Ultraviolet index

The ultraviolet index is a scale of sunburn-effective ultraviolet radiation at Earth’s surface. It indicates how quickly unprotected skin may be harmed by sun exposure.

Erythema action spectrum: A wavelength-by-wavelength measure of how effectively radiation produces skin redness. It supplies the weighting that makes the index track sunburn potential.
Sun protection factor: A measure of how much sunscreen reduces sunburn-producing ultraviolet exposure under test conditions. The index helps determine when sunscreen protection is advisable.
Sunburn: An inflammatory skin injury caused mainly by excessive ultraviolet radiation. Sunburn is the acute harm the index is designed to help prevent.
UV-A: The longest-wavelength band of ultraviolet radiation reaching Earth’s surface in substantial amounts. UV-A contributes differently from shorter wavelengths to the sunburn-weighted total.
Ultraviolet radiation: Electromagnetic radiation with wavelengths shorter than visible light and longer than X-rays. The index summarizes the ultraviolet radiation reaching the ground.
Sunscreen: A topical product that reduces ultraviolet radiation reaching the skin. High index conditions increase the value of applying effective sunscreen.
Skin cancer: A group of cancers arising from abnormal growth of skin cells. Cumulative ultraviolet exposure contributes to several forms of skin cancer.
UV-B: The ultraviolet band between UV-A and UV-C, much of which is absorbed by atmospheric ozone. UV-B has strong sunburn effectiveness and often dominates the index’s weighting.
Solar zenith angle: The angle between the Sun’s direction and the vertical at a location. It changes the atmospheric path sunlight travels before reaching the surface.
Protective clothing: Clothing designed or selected to reduce ultraviolet exposure to the skin. Clothing provides practical protection when the index signals elevated risk.

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