Definitions of UVA and UVB with diagrams
Sunlight is grouped into 3 types of ultraviolet light; Ultraviolet-A (UVA), Ultraviolet-B (UVB), and Ultraviolet-C (UVC), based on the wavelength of the light being emitted (see diagram). UVC is absorbed by the ozone, but UVA and UVB pass to the earth's surface, and are therefore of main concern to us.
UVC radiation has a wavelength of 200-290 nanometers. This wavelength is the most harmful to the skin, so fortunately it is filtered by ozone in the atmosphere. UVB radiation (290-320 nanometers) is the wavelength that is most strongly associated with skin cancer formation. UVA radiation (320-400nm) is the range used for tanning. It is important to remember that there is actual great variation in the biologic effects of different wavelengths within each of these subdivisions.
The amount of ultraviolet radiation that penetrates the skin is largely related to the amount of melanin within the epidermis, but is also dependent on the thickness of the skin, as well as the wavelength of the ultraviolet light. Ultraviolet-A radiation penetrates more deeply than UVB radiation.
The photobiologic effects and damage of wavelengths longer than 320nm are thought to be less than that of shorter wavelengths. However, the sun emits much more radiation in the range of wavelengths greater than 320nm (as in the UVA range), increasing its potential cumulative damaging effects.
The UV Index may be checked daily.
|Rating||Risk||Minutes to burn||Precautions|
|0 - 2||Minimal||60 minutes||Sunscreen, UV sunglasses|
|2 - 4||Low||45 minutes||Sunscreen, UV sunglasses|
|4 - 6||Moderate||30 minutes||Sunscreen, UV sunglasses & Hat|
|6 - 10||High||15 minutes||Sunscreen, UV sunglasses, Hat & Umbrella|
|10 - 15||Very High||10 minutes||Sunscreen, UV sunglasses, Hat, Umbrella & Avoiding mid-day sun|
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