Extensive and consistent epidemiological research on lung cancer mortality in underground miners [BEIR studies] exposed to radon progeny qualify the classification of radon as a known human carcinogen and are the basis on which the estimation of health risks from indoor exposure to radon. Approximately 20 epidemiological studies of occupational exposure to radon and lung cancer risk have been conducted, and all have indicated an increase in lung cancer incidence with exposure to radon. Studies of miners who work in metal, fluorspar, shale, and uranium mines have been conducted in the United States, Canada, Australia, China, and Europe. In addition to consistently showing an increase in lung cancer risk with exposure to radon, a number of studies have had other important findings (Figure 1-1).
The miner studies are based upon working level measurements made in these occupational settings. Exposure times were also recorded to determine a time and dose record for each of the miners. The term that was used to express the time-dose relationship is working level months (WLM). It is an expression of the amount of RDPs to which a person is exposed, multiplied by the duration of exposure expressed in fractions of a month. This term comes from occupational exposure records that pertain to the underground mining industry but is used universally to calculate radon exposure for all radon measurement and mitigation practitioners.
Working Level Month can be calculated mathematically as illustrated below:
The assumption is that the degree of risk is a linear relationship between the concentration of exposure and the duration of exposure. For example, a person exposed at 4 pCi/L for 1 year is at the same risk as someone who is exposed at 2 pCi/L for 2 years or 16 pCi/L for 3 months.
Increased lung cancer risk has been observed at low cumulative exposures (WLM) comparable to cumulative lifetime residential exposures. For example, in a study on uranium miners in Ontario, Canada the death rate from lung cancer was significantly increased by cumulative exposures as low as 40-70 WLM (exposure to a radon concentration of 4 pCi/L in a home over a lifetime of 70 years). (Assuming 75% occupancy and an equilibrium fraction of 50% is equivalent to a cumulative RDP exposure of 54 WLM).
In some studies, as summarized in the WHO Handbook on Indoor Radon, A Public Health Perspective, 2009, low exposures over longer periods of time were associated with greater risk than high exposures of shorter duration. This may be particularly important to the estimation of risk from residential radon exposure, as residential exposures tend to be lower exposures received over longer periods of time, while occupational exposures tend to be higher levels received over shorter time periods. If the lower exposure rates do, in fact, pose more risk, then current radon risk assessments may be underestimating the risk from residential exposures. Despite limited data, a National Academy of Science Committee concluded in its report entitled “Biological Effects of Ionizing Radiation” (BEIR VI) that lung cancer risk increases exponentially with joint exposure to radon and smoking. In other words, the risks from combined exposure to radon and smoking is greater than the sum of the risks from exposure to either active alone. As a result of these studies, the United States EPA developed a risk chart for residential setting for smokers and non-smokers (Figure 1-4 and 1-5).