Skin Health Skin Care & Cleansing Products How to Understand the UV Index By Heather L. Brannon, MD Heather L. Brannon, MD Heather L. Brannon, MD, is a family practice physician in Mauldin, South Carolina. She has been in practice for over 20 years. Learn about our editorial process Updated on March 19, 2022 Medically reviewed Verywell Health articles are reviewed by board-certified physicians and healthcare professionals. These medical reviewers confirm the content is thorough and accurate, reflecting the latest evidence-based research. Content is reviewed before publication and upon substantial updates. Learn more. by Mary Choy, PharmD Medically reviewed by Mary Choy, PharmD LinkedIn Twitter Mary Choy, PharmD, is board-certified in geriatric pharmacotherapy. She currently serves as the director of pharmacy practice of the New York State Council of Health-System Pharmacists. Learn about our Medical Expert Board Print The UV Index was developed in 1994 by the National Weather Service and the U.S. Environmental Protection Agency (EPA). It helps you plan your outdoor activities so you can avoid exposure to intense UV radiation. Stephen Dorey / Stockbyte / Getty Images What Is the UV Index? Every day the UV Index is calculated for the next day for every zip code in the U.S. It predicts the intensity of UV radiation at noon and is reported on a scale of 1 to 11+. On this scale, 1 signifies the lowest risk of overexposure and 11+ indicates the highest risk of overexposure. UV Index numbers are also grouped into exposure levels ranging from low to extreme and each exposure level has a corresponding color code. Factors that Affect the UV Index The intensity of UV radiation, and thus the UV Index, depends on several factors: Season: The UV Index is highest in spring and summer. It goes down in the fall and is the lowest in the winter.Latitude: UV radiation is strongest at the equator and goes down as you move towards the north or south poles.Altitude: Because air at higher altitudes is thinner, UV radiation goes up as you "go up" in altitude.Time of Day: When the sun is highest in the sky, the sun's rays beat straight down on you with very few of the rays getting scattered. This means that the intensity of UV radiation is highest at the time of "solar noon," usually somewhere between noon and 1 p.m. When the sun is at an angle other than 90 degrees to the earth, some UV radiation is scattered lowering the intensity that affects your skin.Ozone: Ozone absorbs UV radiation making it less intense. Ozone levels can fluctuate from day to day.Cloud Cover: Thick, heavy cloud cover can block most UV radiation, however, thin clouds can let most of the UV rays through. Fluffy, fair-weather clouds are deceiving because they reflect the rays and can increase the amount of radiation reaching Earth.Land Cover: It makes sense that structures like trees and buildings lessen the amount of UV radiation that hits your skin.Earth Surface Characteristics: Whatever is coating the surface of the Earth around you can reflect or scatter UV radiation. Snow reflects up to 80%, while sand reflects 15% and water reflects 10%. How to Find Your UV Index You can find your UV Index by visiting the EPA's UV Index site. There you can look up the UV Index for your zip code. There's also a 4-day UV Index Forecast map of the United States to help you plan your outdoor activities for the next couple of days. The UV Index UV Index Number Exposure Level Color Code 2 or less Low Green 3 to 5 Moderate Yellow 6 to 7 High Orange 8 to 10 Very High Red 11+ Extreme Violet Was this page helpful? Thanks for your feedback! Sign up for our Health Tip of the Day newsletter, and receive daily tips that will help you live your healthiest life. Sign Up You're in! Thank you, {{form.email}}, for signing up. There was an error. Please try again. What are your concerns? Other Inaccurate Hard to Understand Submit 6 Sources Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy. United States Environmental Protection Agency. A guide to the UV index. United States Environmental Protection Agency. UV index search. World Health Organization. UV radiation. World Health Organization. Health effects of UV radiation. American Cancer Society. Ultraviolet (UV) radiation. World Health Organization. Ultraviolet radiation: global solar UV index. Additional Reading Kinney, John P, Craig Long, and Alan Geller. "The Ultraviolet Index: A Useful Tool." Dermatology Online Journal. 6(2000): 2. Ramirez, Raymond and Jeffrey Schneider. "Practical Guide to Sun Protection." Surgical Clinics of North America. 83(2003): 97-107. By clicking “Accept All Cookies”, you agree to the storing of cookies on your device to enhance site navigation, analyze site usage, and assist in our marketing efforts. Cookies Settings Accept All Cookies