Sunlight, Ultraviolet Radiation, and the Skin

http://consensus.nih.gov/1989/1989SunUVSkin074html.htm
National Institutes of Health
Consensus Development Conference Statement May 8-10, 1989

Clipped out the following

Sunlight is the greatest source of human UVR exposure, affecting virtually everyone. The extent of an individual's exposure, however, varies widely depending on a multiplicity of factors such as clothing, occupation, lifestyle, age, and geographic factors such as altitude and latitude. There is greater UVR exposure with decreasing latitude. Residing at higher altitude results in a greater UVR exposure such that for every 1,000 feet above sea level, there is a compounded 4 percent increase in UVR exposure. UVR exposure increases with decreased stratospheric ozone. Other factors that influence exposure to UVR include heat, wind, humidity, pollutants, cloud cover, snow, season, and time of day.

Solar flares (sunspots) also alter the amount of UVR reaching the Earth. Solar flares increase ozone concentration in the stratosphere (above 50 km) thereby reducing the amount of surface UVB. This 11-year cycle of solar flares causes as much as a 400-percent variation in UVB at 300 nm reaching the earth. When solar flares are inactive, there is a decrease in the ozone concentration, allowing increased UVB to penetrate to the Earth's surface.