A new study of radon concentrations in homes across the United States has found that the average American's exposure to the radioactive gas is far below recognized danger levels, but that radon still accounts for an estimated 10,000 lung cancer deaths a year, second only to cigarette smoking.
Although the study indicates that 98% of Americans are exposed to relatively low doses of radon, the remaining 2% are exposed to at least twice the acceptable levels, exceeding even the doses received by underground uranium miners.
Radon is a colorless, odorless gas that is produced by the radioactive decay of uranium in the soil. The gas seeps into homes, principally because the warmer air inside the house creates a "chimney effect" and draws up radon gas from the soil.
The findings by four researchers at the University of California's Lawrence Berkeley Laboratory, published today in the journal Science, are believed to be the first to accurately gauge the average level of radon in U.S. households.
"The early estimates didn't have a good appreciation for what the average concentration was, and now we do," lead researcher and nuclear physicist Anthony V. Nero said in a telephone interview from Berkeley.
Nero cautioned that there is still "considerable uncertainity" about the extent of lung cancer risk. Still, the study's estimate of 10,000 radon-caused lung cancer deaths a year is within the range of 5,000 to 20,000 deaths annually previously estimated by the U.S. Environmental Protection Agency and others. By comparison, 108,000 lung cancer deaths each year are attributed to cigarettes and 9,000 to asbestos exposure.
Based on reviewing radon measurements in 1,377 homes in 21 states, Nero said the average concentration of radon was found to be 1.5 picocuries per liter--far below the four picocuries per liter considered acceptable by the EPA.
A picocurie is a trillionth of a curie, which is a common measure of radiation.
One million homes, or 2% of the national total, are estimated to have radon concentrations exceeding eight picocuries per liter of air. The average uranium miner's exposure is five picocuries, according to the researchers.