Levels of cancer-causing ultraviolet light reaching the United States actually declined between 1974 and 1985 despite depletion of the ozone layer that blocks the harmful radiation, scientists report.
The surprising data, gathered at eight field stations nationwide, was reported in the journal Science last week by researchers from the National Cancer Institute, the National Oceanic and Atmospheric Administration and Temple University.
Scientists had expected increases in ultraviolet light because of damage to the stratospheric ozone layer, long thought to be Earth's only screen against the damaging rays from the sun. In May 1985, researchers detected a seasonal hole in the ozone over Antarctica, which has reappeared each spring since.
But average annual ultraviolet light levels, in the most life-damaging wavelengths, actually showed declines at each station, with drops ranging from 2 percent to 7 percent over six years.
Ozone damage is caused by chlorofluorocarbons, gases used as propellants, refrigerants, cleaning agents and foam packing.
The researchers said other factors -- such as cloud cover, air pollution or particles in the atmosphere -- may have stopped the ultraviolet light from reaching Earth's surface, at least in the United States.
In the long run, however, the scientists predicted depletion of ozone will probably still permit more ultraviolet radiation to reach the surface. This will result, they also predicted, in more skin cancer, including the often fatal melanoma.