What Is the UV Index—and How Can It Help You Stay Safe in the Sun?

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.

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. 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.

Below is a chart that explains what each level (from 1-11) means and the level of sun protection you will need. 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.

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. 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.

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.

Skin cancer is treatable if it's caught early, making the need for regular monitoring of your skin and yearly screenings vital.

Sun Sensitivity

Some people may experience an allergic reaction to sun exposure, called photophobia or sun sensitivity. This condition can cause symptoms like rash, bumps, red blotchy areas, or even hives if you are exposed to any UV radiation. Some medications can also increase the likelihood of sun sensitivity, as they can influence your body to burn more easily or turn exceptionally red.

Premature Aging

Too much time in the sun can increase the likelihood of wrinkles, sunspots, and skin discoloration—sometimes much faster than someone who does not spend a lot of time in the sun. In fact, up to 90% of visible skin changes associated with aging are caused by UV radiation.

Eye Damage

Not only can exposure to the sun cause a burn to your cornea, but it also can increase the likelihood of cataracts or raise your risk of blindness. Around 13 million people in the U.S. age 40 and older have cataracts, while more than half of those 65 and older have some evidence of cataracts.

Excess UV exposure may also cause you to experience tissue growth that can block vision, skin cancer around your eyes, and macular degeneration (which affects your central vision).

Immune System Suppression

Exposure to UV radiation can reduce the effectiveness of your immune system. For instance, your skin often mounts a defense against germs you are exposed to, but exposure to UV radiation can weaken this response. This can cause you to be more susceptible to certain infections and cancers—regardless of how easily your skin burns.

Safety Tips

The best way to decrease your chances of complications from the sun is to reduce your exposure to UV radiation. Avoid tanning beds and make sure you are regularly monitoring your skin for signs of skin cancer. Here are some other tips for reducing your exposure to UV radiation.

• Look for sunglasses that offer 99-100% UV protection
• Wear a hat when you are going to be in the sun
• Choose sun-protective clothing or clothes that have a UPF on the label
• Select a broad-spectrum, water-resistant sunscreen with an SPF of 30 or higher and wear it on all exposed skin
• Put sunscreen on 15 minutes before going in the sun and reapply every two hours, as well as when you sweat or swim
• Check the expiration date of your sunscreen, as it only lasts for three years
• Stay in the shade whenever possible, especially if your shadow is shorter than you
• Avoid the sun between 10 a.m. and 4 p.m. when the sun's rays are the strongest
• Drink plenty of water to keep your body and skin hydrated
• Limit or avoid the use of tanning beds
• Regularly check your skin for signs of skin cancer

A Quick Review

The UV index measures how much UV radiation you will be exposed to that day. This measurement, which is on a scale of 1-11, can be used to keep you safe when planning outdoor activities in the sun.

Overall, the UV index is influenced by the weather, the location, time of day, season, and more. Regardless of the UV index, wear sunscreen daily to protect your skin from the sun's rays and limit the risk of heat-related health complications.