The UV Index Explained

Medically Reviewed by Melinda Ratini, MS, DO on May 10, 2022
3 min read

You might be tempted to soak up the sun on a nice, bright day. But those warm rays come with health risks. That’s because the sun gives off ultraviolet (UV) radiation.

Too much UV light can burn your skin in the short term and up your risk for skin cancer over the long run. The good news is there’s a tool you can use to get a sense of how strong UV rays are in your area the UV index. It’s a color-coded scale that goes from 1 to 11+. The higher the number, the higher your odds of UV ray exposure.

It works like this:

  • Low exposure (green): 1-2
  • Moderate exposure (yellow): 3-5
  • High exposure (orange): 6-7
  • Very high exposure (red): 8-10
  • Extreme exposure (violet): 11+

The index only predicts levels of UV radiation – not how hot it’ll be outside. It gives you a forecast of the UV level at noon, when the sun tends to be highest in the sky. That said, the UV level rises and falls as the day goes on.

You can look up your UV index forecast by ZIP code or city and state through the Environmental Protection Agency’s website.

The National Weather Service and the Environmental Protection Agency (EPA) developed it in 1994. The agencies created the index as a way for Americans to plan ahead to protect their skin from the sun outdoors. The National Weather Service calculates the UV index each day.

Certain factors can influence the UV index. They include:

Time of day. UV radiation peaks during the middle of the day. It eases up in the early morning and late afternoon.

Cloud cover. If there’s heavy cover, it can block most UV radiation. If the clouds are thin or broken, most UV rays get through. Puffy clouds during fair weather deflect sun rays and can up the amount of UV radiation that reaches the ground.

Land cover. Things like trees can cut the amount of UV radiation you get.

Ozone. This gas found in the upper atmosphere absorbs UV radiation. The more ozone, the fewer sun rays reach the ground. Ozone found on ground level is the main ingredient in smog and can harm your health. Ozone levels vary each day and throughout the year.

Altitude. UV radiation goes up about 2% for every 1,000-foot increase in elevation because of thinner mountain air.

Seasons. UV radiation peaks during spring and summer (April through August). It drops in fall and is lowest during winter.

Environment. Things on Earth’s surface can reflect or scatter UV rays. The EPA says snow might reflect up to 80% of UV, sand may reflect 15%, and water 10%.

Latitude. This measures the distance north or south of the equator. UV radiation is strongest at the equator and drops toward the North and South poles.

If you have a medium to dark complexion, you’re less sensitive to UV exposure in general. Darker skin has more of a pigment called melanin, which helps block UV rays a bit. But the sun can still damage your skin and raise your chances for skin cancer. So, it’s still important to keep an eye on the UV index and wear sunscreen and protective clothes.

If you have lighter skin, you’re more likely to get skin cancer. Check the UV index and be extra careful in the sun if you have:

  • Pale skin
  • Blond, red, or light brown hair
  • Gotten treatment for skin cancer
  • A family member who’s had skin cancer

If you have pale, sensitive skin, you can still get burned on a day with a UV index of 1 or 2 if you don’t use sunscreen and cover up while you’re outside.

The amount of UV you’re exposed to depends on more than the strength of the sun’s rays. It also matters how long your skin is exposed to the sun and whether you’re wearing protective clothes and sunscreen.

There isn’t a best UV index number for tanning, because there’s no safe or healthy way to get a tan. Getting too much UV can lead to sunburns and premature aging. It can also lead to skin cancer, including melanoma, the most serious type.