At least 17 states were expected to experience elevated ultraviolet (UV) radiation levels on Thursday, according to the National Weather Service and the Environmental Protection Agency (EPA).

Why It Matters

The EPA warns that elevated UV radiation levels greatly increase the risk of skin burns, eye damage and long-term health problems, such as skin cancer.

What To Know

According to the EPA’s UV index forecast for Thursday, solar noon—when the sun is at its highest point in the sky at a specific location—in parts of California, Nevada, Utah, Arizona, Colorado, New Mexico, Texas, Oklahoma, Kansas, Arkansas, Mississippi, Louisiana, Alabama, Georgia, Florida, North Carolina and Virginia could see levels of at least 11, meaning an extreme risk of harm from sun exposure.

The UV index runs on a scale of one to 11. The levels, and their corresponding advice, are as follows:

1-2: Low (minimal risk from UV rays for the average person)

• Wear sunglasses on bright days.
• In winter, snow can reflect UV and almost double exposure.
• Use SPF 30+ sunscreen if you burn easily.

3-5: Moderate (moderate risk of harm from unprotected sun exposure)

• Cover up, wear a hat and sunglasses.
• Use SPF 30+ sunscreen.
• Stay in the shade near midday when the sun is strongest.

6-7: High (high risk of harm from unprotected sun exposure)

• Limit sun time between 10 a.m. and 4 p.m.
• Wear protective clothing, sunglasses and SPF 30+ sunscreen.

8-10: Very High (very high risk of harm; sun protection is essential)

• Take extra precautions, including wide-brimmed hats and long sleeves.
• Minimize exposure between 10 a.m. and 4 p.m.
• Be aware: White sand and similar surfaces reflect UV and increase exposure.

11+: Extreme (extreme risk of harm; unprotected skin can burn in minutes)

• Avoid sun exposure from 10 a.m. to 4 p.m.
• Reapply SPF 30+ sunscreen every two hours.
• Seek shade, wear full coverage and use sun protection.

What People Are Saying

Antony Young, the emeritus professor of experimental photobiology, dermatology, at King’s College, London, previously told Newsweek: “The UV index [UVI] is a measure of the sunburning power of sunlight. … The higher the UVI, the shorter the time you need in the sun to burn.”

Richard Weller, a professor of medical dermatology at the University of Edinburgh, previously told Newsweek that sunlight can cause photoaging—premature aging of the skin as a result of prolonged and repeated exposure to UV radiation. He added, “Sunburn is a risk factor for melanoma skin cancer, which is the most serious of the skin cancers.”

What Happens Next

The UV index forecast map is created each day using data from the National Weather Service.