Many researchers say low levels of ionizing radiation, such as nuclear power plant emissions, are safe. Others contend the data needed to prove otherwise is unavailable to independent researchers.

"Low-level ionizing radiation is possibly dangerous, though it is difficult to prove because the data necessary to determine that has not been widely available to independent researchers," said Daryl Kimball, a research analyst for Physicians for Social Responsibility. "Those allowed access (to the data) have been researchers contracted by the Department of Energy."

Ionizing radiation is any radiation that causes ionization or excitation of the material it passes through.

Low levels of ionizing radiation are emitted from the naturally radioactive Earth, cosmic rays, X-rays, television and nuclear plants.

Though the physical effects of low doses of accidental radiation are not known for sure, some situations (such as the March 1979 Three Mile Island nuclear reactor accident) have given scientists opportunities to study radiation's effects.

George Tokuhata, director of the Division of Epidemiological Research of the Pennsylvania Department of Health, has been studying the health effects of the TMI accident since it occurred.

The State Health Department has so far found no evidence of increased cancer among area residents due to the accident and has also found fewer cases of leukemia than expected.

Leukemia is the only form of cancer associated with ionizing radiation that could have manifested itself within the time period studied because of its comparatively short latency period of two to four years.

Other cancers may have latency periods of 10 to 20 years, requiring long-term study to detect.

"Potential physical health effects (particularly cancer) of low-level ionizing radiation in humans have not yet been fully established," Tokuhata said.

And, Kimball said according to the most recent National Academy of Sciences Committee on the Biological Effects of Ionizing Radiation report, low-level radiation may be more dangerous than expected.

"Exposure to low-level radiation is three to four times more likely to cause cancer than previously believed. This study was based on victims in the outlying areas of Hiroshima and Nagasaki who were exposed to low levels of radiation," Kimball said.

The Three Mile Island accident was classified as a Class 1 accident -- the least severe of the three classes of nuclear accidents.

"The maximum possible dose to a hypothetical person standing unprotected anywhere along the border of the (TMI) plant site for the duration of the accident was estimated as no more than 100 millirems, the approximate equivalent of one year natural background radiation in the area," Tokuhata said in a speech given at the International Symposium on Nuclear Power.

The average likely dose to people living within five miles of the plant was estimated to be nine millirems.

According to the American Nuclear Society, the average American absorbs 180 millirems per year of radiation from natural background sources.

The amount of radiation produced by nuclear reactors is less than one millirem per year, said Edward Kenney, professor of nuclear engineering.

"Routine releases of radiation from normally operating power plants are so low compared to natural radiation in the environment that the health effects are inconsequential," he said.

A rem is a measure of the amount of radiation energy transferred to the body. One rem is equal to one rad (0.01 joule/kg) multiplied by a correction factor to equalize biological effects.

The national radiation-absorption average may vary, however, depending on where people live and their lifestyles. For instance, someone living next to a nuclear power plant would be exposed to higher levels.

The dose due to terrestrial and cosmic rays is dependent upon altitude and latitude. Airline pilots increase their exposure to cosmic rays by leaving the protective atmosphere of the Earth.

Those living in the southwestern United States receive the highest dose with an average 90 millirems per year. The Northeast receives an average of 46 millirems per year.