Weather Explainers

The UV Index: How Much Is Your Skin in Danger?

At a Glance

The sun produces ultraviolet radiation, which can be harmful to human health if overexposed.
Prolonged exposure to direct sunlight can lead to sunburn in the short-term and an increased skin cancer risk in the long-term.
The UV Index ranks the amount of UV radiation on an 11-point scale.
Seeking shaded areas and generously applying sunscreen are some of the best ways to protect your skin from UV rays.

Most people have a general familiarity with the Ultraviolet (UV) Index, but just how dangerous are those UV rays to your skin when the index is high?

Used in the United States, the UV Index scale integrates the international guidelines for reporting established by the World Health Organization (WHO).

Ultraviolet radiation is part of the electromagnetic spectrum emitted by the sun. UVC rays, with wavelengths of 100 to 280 nanometers, are absorbed by the atmosphere's ozone layer, but most radiation in the UVA range (315 to 400 nanometers) and about 10 percent of the UVB rays (280 to 315 nanometers) are able to make it to the Earth’s surface.

UVA and UVB rays are both of major importance to human health. According to the WHO, small amounts of UV radiation are necessary for the production of vitamin D in your body, but overexposure may result in serious health effects to the skin, eyes and immune system.

This map is just an example and does not represent the current UV Index.

(MORE: What is the Heat Index and Why is it Used?)

Overexposure to UV radiation leads to sunburn in the short-term, which is painful and, if severe enough, can be considered a first-degree burn. In the long-term, there is an increased risk of developing skin cancer, which could potentially be fatal.

The UV Index is ranked on a scale from 0 to 11-plus, with 0 being the lowest and 11-plus being the most extreme. Those numbers are grouped as follows:

The UV Index ranks the amount of UV radiation on an 11-point scale.

(MAPS: Current UV Index)

When the UV Index is 9, for example, you can get a sunburn in 20 minutes or less. Even when the UV Index is only 4, sunburn is still possible within 50 minutes. Sunburn can still occur when the UV Index is low – on a cloudy day, for example – but it typically takes an hour or longer.

The Environmental Protection Agency offers some tips on how to stay safe from UV rays:

Minimize sun exposure between 10 a.m. and 4 p.m.
If outdoors, seek shade and wear protective clothing, a wide-brimmed hat and UV-blocking sunglasses.
Generously apply broad spectrum SPF 30 or higher sunscreen every two hours, even on cloudy days, and after swimming or sweating.
Watch out for bright surfaces, such as sand, water and snow, which reflect UV rays and increase exposure.

The map below shows the current UV Index across the Lower 48 states.

The UV Index is highest in the summer, when the Northern Hemisphere is tilted toward the sun and receives the greatest amount of direct sunlight and UV radiation.

Brian Donegan is a meteorologist at weather.com. Follow him on Facebook, Twitter and Instagram.