Radiation and the UV Index
The UV Index is a forecast of the amount of skin damaging UV radiation (UltraViolet radiation) expected to reach the earth’s surface at the time when the sun is highest in the sky (solar noon). The amount of UV radiation reaching the surface is mostly related to the elevation of the sun in the sky, the amount of ozone in the stratosphere, and the amount of clouds present.
The UV Index can range from 0 (at night) to 15 or 16 (in the tropics at high elevations under clear skies). UV radiation is greatest when the sun is highest in the sky and rapidly decreases as the sun approaches the horizon. The higher the UV Index, the greater the dose rate of skin damaging (and eye damaging) UV radiation. Consequently, the higher the UV Index, the smaller the time it takes before skin damage occurs.
UV Index – Minutes to Skin Damage
1) Find today’s UV Index on the bottom axis of the chart.
2) Move up to the appropriate shaded box for your ‘burn frequency’.
Your Burn Frequency (darker shade to lighter shade)
10-30 minutes (Always burn)
30-50 minutes (Usually burn)
40-75 minutes (Sometimes burn)
50-120 minutes (Rarely burn)
3) Look at the left axis to determine your skin damage time range.
There are two prices to pay for overexposure to UV radiation:
1) A severe sun burn following an intense short term overexposure.
2) Skin cancers developing after long term overexposure. Melanoma, the more deadly of the two types of skin cancer occurs when someone has been subjected to several intense short term over-exposures. Non-melanoma skin cancers, which are almost 100% curable, occur in people who are overexposed for very long periods of time, like construction workers, farmers, or fishermen. Long term overexposure to UV radiation has been linked to the formation of cataracts in the eyes as well.
Exposure Category – Minimal
UV Index (0, 1, 2)
Protective Actions: Apply skin protection factor (SPF) 15 sun screen.
Exposure Category – Low
UV Index (3, 4)
Protective Actions: SPF 15 & protective clothing (hat)
Exposure Category – Moderate
UV Index (5, 6)
Protective Actions: SPF 15, protective clothing, and UV-A&B sun glasses.
Eposure Category – High
UV Index (7, 8, 9)
Protective Actions: SPF 15, protective clothing, sun glasses and make attempts to avoid the sun between 10am to 4pm.
Exposure Category – Very High
UV Index (10+)
Protective Actions: SPF 15, protective clothing, sun glasses and avoid being in the sun between 10am to 4pm.
UV Index Map – United States
credit: Weather Underground
UV Alert Forecast – United States
credit: NOAA / National Weather Service
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