Ultraviolet index

[1] The purpose of the UV index is to help people effectively protect themselves from UV radiation, which has health benefits in moderation but in excess causes sunburn, skin aging, DNA damage, skin cancer, immunosuppression,[2] and eye damage, such as cataracts.

For a wide range of timescales, sunburn in response to controlled UV radiation occurs in proportion to the total number of photons delivered, not varying with the intensity or duration of exposure.

Although this may be in error (especially when cloud conditions are unexpectedly heavy or light), it is usually within ±1 UV index unit as that which would be measured.

Predictions are made by a computer model that accounts for the effects of sun-earth distance, solar zenith angle, total ozone amount, tropospheric aerosol optical depth, elevation, snow/ice reflectivity and cloud transmission, all of which influence the amount of UV radiation at the surface.

This gives a weighted figure called the Diffey-weighted UV irradiance (DUV) or the erythemal dose rate.

Thus, for convenience, the DUV is divided by 25 mW/m2 to produce an index[13][14] nominally from 0 to 11+, though ozone depletion is now resulting in higher values.

[14] After sporadic attempts by various meteorologists to define a "sunburn index" and growing concern about ozone depletion, Environment Canada scientists James B. Kerr, C. Thomas McElroy, and David I. Wardle invented the modern UV index in Toronto, Ontario.

A global UV index, first standardized by the World Health Organization and World Meteorological Organization in 1994,[17] gradually replaced the inconsistent regional versions, specifying not only a uniform calculation method (the Canadian definition) but also standard colors and graphics for visual media.

[18] On December 29, 2003, a world-record ground-level UV index of 43.3 was detected at Bolivia's Licancabur volcano,[19][20] though other scientists dispute readings higher than 26.

Generously apply broad spectrum SPF 50+ sunscreen every 1.5 hours, even on cloudy days, and after swimming or sweating.

Generously apply broad spectrum SPF 50+ sunscreen every 1.5 hours, even on cloudy days, and after swimming or sweating.

Generously apply broad spectrum SPF 50+ sunscreen every 1.5 hours, even on cloudy days, and after swimming or sweating.

Generously apply broad spectrum SPF 50+ sunscreen every 1.5 hours, even on cloudy days, and after swimming or sweating.

These use the UV index and Fitzpatrick scale skin type to calculate the maximum exposure time before receiving a sunburn.

Average UV at noon 1996-2002 ( European Space Agency )
Typical variation of UV index by time of day and time of year, around 40.71 -74.01, based on FastRT UV Calculator [ 4 ]
Sunburn effect (as measured by the UV index) is the product of the sunlight power spectrum (radiation intensity) and the erythemal action spectrum (skin sensitivity) across the range of UV wavelengths. [ 9 ] [ 10 ]