The UV Index: Know Your Risk

The Skin Cancer Foundation

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You may have heard of the UV index, or seen it on an app or on TV, but what is it, really? Where does it come from, and how can it help you protect your skin? We speak with two experts, one from hot and sunny Phoenix, the other from cooler and cloudier Boston, to help make sense of this tool explain how it can be most useful.


The UV index is a measure of heat.

False. The UV index measures the intensity of ultraviolet (UV) radiation from the sun at a location. It’s calculated using the latitude and altitude of that place, time of day, time of year, ground conditions, cloud cover and state of the ozone layer in the atmosphere. The UV index is reported as a whole number between 0 and 11(+), with 0 indicating absolutely no sunlight (used only at night!) and 11 indicating extreme radiation, when you can burn in less than 10 minutes.


But when the UV index is high, it will also be hotter outside.

Also false! It may be hotter outside when the UV index is high because UV is higher in the summer and when there are fewer clouds, things that make the temperature rise, too. But you could be in Colorado, for example, at an elevation of 6,000 feet, with cool temperatures and a high UV index. That’s because the atmosphere filters out some UV radiation before it hits Earth, and the atmosphere is thinner at higher elevations, making it a less effective filter.


If the UV index is low, I won’t get a sunburn. 

Possibly true. How quickly and easily you burn depends on your skin type. Someone with pale, sensitive skin can burn on a day with a UV index of 1 if they spend a couple of hours outside without sun protection. But on a UV index 2 day, just one step up on the scale, someone with Fitzpatrick Skin Type IV can burn under similar circumstances.

Two types of ultraviolet light reach us here on Earth: Ultraviolet B (UVB) rays cause sunburn and damage to DNA in skin cells, which can lead to skin cancer. Ultraviolet A (UVA) rays are the ones mostly responsible for tanning and premature signs of aging but can also contribute to sunburn and skin cancer. Although the UV index is billed as “sunburn protection,” a lot of the UV that reaches the Earth’s surface is UVA. Throughout the year, UVB fluctuates a lot, while the amount of UVA that reaches us is much steadier.

How to Use the UV Index

“The UV index gives people a way to easily quantify and understand their risk,” says Elizabeth Buzney, MD, associate vice chair of clinical affairs for the Department of Dermatology at Brigham and Women’s Hospital and assistant professor of dermatology at Harvard Medical School. “Notifying us about extreme risk and getting people to rethink their typical routine is a great use of this forecast measurement.”

“For example, if the UV index is really high, it might be wiser to just not go outside,” adds Dr. Buzney. “Being aware of the UV index could help you determine when is a safer time, with a safer UV level, to go out and do what you need to do.” That might mean going earlier or later in the day or even at night.

Dr. Buzney is based in Boston, where the monthly average UV index peaks at 6 in June, July and August. But in Phoenix, where Harper N. Price, MD, division chief of pediatric dermatology at Phoenix Children’s Hospital, is based, the monthly average UV index is 8 during those same months, and never goes below 3 (in December). “I think about the UV index almost like a weather report,” Dr. Price says, “I use it as a register of how bad it is going to be out there today — or should I even be out there?”

Dr. Price believes in basking in the great outdoors without basking in too much UV radiation!


In fact, the UV index is calculated by the National Weather Service (and published by the Environmental Protection Agency, or EPA), so adding it to your daily weather check makes a lot of sense. The same way you might bring an umbrella if it’s going to rain, or stay inside if there is a thunderstorm looming, you can also be prepared for the different levels of UV that you may encounter. You can check your UV forecast, for the day, or by the hour, here.

Another use of the UV index is if you are already at high risk of developing a skin cancer. If you know you are at high risk, it can be especially important to avoid exposure during peak UV hours. Using the UV index can help you determine a safer time to be outdoors.

Limitations of the UV index

The bottom line, however, is that everyone needs sun protection during daylight hours all year long, regardless of their skin type or the UV forecast for the day. As Dr. Buzney tells her patients: “If you can see, then there’s UV!”

“People don’t realize it takes very little UV to actually induce DNA damage,” says Dr. Price, a melanoma survivor who is also avid about outdoor activity. To protect herself from the Phoenix sun, she always covers up with a wide-brimmed hat and UPF long sleeves and pants, even for short walks, such as from a store to her car in the parking lot. Any exposed skin gets a good layer of sunscreen.

“It would never occur to me to say, ‘Oh, it’s just moderate UV, or low UV, so we don’t have to do anything to protect ourselves,’” says Dr. Buzney. “No! You need protection every time you go outside.”

And if you want to know when you should absolutely avoid being outside, the healthiest thing you can do for yourself may be to check that UV index!

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