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The ultraviolet (UV) index is a daily measurement of the expected intensity of UV radiation from the sun calculated by the National Weather Service for most zip codes in the United States. Published by the Environmental Protection Agency (EPA), this measurement, which is usually on a scale of 1-11, can be useful for planning outdoor activities and protecting yourself.
To get this daily measurement, the National Weather Service (NWS) uses a computer model to predict the intensity of UV exposure someone will receive that day. This computer model considers the strength of solar UV radiation, ozone concentration, ground elevation, and cloud amounts in each area.
When UV levels are expected to be higher than normal, the NWS may issue an alert for that particular area as a warning to be mindful before going outside.1
UV Index Scale
The UV index was first developed in Canada in 1992. In 1994, it was adopted by the NWS, EPA, the World Meteorological Organization, and the World Health Organization.2 While minor tweaks have occurred over the years, it has remained essentially the same since then. People can look to the scale to determine just how strong the sun’s rays will be that day and how to protect themselves.3
Below is a chart that explains what each level (from 1-11) means and the level of sun protection you will need.3 Keep in mind, however, that the American Academy of Dermatologists Association and the American Cancer Society recommend wearing a broad-spectrum sunscreen of SPF 30 or higher every day regardless of the UV index.45
UV Index Exposure Guidelines
1-2 Low Wear sunglasses and sunscreen
3-5 Moderate Take precautions, wear sunscreen, and use sunglasses
6-7 High Reduce time in the sun, cover up, seek shade, and wear sunscreen, sunglasses, and a hat
8-10 Very High Take extra precautions as unprotected skin can be damaged, cover up, seek shade, and wear sunscreen, sunglasses, and a hat
11+ Extreme Take all precautions as unprotected skin will burn in minutes, stay out of the sun if you can, cover up, seek shade, and wear sunscreen, sunglasses, and a hat
Factors Affecting the UV Index
There are several factors that can influence the UV index on any given day. For this reason, the NWS and EPA calculate the UV index based on location and the specific weather conditions for that day. The following can affect the UV index in your area:
Cloud cover: While heavy clouds can sometimes block UV radiation, puffy clouds can actually increase the amount of UV radiation reaching the Earth.
Altitude: For every 1,000-foot increase in elevation, there is a 2% increase in UV radiation. This means while hiking at higher altitudes may be cooler, the UV radiation could still be higher.
Latitude: UV radiation is strongest at the equator and decreases the closer you get to either the North Pole or the South Pole.
Season: In the U.S., UV radiation is highest from April through August (spring and summer) and declines in the fall and winter.
Ozone: Ozone absorbs UV radiation and helps keep the rays from reaching the surface, but if the ozone layer has been depleted, more UV radiation is getting to Earth.
Time of day: UV radiation peaks midday, which is usually around 10 a.m. to 4 p.m. It tends to lessen in the morning and evening hours.
Earth’s surface: Where you are on Earth can affect the UV radiation you are exposed to. For instance, snow may reflect up to 80% of the UV radiation, while sand reflects 15%, and water reflects 10%—which means you may be exposed to more radiation if you are in any of these environments.
Land cover: Trees and structures can reduce the amount of UV radiation you are exposed to.
How a High UV Index Can Affect Your Health
While spending time in the sun can be fun and boost your mood, there are also some risks of being outside that can affect your health.6 Too much sun can cause immediate issues like heat-related illnesses or sunburn. Too much sun time on a high UV index day can also increase your risk of conditions like skin cancer and cataracts.7
Skin Cancer
When UV radiation hits your skin, it causes genetic mutations in your cells. Over time, this exposure (alongside frequent sunburns) can increase the risk of skin cancer. In fact, in the U.S., more than one million people in the United States are diagnosed with skin cancer each year.3
Skin cancer is treatable if it’s caught early, making the need for regular monitoring of your skin and yearly screenings vital.