Radon is a radioactive gas that is ubiquitous in earth's atmosphere. Itis formed from the radioactive decay of radium-236. Radium is found in substantial but varying amounts in soil and rocks and ends up in some building materials. Various parts of the country have varying amounts, as do certain localities within a small geographic area. There is extensive epidemiologic evidence that exposure to high levels of radon produces bronchogenic carcinoma (reviewed in Reference 115), most of which comes from studies of workers involved in deep mining of uranium and other ores. Because this epidemiologic evidence is quite compelling and because radon is so widespread, the potential that large numbers of people might develop lung cancer from such exposure has produced a radon scare in the United States, not unlike the asbestos scare.

An increased incidence of lung cancer in deep-well miners was observed in uranium and other ore miners in eastern Germany and western Czechoslovakia over 60 years ago. Exposure to radon among these miners was very high, approaching 3000 picocuries (pCi) per liter of air. In the early 1950s, an increase in lung cancer incidence was noted among uranium miners in Colorado. Later, an increased rate of lung cancer was also noted among miners working in iron, zinc, tin, and fluorspar mines. In these mines, radon levels were also high. Although these miners were also exposed to other potentially carcinogenic dusts, the common feature was exposure to radon. The excess number of lung cancer deaths in these miners (compared to nonminers) ranges from 0.3% to 13% and varies depending on the ambient air concentration.115 This risk goes up in more than an additive manner for individuals who are also smokers.

Monitoring of homes began in a rather haphazard fashion in the 1980s. Nevertheless, some regions with high indoor levels were found, including the Reading Prong geological region extending from Pennsylvania to New York. Estimates of risk for lung cancer from radon exposure in residences is based on extrapolations from miner risk data. These estimates may or may not be realistic, as there is some evidence both ways. In a case-control study of 400 women with lung cancer, performed by the Department of Health in New Jersey, an increased risk was found for exposure levels of 2 pCi/L of air, but the results were not statistically si gnificant.116 A study done in China, in which median household radon levels ranged from 2.3 to 4 pCi/L of room air, no positive associations between radon levels and lung cancer was found,117 a finding suggesting that projections of lung cancer risk from surveys of miners exposed to high-radon levels are overestimates.

Estimates of increased risk due to radon residential exposure very widely. For an average lifetime exposure to 1 pCi/L, estimates vary from 5000 to 20,000 excess lung cancer deaths per year in the United States.115 These estimates uphold the conservative tradition ofradiation protection. To put this in some perspective, a lifetime exposure to 4 pCi/L is estimated to cause a 1% increase in lung cancer, whereas the risk of smoking cigarettes increases the risk of lung cancer at least 10-fold over that of nonsmokers. Most homes in the United States have indoor radon levels less than 2 pCi/L and these are levels usually found in basements.118

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