How to read the UV index scale and why it's crucial for preventing skin cancer

Radiation from the sun's ultraviolet (UV) rays can cause lasting damage to your skin. In fact, UV rays are the leading cause of skin cancers, like melanoma and basal cell carcinoma, says Beth Goldstein, MD, a dermatologist with Central Dermatology Center.

Therefore, it's important to monitor the UV index in your area before spending time outside. This can help protect you from sun damage and subsequent health consequences down the line. 

What is the UV index? 

The ultraviolet index or UV index forecasts the intensity of the sun's UV rays on a scale of 0 to 11+. 

Understanding the index helps you predict how quickly your skin will burn if exposed to the sun and what kind of preventative actions you should take if planning to be outside. For example, the higher the UV index number, the stronger the UV rays, and the faster you are likely to burn. 

The US National Weather Service (NWS) and the EPA measure UV levels daily using a computer model that analyzes ground-level strength of UV radiation along with forecasted cloud cover and ozone concentration. You can see the UV index forecast for your location on the EPA's website.

To predict the intensity of UV radiation, the UV index takes into account:

• The time of day. UV radiation is strongest between 10 am and 3 pm when the sun is at its highest peak in the sky.
• The season. UV radiation is highest in the spring and summer because this is when the northern hemisphere is tilted toward the sun. UV radiation declines in the fall and is lowest in the winter as the northern hemisphere tilts away from the sun.  
• Cloud cover. Heavy clouds can block UV radiation and lower the UV index. 
• Latitude. Latitude measures how close you are to the equator. The closer to the equator, the more intense the UV radiation. 
• Altitude. The higher the elevation, the stronger the UV rays. In fact, UV increases about 2% for every 1,000-foot elevation gain.
• Environmental factors. Trees or tall structures that block UV rays can lower UV radiation levels. On the other hand, reflective surfaces, like water and snow can intensify UV radiation. 

UV Index of 0 to 2 

A UV index of 0 to 2 means you can expect a low level of UV radiation with minimal risk of getting sunburned, says Debra Jaliman, MD, a dermatologist with her own private practice and an assistant professor of dermatology at the Icahn School of Medicine at Mount Sinai. 

While the average person can lay in the sun for an hour without getting burnt, people with very fair skin should take precautions as they may burn within 30 minutes, says Jaliman.

UV Index of 3 to 5 

This is a moderate level of UV radiation, meaning there is a moderate risk of getting burnt if you are outside without sun protection. 

The time it takes for your skin to burn at this level depends on your skin type, Jaliman says, but generally speaking, a UV index of 3 to 5 could result in sunburn in about 30 to 45 minutes on average.

To protect your skin, the EPA recommends:

• Seeking shade in the late morning through the mid-afternoon. 
• Using sunscreen with at least SPF 30 on exposed skin.
• Wearing a wide-brimmed hat, sunglasses, and protective clothing that helps block UV rays. This can include fabric with an ultraviolet protection factor (UPF) of 30 or more, which means it is tightly woven and helps block UV rays. 

UV Index of 6 to 7 

This is a high level of UV radiation, meaning the average person is likely to burn in about 15 to 25 minutes without sun protection. 

At this level, the EPA recommends:

• Applying sunscreen with at least SPF 30 every two hours.
• Wearing a wide-brimmed hat, sunglasses, and sun-protective clothing, like UPF fabric or long sleeves and pants. 
• Avoiding time in the sun during the hottest parts of the day — between 10 a.m and 3 p.m. If you must be outside during this time, try to stay in the shade. 

UV Index of 8 to 10 

This is a very high level of UV radiation with a high risk of getting sunburnt. On average, you can expect to burn in about 10 minutes or less at this level of UV radiation, Jaliman says. The EPA recommends you take extra precautions if you are outside, including all the measures listed above. 

"Do not go [outside] without serious protection, or just do not go out at all," Goldstein says.

UV Index of 11+ 

This is an extreme level of UV radiation and sun exposure without protection at this level is dangerous. All precautions should be taken, including wearing sunscreen, sunglasses, a wide-brimmed hat, and protective clothing. At this level, unprotected skin can burn in minutes. 

Our top sunscreen picks to keep you protected at any UV index

It's clear that no matter the UV index, your best bet is to use at least an SPF 30 when in sun exposure to stay optimally protected. We've tested the best sunscreens that offer lasting, comfortable coverage for any skin type. Whether you're looking for the best sunscreen moisturizer specifically for the face or the best sunscreens for darker skin tones, our rigorous testing in sun, sweat, and water has put each of these at the top of our list. And don't forget about your SPF lip balms; they're susceptible to burning as well.

Insider's takeaway 

The UV index provides a measure of how strong UV rays are in a specific location. This information can help you plan when to spend time outside and what kind of sun protection to use. 

Your risk of burning increases the higher the UV index reads, Jaliman says, but regardless of what the index says you should always wear sun protection when spending time in the sun. 

This can include wearing sunscreen with at least SPF 30 or wearing protective clothing, like a hat, sunglasses or clothes with UPF fabric.