The UV Index
Scientist and medical experts have determined that ultraviolet radiation ("UV") causes sunburn, skin aging, cataracts and skin cancer. It also exacerbates the symptoms of lupus. To help individuals understand and respond to the potential hazard from UV, the UV Index was introduced in a number of countries in 1995/1996. In the U.S., it is published on a daily basis and provides an estimate of the maximum potential hazard from UV for the next day.
A number of organizations publish the UV index. Here are links to three web sites:
- Environmental Protection Agency (search by your zip code)
- Climate Prediction Center of the National Weather Service
- Interactive Weather Information Network (most reliably updated forecast)
The Environmental Protection Agency helped develop the UV Index and offers the following interpretation and recommended actions.
Exposure Category UV Index General Recommended Action (All Skin Types) Minimal 0, 1, 2 Apply SPF 15+ sunscreen. Low 3, 4 Apply SPF 15+ sunscreen and wear protective clothing including a hat. Moderate 5, 6 Apply SPF 15+ sunscreen, wear protective clothing including a hat and protective sunglasses. High 7, 8, 9 Apply SPF 15+ sunscreen, wear protective clothing including a hat and protective sunglasses, avoid the sun between 10 am and 4 pm. Very High 10+ Apply SPF 15+ sunscreen, wear sun protective clothing including a hat and protective sunglasses, avoid the sun between 10 am and 4 pm.
UV is next to visible light on the solar specturm. It can be broken into 3 parts. UVC has wavelengths less than 280 nm but is completely absorbed by stratospheric ozone and does not reach the earth's surface. UVB ranges in wavelength from 280 nm to 320 nm. UVB is absorbed by ozone although some reaches the earth under normal conditions and even more will reach the surface because of ozone depletion. UVA spans from 320 nm to 400 nm and is not significantly absorbed by ozone so most passes through to the earth's surface.
A number of factors influence UVB levels including: latitude, ozone levels, elevation, time of year, time of day and cloud cover.
To see how UVB varies by the time of year for your location click on the following link:
National Weather Service: UV Index for 58 U.S. Cities