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The UV index is an international standard measurement of how strong the ultraviolet (UV) radiation from the sun is at a particular place on a particular day. Its purpose is to help people to effectively protect themselves from UV light, which causes sunburns, eye damage such as cataracts, skin aging, and skin cancer (see the section health effects of ultraviolet light). Public-health organizations recommend that people protect themselves (for example, by wearing sunscreen and a hat) when the UV index is 3 or higher; see the table below for complete recommendations.

Contents 1 Description 2 History 3 How to use the index 3.1 Cautionary notes 4 External links

Description

The index has a scale from 1, indicating a very weak sun, to 11, indicating an extremely strong sun. An index of 0 is theoretically possible, but unlikely to actually occur, as is an index greater than 11. The numbers are directly related to the amount of UV radiation reaching the surface of the earth, measured in W/m².

The UV index announced in weather forecasts is a prediction of how strong the actual UV intensity will be at the sun's highest point in the day, which typically occurs during the four-hour period surrounding solar noon. The prediction is made by a computer model that accounts for the effects of altitude and inclimate weather (clouds), which increase or decrease, respectively, the amount of UV radiation that will reach the surface. The calculations are weighted in favor of the UV wavelengths that human skin is most sensitive to. Providing the public with an easy-to-understand daily forecast of UV intensity is the main purpose of the UV index.

Less frequently than the predictive use, the UV index can also be used to report actual observed UV radiation at various times during the day or year. In this case, the index represents a simple arithmetical transformation of the UV intensity (W/m²), and should not be directly compared to the UV index reported by weather forecasters.

History

In 1992, three scientists from Environment Canada developed the UV index, and made Canada the first country in the world to broadcast forecasts of the predicted daily UV levels for the next day. Several other countries followed suit with their own UV indices, among them the United States in 1994. Until recently, the methods of calculating and reporting a UV index varied from country to country. Today, a worldwide UV index, standardized by the World Health Organization (WHO), has replaced the inconsistent regional methods. The international UV index not only specifies a uniform calculation method but also standard colors and graphics for printed media. In the USA, the WHO index replaced the original US index in 2004.

How to use the index

Recommendations for protection when the day's predicted UV index is at various values are:

UV Index	Description	Media Graphic Color	Recomended Protection
0–2	Low danger to the average person	Green	Wear sunglasses; wear sunscreen if there is snow on the ground, which reflects UV radiation, or if you have particularly fair skin.
3–5	Moderate risk of harm from unprotected sun exposure	Yellow	Wear sunglasses and sunscreen, cover the body with clothing and a hat, and seek shade around midday when the sun is most intense.
6–7	High risk of harm from unprotected sun exposure	Orange	Wear sunglasses and sunscreen having SPF 15 or higher, cover the body with clothing and a wide-brim hat, and reduce time in the sun from two hours before to three hours after solar noon (roughly 11:00 to 16:00 during summer in zones that observe daylight saving time).
8–10	Very high risk of harm from unprotected sun exposure	Reddish-purple	Same precautions as above, but take extra care — unprotected skin can burn quickly.
11+	Extreme risk of harm from unprotected sun exposure	Violet	Take all precautions, including: wear sunglasses and sunscreen, cover the body with a long-sleeve shirt and pants, wear a broad hat, and avoid the sun from two hours before to three hours after solar noon.

(Source: United States Environmental Protection Agency [1])

Cautionary notes

Be aware when interpreting the UV index and recommendations that:

1. The intensity of UV radiation reaching the surface of the earth varies greatly depending on how high the angle of the sun is in the sky. The sun reaches its highest angle at solar noon, which rarely corresponds to 12:00 on clocks. This is because time zones are identical over large regions of the earth, while the angle of the sun at any given moment varies continually with longitude.
2. The recommendations given are for adults with dark to moderately fair skin. Children and particularly fair-skinned people or those who have sun sensitivity for medical reasons need to take extra precautions.
3. Damage from sun exposure is cumulative over the day.

External links

• Sunwise by the US Environment Protection Agency — background information and UV-index forecasts
• World Health Organization UV Radiation program

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