Understanding the UV Index—and What It Means for Your Health

Woman wears a large hat and sunglasses outdoors


Have you heard of the UV index? By now, most of us understand that ultraviolet (UV) radiation from the sun damages the skin. In particular, UV radiation creates oxidative stress. Severe damage can lead to sunburns in the short term. In the long term, UV-induced oxidative damage can lead to the formation of skin cancer and early presentation of fine lines, wrinkles, and uneven skin tone. Importantly, these long-term negative effects are a result of lifetime UV exposure, not just the number or intensity of sunburns.

The simple answer to minimize UV-induced skin damage is to minimize UV exposure. It's also always good to wear sunscreen. The UV index is a useful tool to help us maximize fun outdoors while minimizing UV exposure. I spoke with my dear friend Dr. Jasmine Obioha, MD, a board-certified dermatologist practicing outpatient medical and cosmetic dermatology at Cedars-Sinai in Beverly Hills, California, to review the UV index. 

What Is the UV Index?

The UV index is a scale that predicts UV radiation at the Earth’s surface on any given day.

What Is the UV Index?

The UV index is a scale that predicts UV radiation at the Earth’s surface on any given day. It is analogous to a weather report. Since the intensity of UV exposure is variable by location, season, and weather, it will vary day by day and region by region.

UV indexes are available globally, as recommended by the World Health Organization (WHO). In the United States, our UV index is managed by the National Weather Service (NWS) and Environmental Protection Agency (EPA). 

"The UV index is an international measure of UV exposure from the sun and can help guide safe sun exposure," adds Obioha.

How Does the UV Index Relate to Skin?

Ultraviolet radiation is responsible for sunburns in the short term. In the long term, ultraviolet radiation is the number one risk factor for the development of skin cancer and common signs of skin aging. Obioha notes that “the higher the UV index, the higher the risk of sunburns, photoaging, and UV-induced skin cancers such as basal cell carcinoma, squamous cell carcinoma, and melanoma.” In addition, damage occurs quicker when the UV index is high. 

Although we should be in the regular habit of protecting our skin from ultraviolet radiation, the UV index provides us with additional beneficial data. Those with sun-sensitive skin may choose to avoid beach time or outdoor runs on days when the UV index is predicted to be high. 

Understanding the UV Index

The UV index is reported using a Global Solar UV Index that ranges from 1-11+ and predicts UV exposure levels at noon, when UV exposure is naturally at its maximum within the day. It is typically available with your local weather information, but you can also access it through the EPA UV Index phone app and through the EPA search widget add-on.

A UV index of 1 represents expected low levels of UV radiation for that day, while a value of 11+ represents very high expected UV exposure. 

The UV index fluctuates throughout the day, peaking at noon and slowly decreasing as the day winds down. A way to follow this daily fluctuation is to take a look at your shadow. If your shadow is shorter than you, UV radiation is high for the day. “The shorter your shadow, the higher your UV exposure is when in the sun," says Obioha.

The UV Index:

  • Low (1-2): UV exposure is relatively low on these days and the risk of sunburn is low even for those with sensitive skin. Routine precautions should be taken. Obioha notes that this is the ideal time to be outside. 
  • Moderate (3-5): Routine UV protection measures should be taken. Seek shade during mid-day (11 a.m.-4 p.m.). 
  • High (6-7): Diligent sun protection is needed to protect against sunburns in those who are susceptible to sunburns. 
  • Very High (8-10): Unprotected skin can burn quickly on these days. Take extra precautions to protect the skin against burn and consider avoiding outdoor activities in mid-day. 
  • Extreme (11+): Unprotected skin can burn within minutes. Minimize sun exposure if possible. 

Note that sand, snow, and other bright surfaces can nearly double UV exposure. 

What Should You Do if the UV Index Is High?

Those who tend to burn should practice extra diligent sun protection on days when the UV index is high. This will include:

  • Avoiding outside activities between 11 a.m.-4 p.m. 
  • Wearing SPF 30+ sunscreen on sun-exposed areas while outside.
  • Reapplying sunscreen every 2 hours of sun exposure and after high sweat and water activities.
  • Seeking shade when possible. 
  • Wearing sun-protective clothing when outside. 
  • Protecting eyes, ears, and the scalp with sunglasses and wide-brimmed hats. 

Those who burn very easily should likely avoid mid-day sun exposure on very high and extreme sun exposure days. And remember that on high UV index days, the risk of skin damage is greater and it can occur quickly.

The Takeaway 

UV radiation is responsible for the development of sunburns, skin cancer, and fine lines and wrinkles. The risk of these conditions can be significantly decreased by protecting the skin against UV radiation. Routine sun protection should be implemented on all days, but additional precautions should be taken on days with high, very high, and extremely high UV index ratings. In contrast, low UV index days are excellent for spending time outdoors. Keep in mind that snow, sand, and other bright objects can reflect UV radiation and effectively double the predicted UV Index rating.

Article Sources
Byrdie takes every opportunity to use high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial guidelines to learn more about how we keep our content accurate, reliable and trustworthy.
  1. Amaro-Ortiz A, Yan B, D'Orazio JA. Ultraviolet Radiation, Aging and the Skin: Prevention of Damage by Topical cAMP ManipulationMolecules. 2014;19(5):6202-6219. doi:10.3390/molecules19056202

  2. United States Environmental Protection Agency. Guide to the UV Index.

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