Primary Uses or Exposures: Of the band within the optical radiation spectrum, broad-spectrum ultraviolet radiation (UVR) is the strongest and most damaging to living things. UVR is divided into wavelength ranges identified as UVA (315 to 400 nm), UVB (280 to 315 nm), and UVC (100 to 280 nm). Of the solar UV energy reaching the equator, 95% is UVA and 5% is UVB. No measurable UVC from solar radiation reaches the earth's surface, because the shortest UV wavelengths are completely absorbed by ozone, molecular oxygen, and water vapor in the upper atmosphere. Sunlamps and sunbeds emit broad-spectrum UVR. Sunbeds now chiefly emit UVA; however, before the mid 1970s, they more commonly emitted UVB and UVC. The greatest source of human exposure to broad-spectrum UVR is solar radiation; however, the exposure varies with geographical location. Approximately 25 million people in the United States use sunbeds each year, and one to two million people visit tanning facilities as often as 100 times a year.