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The UV Index - A Measure of the Intensity of Ultraviolet Radiation and Your Health
What is the UV Index?
Some exposure to sunlight can be enjoyable; however, too much could be dangerous. Overexposure to the sun’s ultraviolet (UV) radiation can cause immediate effects such as sunburn and long-term problems such as skin cancer and cataracts. Developed by the National Weather Service and the U.S. Environmental Protection Agency (EPA), the Ultraviolet (UV) Index provides important information to help you plan your outdoor activities to prevent overexposure to the sun’s rays.The UV Index provides a daily forecast of the expected risk of overexposure to the sun. The Index predicts UV intensity levels on a scale of 0 to 10+, where 0 indicates a minimal risk of overexposure and 10+ means a very high risk. Calculated on a next-day basis for dozens of cities across the United States, the UV Index takes into account clouds and other local conditions that affect the amount of UV radiation reaching the ground in different parts of the country.
USING THE UV INDEX
The UV Index can help the public be aware of the level of UV radiation exposure expected on a given day. As a result, people can use simple sun protective behaviors to reduce their lifetime risk of developing skin cancer and other sun-related illnesses. What follows is a description of each UV Index level and tips you can give to help people prepare.
0 to 2: Minimal | A UV Index reading of 0 to 2 means minimal danger from the sun's UV rays for the average person:
Look Out Below | ||||||||
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3 to 4: Low | A UV Index reading of 3 to 4 means low risk of harm from unprotected sun exposure. Fair-skinned people, however, might burn in less than 20 minutes:
Me and My Shadow
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5 to 6: Moderate | A UV Index reading of 5 to 6 means moderate risk of harm from unprotected sun exposure. Fair-skinned people might burn in less than 15 minutes. Apply a sunscreen with an SPF of at least 15. Wear a wide-brim hat and sunglasses to protect your eyes:
Made in the Shades | ||||||||
7 to 9: High | A UV Index reading of 7 to 9 means high risk of harm from unprotected sun exposure. Fair-skinned people might burn in less than 10 minutes. Minimize sun exposure during midday hours, from 10 a.m. to 4 p.m. Protect yourself by liberally applying a sunscreen with an SPF of at least 15. Wear protective clothing and sunglasses to protect the eyes:
Be careful during routine outdoor activities such as gardening or playing sports. Remember that UV exposure is especially strong if you are working or playing between the peak hours of 10 a.m. and 4 p.m. Don't forget that spectators, as well as participants, need to wear sunscreen and eye protection to avoid too much sun. | ||||||||
10+ Very High | A UV Index reading of 10+ means very high risk of harm from unprotected sun exposure. Fair-skinned people might burn in less than 5 minutes. Outdoor workers are especially at risk as are vacationers who can receive very intense sun exposure. Minimize sun exposure during midday hours, from 10 a.m. to 4 p.m. Apply sunscreen with an SPF of at least 15 liberally every 2 hours:
If possible, stay indoors on days when the UV Index is very high. Take the opportunity to relax with a good book rather than risk dangerous levels of sun exposure. Try not to pursue outdoor activities, whether at work or at play, unless protected with sunscreen, hat, and sunglasses. |
By taking a few simple precautions, you can greatly reduce your risk of sun-related illnesses. In general, consider the following steps:
 | Limit your time in the sun between 10 a.m. and 4 p.m. |
| Whenever possible, seek shade. |
| Use a broad spectrum sunscreen with an SPF of at least 15. |
| Wear a wide-brimmed hat and if possible, tightly woven, full-length clothing. |
| Wear UV-protective sunglasses. |
| Avoid sunlamps and tanning salons. |
| Watch for the UV Index daily. |
| More detailed tips |
While you should always take precautions against overexposure to the sun, please take special care to adopt the safeguards when the UV Index predicts levels of moderate or above. Watch for UV Index reports in your local newspapers and on television! For more information, call EPA’s Stratospheric Ozone Information Hotline at 800 296-1996.
