New statistical techniques alter radon risk estimates

Wednesday, May 24, 1989

Rick Borchelt, NRC News Report, 38(2), 8; 1988

Radon, an odorless, colorless radioactive gas produced as a breakdown product of radium and other radioactive elements, has reached levels high enough to warrant corrective action in 8 million of the nation’s 70 million homes, according to Environmental Protection Agency (EPA) estimates. Radon and its decay products are known to produce lung cancer and constitute a hazard, yet federal standards for radon in houses have not been set. EPS guidelines are broad and do not take account of different risks due to age, gender or smoking habits. Until now, no clear picture had emerged about the effect on the risk estimates due to these differences.

A new Research Council report concludes that the risk is more serious than one previous study had estimated, but less serious than an earlier Research Council committee had calculated. The report also provides new information about the relative risk as a function of age, time since exposure occurred, and smoking status.

The Nuclear Regulatory Commission (NRC) and EPA requested the just-published Research Council report in 1984. In addition to estimating risk of lung cancer from radon, the cuttent study committee also examined health effects of polonium, radium, thorium, uranium, and transuranic elements such as plutonium.

The report addresses health effects only; it does not estimate radon levels in home or occupational environments. In addition, the committee pointed out that while its estimates may be helpful in determining health standards for radon, its conclusions do not constitute regulatory guidance on exposure levels to radon. Such estimates were beyond its charge and expertise, the committee said.

The radionuclides studied by the committee are all alpha-emitters – meaning that their radioactive decay produces alpha particles. Since alpha particles are highly ionizing but very short-ranged, they can only damage bone or other organs if they are inhaled or ingested into the body. Some alpha-emitters studied, such as uranium and thorium, persist for millions of years. Others, such as radon and its decay products, live only a few days or hours. The alpha particles emitted by these radionuclides produce damage in only three of four layers of cells in the immediate vicinity of the radioactive material, the committee explained.

A large portion of the report discusses radon, a radioactive element that is ubiquitous in the Earth’s crust and can accumulate in the air of buildings and mines.

The committee’s findings are based on the application of recently developed statistical techniques it adopted to analyze four major epidemiological studies of radon and lung cancer. These studies, of uranium miners in Ontario, Saskatchewan, and the Colorado Plateau, and of metal miners in Sweden, were statistically combined to yield more comprehensive information about lung cancer risk.

Most previous models for estimating an individual’s lung cancer risk due to radon and its decay products have taken account only of the person’s total exposure. The Research Council committee’s model incorporates both the effects of age at risk and time since the exposure occurred. Both of these factors "modulate" the risk on a relative risk scale, that is, they affect the lung cancer rate compared with the baseline age-specific lung cancer rate.

According to the committee’s new estimates, lifetime exposure to one working-level-month (WLM) of radon decay products per year may increase an individual’s chances of dying from lung cancer by 1.5 times, compared with someone who is exposed only to background levels of radon. (A WLM denotes exposure to a specific amount of alpha particle energy per liter of air for a 170-hour work month.) The committee also estimated for that every 1 million people exposed to one WLM over their lifetime (a much smaller exposure rate than the example above), about 350 would die from lung cancer.

Most measurements of radon in homes, however, are made in terms of picocuries of radon per liter of air. In most homes, four picocuries per liter is equivalent to 0.02 working levels. Using this conversion figure, someone who lives in a home with a radon level of four picocuries per liter and stays at home an average of 12 hours per day, would receive an annual exposure of about 0.5 WLMs.

The committee stressed, however, that "most of the increased risk is in smokers, in whom the risk is 10 or more times greater than in non-smokers." Only 5 to 10 percent of all lung cancers occur in lifelong non-smokers, the report notes.

Many US radiation experts currently estimate radon lung cancer risks using a 1984 study by the National Council on Radiation Protection. That study indicated that one WLM of radon exposure over a lifetime would produce about 130 lung cancers per million people exposed. A 1980 Research Council Report, The Biological Effects of Ionizing Radiation III (Beir III), concluded that lifetime exposure to one WLM of radon would produce about 730 lung cancer deaths per million people.

Health effects other than lung cancer due to exposure to radon and its decay products are largely unstudied, the committee said. "Data are sparse and associations are weak," it concluded.

The committee also studied the health effects of other sources of inhaled and ingested alpha emitters.

Radium, thorium and uranium are radioactive elements that have been relatively well studied, and there is little indication from the committee’s most recent analysis that scientific thought on their health effects should be revised. Radium was widely used in watches and electronic gadgets during the 1940s and 1950s as the compound that made luminous watch hands "glow" in the dark. Thorium was routinely used as a contrast medium in the 1940s by patients getting x-rays. Uranium, while ubiquitous in the Earth’s crust, is seldom encountered in significant quantities; human exposure is usually restricted to uranium miners and millers.

Other radioactive elements – the so-called transuranic elements – have been studied principally in animal models, the committee said. For these elements – including plutonium, curium, neptunium, and americium – sufficient human data are not available to provide accurate risk assessments for exposure, the committee found.

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