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UV Radiation

SERC's meteorological tower in the middle of a forest.

Ultraviolet radiation (UV) is, at least to humans, an invisible part of sunlight. But as you well know from a sunny day at the beach, it's biologically very active. UV radiation can have severe effects on exposed skin and eyes, cause cancer, weaken immune systems, and affect plants, animals, and ecosystems.

Picture: Meteorological tower at SERC. Every four seconds, the radiometer at the meteorological tower at the Smithsonian Environmental Research Center (SERC) measures the incoming UV radiation.

One of the main biological impacts of UV radiation on plants is that it reduces their rate of photosynthesis, the process that plants use to trap carbon dioxide from the air to create sugars for food, releasing oxygen in the process. UV radiation with a short wavelength, called UV-B, is most effective at reducing the photosynthesis rate.

This reduced photosynthesis rate can directly affect the plants' ability to grow. Because different species may vary in their sensitivity to UV radiation, this may eventually affect biodiversity and change the structure of an ecosystem.

Although UV radiation has always been present throughout the evolution of earth and life, research is showing that we are now exposed to higher levels of UV because of the thinning of the ozone layer. The ozone layer, a concentration of ozone molecules in the stratosphere, protects us from UV by filtering the sun's radiation. Ozone molecules in this layer are constantly being produced and broken down, but chemicals like halons and chlorofluorocarbons (CFCs) that are used in refrigerators and air conditioners accelerate the breakdown of ozone. As a result, the ozone layer is getting thinner, allowing more UV to reach the earth's surface.
(For more information about the thinning of the ozone layer, read EPA's Questions and Answers on Ozone Depletionoutside link.)

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UV index indicates risk

To get an indication of your risks from exposure to UV radiation, you can look at UV index predictions that are published in many papers. If you live near Edgewater, MD you can also use the live UV Indexoutside link from the Photobiology and Solar Radiation Laboutside link at the Smithsonian Environmental Research Center(SERC).

The UV index provides a daily forecast or description 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. When the index is high, you can quickly get a sunburn.

UV Index Scale, showing on a scale of 1-10 the risk of overexposure to the sun.   From 0 to 2, the risks are minimal and  it will take between 35 and 105 minutes to get a sunburn. From 2 to 4, risk is low,  18 to 65 minutes to sunburn. From 4-7 risk is moderate, from 11 to 40 minutes to sunburn. From 7-11, risk is high, from 9 to 22 minutes to sunburn. And index higher than 11 indicates very high risks, from 7 to 20 minutes exposure can cause a sunburn.Picture: The UV index indicates, on a scale of 1 to 10+, the expected risk of overexposure to the sun.

To create a live UV index, the radiometer at the meteorological tower at SERC in Edgewater, MD measures the incoming UV radiation every four seconds.

The graphic below shows the current value of the UV index and, in the afternoon, a daily value of the UV index. This daily value is calculated based on a one hour period around solar noon, which is around 1. p.m. The current value changes all the time and represents the current readings of the radiometer. You can use both values to assess you risk of exposure to UV.

(Click on the graphic to view a large version of the UV index graphoutside link.)

Grpah showing the UV index for today.

To read more about the effects of exposure to UV radiation and general safety suggestions, visit EPA's Ozone Depletion Home Pageoutside link.

References and further reading

For more information about the ecological effects of ultraviolet radiation, visit the website of SERC's Photobiology and Solar Radiation Laboutside link where you can also find SERC's live UV indexoutside link.

To learn more about ozone depletion, the harmful effects of UV radiation, and what you can do to protect yourself, visit EPA's Ozone Depletion Home Pageoutside link.