15.
Radon
Authors
The information in this chapter was prepared by Mark Coombs, Ph.D., Radiation Protection Facility, Arizona State University
Introduction
Radon is a radioactive disintegration product of radium. Commonly detected in rock and soil, radon is in ambient air in infinitesimal amounts and readily gains entrance into homes. The EPA has established a maximum of four picocuries per liter of air (4piCi/L) as a safe level of exposure in homes throughout the U.S.
The radon risk assessment presented here is an estimate of the number of lung cancer deaths which occur each year in Arizona from exposure to residential radon.
Radon Concentrations in Arizona Homes
The formula used by the Committee in this study calculates that about 243 Arizona residents will die each year from lung cancer caused by residential radon.
The radon concentration used in this assessment was taken from measurements conducted by the Arizona Radiation Regulatory Agency in cooperation with the EPA. The measurements were conducted throughout the state during the winter and spring of 1987-1988 in a population-based study.
Two measurement techniques were used:
o Activated charcoal canisters were placed in closed homes for a period of 48 hours to ensure the discovery of any existing high radon concentrations. Canisters were placed in the lowest livable level, where reading within a home would typically be the highest. As a result, the estimates of indoor radon concentrations using charcoal canisters do not accurately reflect a true annual average.
o More accurate is the second measurement, whereby Alpha Track Detectors were placed in ten percent of the homes. The Alpha Track Detectors were exposed for a twelve-month period.
The average of the charcoal canister measurements was 1.0 pCi/l. The average for alpha-track detectors was 0.8 pCi/L (Spencer, 1993). These results are somewhat lower than the national average estimated to be near 1.3 pCi/l (EPA, 1993).
Conversion of pCi/L to Working Level Month
Estimates of the risk associated with residential radon exposure have been based on extrapolations from lung cancer data for underground miners exposed to elevated levels of radon and radon daughters.
The units traditionally used for radon exposure in mines have been the working level, and the working level month.
o The working level (WL) is defined as any combination of the short-lived radon daughters in one liter of air that results in the ultimate release of 1.3x105 MeV of potential alpha energy.
Exposure of a miner to this concentration for 170 hours (a working month) results in an exposure of one working level month (WLM).
In this assessment it has been assumed that the equilibrium between radon and radon daughters is approximately 50%. It has also been assumed that people spend 75% of their time in their homes.
This results in an exposure of approximately 0.19 WLM each year for an average radon concentration of 1 pCi/L (EPA, 1987; NRC, 1988).
Lung Cancer Deaths per WLM
Estimates of the magnitude of the risk from residential radon exposure have been based on extrapolations from lung cancer data for underground miners exposed to elevated level of radon and radon daughters.
Epidemiological studies of lung cancer from residential radon exposure have been inconclusive or contradictory (ICRP, 1987).
A number of studies have been conducted on groups of underground miners in which there has been elevated incidents of lung cancer deaths. These studies have shown a strong correlation of radon and radon daughter exposure with increased lung cancer risk. Lung cancer deaths in these groups could not be explained by exposure to non- radioactive pollutants in the mines (ICRP, 1987).
Several important reviews of the miner studies have been conducted. The results are expressed as the number of lung cancer deaths that would be expected each year in Arizona from radon exposure. For three of the studies, for Arizona, the results are as follows:
o The ICRP 1993 study estimates 200 Arizona lung cancer deaths annually from radon.
o The NRC 1988 study estimates 247 Arizona lung cancer deaths annually from radon.
o The EPA (Puskin & Yang 1988) study estimates 325 Arizona lung cancer deaths annually from radon.
Uncertainties in the Risk Assessment
Uncertainties in the risk assessment process are numerous and have been effectively discussed by several observers (ICRP, 1987; NRC, 1988; Cole, 1993). The following areas of uncertainty were identified:
o Estimates of exposure to underground miners
o Extrapolation of dosimetry from mine to residential environments including:
Aerosol particle size
Unattached radon daughter fraction
Radon daughter equilibrium factor
Minute volume
Aerosol deposition
o Effects of smoking
o Effects of gender
o Effects of age at the time of exposure
o The temporal expression of risk
o Differences in mining and residential environments, especially with respect to silicon, metals, organic vapors, and other toxins inhaled by miners in an underground mine environment
o Extrapolation from relatively high doses in mines to lowers doses typical of residential environments.
The last two areas of uncertainty are particularly profound and have generated much controversy about the risk of lung cancer from exposure to residential radon. Some observers have implied that there may not be a health risk associated with radon in homes (Cole, 1993), while others have emphasized the need to validate and refine risk assessments for residential radon exposure (ICRP, 1987).