UV index

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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. It is a scale primarily used in daily forecasts aimed at the general public.

Its purpose is to help people to effectively protect themselves from UV light, of which excessive exposure 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 applying sunscreen to the skin and wearing a hat) when the UV index is 3 or higher; see the table below for complete recommendations.

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[edit] Description

The index is an open-ended scale, with higher values representing a greater risk of skin damage due to UV exposure. An index of 0 is theoretically possible, but unlikely to actually occur, indices greater than 11 are quite common in the southern hemisphere where the Ozone layer is depleted. Values as high as 17 have been recorded in Carnarvon, Western Australia.[citation needed] 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 inclement 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.

[edit] 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. In 2005, the United States [3] and Australia [4] launched the UV Alert. While the two countries have different baseline UV intensity requirements before issuing an alert, the goal is to raise awareness of the dangers of overexposure to the sun on days with intense UV radiation.

[edit] 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 Recommended Protection
0–2 Low danger to the average person Green Wear sunglasses; use 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 use 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 use sunscreen having SPF 15 or higher, cover the body with sun protective 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 use 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 [5])

[edit] 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 one's lifetime.[1][2][3][4]

[edit] References

  1. ^ A popular article in a boating magazine said, 'Sun damage never "heals" and is cumulative over one's lifetime; the more exposure, the more damage and the higher the chances of getting skin cancer.' [1]
  2. ^ Cumulative exposure to the sun imparts damage to the epidermis (the outer layer) and the dermis, (the deeper layer where the skin's framework exists). Damage to the dermal layer changes the structural components, causing Elastin fibers to thicken and become more numerous. Collagen is damaged and degraded and 'reticulin' fibers appear throughout the dermis rather than outlining the specific dermal-epidermal junction. Ad website for Fallene, a sunblock product.
  3. ^ Medical evidence suggests that the sun's harmful ultraviolet rays damage the skin and eyes. Moreover, this harm accumulates during one's lifetime. Ohio State University Fact Sheet
  4. ^ UV-initiated cellular damage early in life can be promoted by subsequent exposures to convert and progress into tumors later in life. For example, the nonmelanoma skin cancer, SCC, is known to be dependent on the cumulative UV dose. [2]

[edit] External links

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