical objects.

AccuWeather Meteorologist Evan Duffey said the UV Index is certainly a good tool to utilize before going outside.

"A lot of things contribute to the UV Index, but the main factors are the amount of sunshine, the solar angle, the amount of ozone in the atmosphere, elevation and the reflectivity of the ground," Duffey said.

Technically, the scale goes to infinity because it is an open-ended scale. However, for most locations and days, it will be below a 12.

"The highest UV Index ever recorded on Earth was 43.3 in Bolivia on a volcano, during the peak of summer," Duffey said.

The World Health Organization (WHO) uses the following scale:

UV Index infographic (via

The WHO suggests wearing sunscreen at every index level. Those especially susceptible to sunburn should take extra precautions even on moderate days.

Wearing sunglasses, sunscreen and proper clothing is crucial on days when the UV Index is moderate or higher for most people. If you burn easily, then it's important to wear sunscreen and take extra precautions on low UV Index days as well.

Duffey said to remember that sunscreen needs to be re-applied, generally every two hours, or more frequently if you are swimming or sweating.

"It is, however, recommended to reduce sun exposure regardless of precautions on days with a UV Index of a 6 or higher and to minimize sun exposure on days with an 8 or higher," Duffey said.

Assuming you are 100 percent in the shade, you will generally be much safer than in direct sunlight. However, UV radiation can reflect or bounce off objects and still pose dangers.

"Beach sand and water can both reflect enough UV to still give you a sunburn despite being under a beach umbrella, for example. The best thing to do is to take precautions regardless," Duffey said.

As for swimming, water does absorb UV radiation, so swimmers are still susceptible to sunburn.

"Snorkeling is especially dangerous as your back is facing the sun, while the water is working to remove any sunscreen you have applied. Applying sunscreen in advance of getting in the water and wearing a shirt while swimming will significantly reduce your potential to be burned," Duffey said.

After a sunburn

(Mehmet Hilmi Barcin/iStockphoto/Getty Images)

According to Duffey, you can get sunburned in the winter because snow can reflect UV and thus can lead to a higher UV Index.

Also, it's important to protect your eyes on a high UV Index day. Your cornea and the whites of your eyes can both be sunburned.

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"While sunscreen is what people usually grab when looking to protect themselves outside in the summer, long clothing is much more protective and doesn't require the hassle of frequent re-application," he said.

Wearing lightweight and well-ventilated UV Protection Factor (UPF) clothing designed for summer adds another layer of protection without making you too hot.

Even if it is rather cloudy with a low UV Index much of the day, a short window of sunshine is all it takes to burn.

To check your area's UV Index visit AccuWeather and enter your specific location or visit the EPA's UV Index forecast page.

For more safety and preparedness tips, visit

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