Ultraviolet –B

Ultraviolet-b is part of the radiation reaching the earth's surface from the sun. Solar UV radiation is divided into three regions; UV-a (400-315 nanometers (nm)), UV-b (315-280nm) and UV-c (280nm to x-ray region). Because of the absorption of the upper atmosphere no light with a wavelength shorter than 280nm reaches the earth's surface. The majority of the UV-b is also absorbed by the ozone layer and the atmosphere. The amount of UV-b that actually reaches the earth's surface is further affected by ground level ozone, particulates and other pollutants, clouds and the elevation of the surface. In addition, the effects of the UV-b reaching the earth's surface are magnified by a reflective ground cover such as snow or water.

Light in the UV-b wavelength region has been shown to be responsible for many biologically harmful effects in both plants and animals. In plants overexposure to UV-b is known to cause DNS damage and decrease agriculture yields. In animals overexposure to UV-b is known to cause skin cancer, eye damage, suppression of the immune response system, sunburns and premature wrinkling of the skin.

There are currently no standards for UV-b. Data from UV-b monitoring is reported as MED (Minimum Erythemal Dose) units. MED is a measurement based on the dose of effective radiation received and its ability to cause damage, such as sunburn, to the human skin. MED units are based on the time it took to cause erythemia (reddening) of the skin of the average Caucasian using differing wavelengths of UV-b. Data is reported as the number of MED's received that hour. For example, if for a particular hour 4 MED's were reported than that means a person would have received four times the amount of sunlight it takes to cause sunburn in the average Caucasian. Various agencies around the world have developed indices for rating the amount of exposure to UV-b that individuals receive. For example less than 2 MED's/hour might be considered low whereas 6 or more MED's/hour might be considered very high exposure.

During 1993 a monitoring site in Howland was equipped with a UV-b monitor and a total solar monitor. This site was chosen because of its rural location, stable land use, freedom from obstructions and the availability of other data from instruments already installed at the site. Some problems were experienced during this first year of operation, most notably frost forming on the dome of the total solar monitor. The lack of a Quality Assurance plan has also caused minor problems. Quality assurance checks currently performed are based on our present knowledge of the methodology. On June 1, 1995 the monitoring equipment was removed from Howland and set up the next day at a site in Cape Elizabeth . The Howland site was difficult to maintain due to telecommunications problems and winter access to the site. The Cape Elizabeth site is located in the major population center of the State and the issuance of public advisories during high UV episodes will be more accurate for the majority of the people in the State. Current data from this monitor can be viewed on the air quality web site .