Ultraviolet (UV) radiation represents 3% to 5% of the total solar radiation that penetrates the earths surface (1). Of the types of radiation emitted from the sun, UV radiation is believed to be almost entirely responsible for the deleterious effects sunlight can have on biological systems (2). In humans, these effects include acute effects, such as, sunburns, as well as more serious chronic effects including skin cancer, suppression of the immune system, cataracts, and premature aging of the skin. The amount of UV radiation that actually reaches the earths surface depends on a number of factors including the following (2,3):
Time of Day
Time of Year
Reflection by Ground Surfaces
UV radiation is classified into three bands based on varying wavelengths of light. Ultraviolet A (UVA) spans the electromagnetic spectrum of wavelengths between 320-400nm, ultraviolet B (UVB) spans the spectrum between 290-320nm, and ultraviolet C (UVC) spans the spectrum between 200-290nm. Ozone in the stratosphere completely absorbs UVC and partially absorbs UVB; it does not absorb any UVA.
Ozone is a naturally occurring gas that comprises one molecule out of every two million in the atmosphere and is found in two different layers of the atmosphere (2,4). In the region of the atmosphere near the earths surface called the troposphere, ground-level ozone is an air pollutant that contributes significantly to urban smog. However, 90% of ozone is located in the upper atmosphere, or stratosphere. Here ozone is most concentrated 6 to 30 miles above the earths surface and is often referred to at the ozone layer (4). It is this layer that is responsible for filtering harmful UV rays and preventing much UV radiation from reaching the earths surface. The mechanism by which filtration occurs is by the natural creative and destructive processes of ozone that are occurring continuously in the atmosphere. Ozone is a molecule composed of three oxygen atoms (O3). It is formed by the dissociation of oxygen (O2) by short wavelength UV radiation (mainly UVC radiation). Absorption of UV radiation at wavelengths up to 320nm converts O3 back to O2 and O. By the natural balance of these creative and destructive processes, UVC is completed filtered and UVB is partially filtered (2).
In the past two to three decades, we have learned chlorofluorocarbons (CFCs) and other gases released by humans mainly in industrial processes alter the natural balance of ozone in the stratosphere and are leading to the depletion of the ozone layer. In 1985, it was reported that there is a hole in the ozone layer over Antarctica and there is additional evidence that ozone is being lost at nearly all latitudes outside of the tropics (2,5). These discoveries prompted the adoption of an international treaty called the Montreal Protocol, which was designed to phase out the production of and use of ozone-depleting substances. According this treaty, the production and consumption of compounds that deplete ozone in the stratosphere including CFCs, halons, and carbon tetrachloride were to be phased on by 2000 and methyl chloroform by 2005 (6). While this is an enormous step in the right direction, the effects of ozone depletion, namely increased UVB and possibly UVC exposure, will continue for many more years. Scientists expect ozone depletion to peak by the end of this decade and then it is believed that natural atmospheric processes will repair the ozone layer by the middle of the 21st century (4). We can expect increased UV radiation levels and a subsequent increase in health effects from this exposure until that time.
Time of Day
The sun is highest in the sky around noon, and at this time, the suns rays have the least distance to travel through the atmosphere and UVB levels are at their highest (3). In general, 20-30% of the total daily UV radiation is received one hour on either side of noon during summer months, and approximately 75% of the total daily UV radiation is received between 9:00am and 3:00pm (2). Less UV radiation is received very early in the morning or in late afternoon because the suns rays pass at this time through the atmosphere at an angle, which greatly reduces their intensity (3).
Time of Year
As the suns angle and hence its intensity varies with time, it also varies with season. UV intensity tends to be highest during the summer months (3). While this is certainly true of temperate regions where there is strong seasonal dependence, there is much less seasonal variation the closer to the equator one gets (2).
The suns rays are strongest at the equator because the sun is more directly overhead and therefore UV rays travel less distance to reach the earths surface here. Also, there is naturally less ozone in the stratosphere over the tropics so there is less absorption of UV radiation. At higher latitudes, UV rays must travel longer distances to get to the earths surface and through more ozone resulting in higher latitudes receiving less UV radiation (3).
At higher altitudes, there is less atmosphere to absorb UV radiation so UV intensity in these regions is higher. In general, each 1 km (0.62 mile) increase in altitude increases the ultraviolet flux by 6% (2).
Cloud cover can reduce solar irradiance of the earths surface, however, solar infrared radiation is attenuated much more than UV radiation by water in clouds. In general, light clouds scattered over a blue sky have little effect on reducing UV levels, while light clouds completely covering the sky can reduce UV levels by half. Heavy storm clouds during the summer can virtually eliminate UV levels if directly covering the sun (2). However, it is important to note that risk of overexposure to the sun can be greater on cloudy days because you may not feel your skin getting warm like you would on a clear, sunny day.
Reflection by Ground Surfaces
Some ground surfaces including water, sand, and snow reflect UV rays thereby increasing UV intensity in the surrounding areas. Reflection by water has been estimated at less than 7%. Reflection by a particular type of sand, gypsum sand, has been estimated at 25% of incident UVB. The reflection of UVB rays by snow has been estimated by various sources to be as little as 30% or as much as 80% or more (2).
1. Kane AB, Kumar V. (1999). Environmental and Nutritional Pathology. In Cotran RS, Kumar V, Collins T. (Eds.), Robbins Pathologic Basis of Disease- Sixth Edition (pp. 403-458). Philadelphia, PA: W.B. Saunders Company.
2. Diffey BL. Solar ultraviolet radiation effects on biological system. Phys Med Biol. 1991; 36(3): 299-328.
3. SunWise School Program. (1999). UV Radiation. U.S. Environmental Protection Agency.
4. SunWise School Program. (1999). Ozone Layer. U.S. Environmental Protection Agency.
5. CIESIN Thematic Guides. Ozone Depletion and Global Environmental Change. Center for International Earth Science Information Network, Columbia University.
6. CIESIN Thematic Guides. The Montreal Protocol on Substances That Deplete the Ozone Layer. Center for International Earth Science Information Network, Columbia University.