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UV (Ultra Violet) Radiation

Energy from the sun reaches the earth as visible, infrared, and ultraviolet (UV) rays. Exposure to UV rays can cause numerous health problems, such as sunburn, skin cancer, skin aging, cataracts, and can suppress the immune system. There are two way of displaying this:

UV Index, an intensity measurement first defined by Environment Canada and since been adopted by the World Meteorological Organization. UV Index assigns a number between 0 and 16 to the current UV intensity. The US EPA categorizes the Index values as shown in table belowThe lower the number, the lower the danger of sunburn. The Index value published by the U.S. National Weather Service is a forecast of the next day’s noontime UV intensity. The index values displayed by this weather station are real-time measurements.

The diagram aboce shows the total minutes it will take to get sunburned by hour, these are average numbers.

UV MEDs, MED (Minimum Erythemal Dose) is defined as the amount of sunlight exposure necessary to induce a barely perceptible redness of the skin within 24 hours after sun exposure. In other words, exposure to 1 MED will result in a reddening of the skin. Because different skin types burn at different rates, 1 MED for persons with very dark skin is different from 1 MED for persons with very light skin.

Both the U.S. Environmental Protection Agency (EPA) and Environment Canada have developed skin type categories correlating characteristics of skin with rates of sunburn.
Index Values Exposure Category
0 - 2 Low
3 - 4 Moderate
5 - 6 High
7 - 9 Very High
10+ Extreme
Skin Phototype Skin Color Tanning & Sunburn history Min UV MED
1 - Never tans, always burns
Pale or milky white; alabaster
Develops red sunburn; painful swelling, skin peels
0.5 - 1.5
2 - Sometimes tans, usually burns
Very light brown; sometimes freckles
Usually burns, pinkish or red coloring appears; can gradually develop light brown tan
1.5 - 2.5
3 - Usually tans, sometimes burns
Light tan; brown, or olive; distinctly pigmented
Rarely burns; shows moderately rapid tanning response
2 - 3.5
4 - Always tans; rarely burns
Brown, dark brown, or black
Rarely burns; shows very rapid tanning response
2.5 - 5.5