What is the UV Index?
The Global Solar UV Index (UVI) is a simple numerical indicator of the maximum solar ultraviolet (UV) radiation received on the earth’s surface during the day. It was developed jointly by the World Health Organization, the United Nations Environment Programme, the World Meteorological Organization, and ICNIRP. The UV Index can either be calculated using computer models that account for factors such as ozone and the amount of cloud cover that affect daily solar UV radiation or derived from measurements. The UV irradiation is weighted by the sunburn response of human skin (280 to 400 nm wavelength range). Values of the UV Index range from zero to over 20. The higher the UV Index, the greater is the intensity of UV radiation that is damaging to skin and eyes.
The UV Index is an important public awareness tool for informing people about the amount of harmful UV radiation during the day at a specified location. Weather forecasts in newspapers, on TV and radio often present the forecast local UVI together with the local weather report. Since the UVI was designed to be easily understood, it is usually reported as the current or forecast maximum UV radiation on a given day, although UV radiation levels will vary throughout the day. Under clear skies, peak UV levels occur in the middle hours of the day, from around 10am to around 2pm (or if there is day-light saving, 11am to 3pm); but heavy cloud-cover may modify this. Throughout the year, peak UV levels occur around the summer solstice.
How does the UV Index relate to UV protection messages?
The UV Index is grouped into 5 different categories and each category has a color code so that people can visualize the level of UV hazard and take suitable precautions. The values 0-2 are classified as Low (green); 3-5 as Moderate (yellow); 6-7 as High (orange); 8-10 as Very High (red); 11+ as Extreme (violet). Each level is accompanied by recommended ways to protect the skin and eyes. The main ways to achieve sun protection are to wear protective clothing, including a broad-brimmed hat and sunglasses with side panels, and to seek shade. Additionally, broad-spectrum (i.e. protecting both from UVA and UVB) sunscreen with a sun protection factor (SPF) of at least 15 should be applied and reapplied after sweating, swimming or after any activity that could wipe off the sunscreen. Babies and young children especially should be well protected from intense sun.
The sun-protection messages given with the UV Index can be modified at national or local levels to meet the particular needs of the population concerned, according to climatic, cultural and other relevant factors. For example the UVI classification scheme is based on the likelihood of harmful effects especially to pale skin and is aimed to alert people with sun-sensitive skins to their risks of skin damage if they go outdoors. For people with darkly pigmented skin the UV Index is more useful to indicate the risks of harmful eye effects from solar UV.
ICNIRP recommends the use of the UV Index to raise public awareness of the risks of exposure to UV radiation. ICNIRP also supports the need for further research into health behavior models so that more successful strategies can be developed for improving sun protection behavior in relation to the use of the UV Index.