Today's UV Index
The UV Index is calculated daily for many US cities. It is provided in two formats on the NWS Web site:
| Text Bulletin listing TODAY'S UV Index for many cities |
| Graphical Map listing TODAY'S UV Index for many cities |
The following archives and other information are also available from NWS:
| Past text bulletins |
| Graphs of yearly values for each city |
Types of UV Radiation
Scientists classify UV radiation into three types or bands—UVA, UVB, and UVC. The stratospheric ozone layer absorbs some, but not all, of these types of UV radiation:
| UVA: Not absorbed by the ozone layer. |
| UVB: Mostly absorbed by the ozone layer, but some does reach the Earth’s surface. |
| UVC: Completely absorbed by the ozone layer and oxygen. |
UVA and UVB that reach the Earth’s surface contribute to the serious health effects listed above.
UV Levels Depend on a Number of Factors
The level of UV radiation that reaches the Earth’s surface can vary, depending on a variety of factors. Clouds, air pollution, haze and elevation all have affects on the amount of ultraviolet (UV) radiation reaching the surface. UV radiation reaches the surface as a sum of its direct component (normal to the sun) and its diffuse component (from all directions). UV radiation reaches the top of the troposphere in mostly its direct component. This is because there are few molecules to scatter the radiation. Decreases in UV radiation intensity has resulted due to absorption by ozone. Once the UV radiation reaches the troposphere it encounters much greater numbers of scattering air molecules and dust.Each of the following factors can increase your risk of UV radiation overexposure and its consequent health effects.
Stratospheric Ozone
The ozone layer absorbs most of the sun’s UV rays, but the amount of absorp-tion varies depending on the time of year and other natural phenomena. That absorption also has decreased, as the ozone layer has thinned due to the release of ozone-depleting substances that have been widely used in industry.Time of Day
The sun is at its highest in the sky around noon. At this time, the sun’s rays have the least distance to travel through the atmosphere and UVB levels are at their highest. In the early morning and late afternoon, the sun’s rays pass through the atmosphere at an angle and their intensity is greatly reduced.Time of Year
The sun’s angle varies with the seasons, causing the intensity of UV rays to change. UV intensity tends to be highest during the summer months.Latitude
The sun's rays are strongest at the equator, where the sun is most directly overhead and UV rays must travel the least distance through the atmosphere. Ozone also is naturally thinner in the tropics compared to the mid- and high-latitudes, so there is less ozone to absorb the UV radiation as it passes through the atmosphere. At higher latitudes the sun is lower in the sky, so UV rays must travel a greater distance through ozone-rich portions of the atmosphere and, in turn, expose those latitudes to less UV radiation.Altitude / Elevation
UV intensity increases with altitude because there is less atmosphere to absorb the damaging rays. Thus, when you go to higher altitudes, your risk of overexposure increases. In the troposphere, air molecules and dust increase as the UV radiation travels from the stratosphere to the troposphere. The further down in to the atmosphere UV radiation travels, the more the direct component is reduced and the more the diffuse component is increased. As more UV radiation is scattered, the smaller the amount that reaches the surface. As a result, there is more UV radiation at higher elevations than at lower elevations.Weather Conditions / Clouds
Cloud cover reduces UV levels, but not completely. Depending on the thickness of the cloud cover, it is possible to burn—and increase your risk of long-term skin and eye damage—on a cloudy summer day, even if it does not feel very warm. And that is the danger: because you do not feel hot, you stay in one position longer and out in the sun more - yet you receive more radiation.Made up of millions of water droplets, clouds can transmit, reflect and scatter UV radiation. The amount of each is dependant upon the thickness of the cloud and its morphology. Generally, the larger and thicker the cloud is the lesser amount of UV radiation that is transmitted. UV radiation can and does reflect off the sides of towering cumulus clouds. Such conditions result in actual enhancements of surface UV radiation.
Air Pollution/Smog
This encompasses many greenhouse gases. Emissions from traffic and manufacturing plants form smog as UV radiation and heat cause the necessary chemical reactions to take place. As a result, the amounts of UV radiation reaching the surface is smaller under these conditions.
Dust/Haze
These two conditions act on UV radiation the same way. They both scatter UV radiation. Enough UV radiation is scattered that on hazy or dusty days there is less UV radiation reaching the surface than would otherwise be there on a clear day.
Reflection
Some surfaces, such as snow, sand, grass, or water can reflect much of the UV radiation that reaches them. Because of this reflection, UV intensity can be deceptively high even in shaded areas.
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