The overall method used to estimate the fraction of lung cancers attributed to occupational radon exposure was similar to the method used to estimate occupational cancer burden from diesel engine exhaust exposure and other exposures in the same study (Kim et al. 2018; Labreche et al. 2019). In brief, the number of incident lung cancer cases attributed to occupational radon exposure was calculated as the product of the Population Attributable Fraction (PAF) and the number of incident lung cancers in Canadians aged 25 years or older in 2011 (Statistics Canada). Levin’s Equation [PAF = PrE(RR−1)/(1 + PrE(RR−1))] was used to calculate the PAF, where PrE equals the proportion of workers ever exposed and RR is the relative risk of lung cancer associated with radon exposure derived from the Biological Effects of Ionizing Radiation (BEIR) VI exposure-age-concentration risk model (National Research Council (US) Committee on Health Risks of Exposure to Radon 1999). Results from CAREX Canada’s radon exposure assessment, along with historical data from the Canadian Census and Labour Force Survey were used to estimate the total number of exposed Canadian workers during the risk exposure period (REP), defined as 1961–2001 assuming a latency of 10–50 years prior to lung cancer diagnosis. For the PAFs, 95% confidence intervals were calculated using Monte Carlo simulation of the PrE and the RR, described in detail elsewhere (Kim et al. 2018; Labreche et al. 2019). Our work received an ethics exemption from the University of Toronto Research Ethics Board and was performed in accordance with the ethical standards as laid down in the 1964 Declaration of Helsinki and its later amendments.
Radon exposure assessment was performed as part of the CAREX Canada project (Peters et al. 2015). Briefly, exposure was assessed on the population reported in the 2006 Canadian census, where information for the Canadian workforce was available by sex, province, industry, and occupation. Exposure to radon was estimated for Canadian workers in two distinct exposure scenarios: high-risk occupations where workers have the potential for high exposure (> 800 Bq/m3) in underground/confined workspaces, and low-risk occupations where workers are exposed to radon in indoor air at lower levels (> 0 to > 800 Bq/m3). For the high-risk occupations, two authors (CG, CP) identified occupations and industries with potential for high radon concentrations using information from the Canadian National Dose Registry (that contains doses experienced by radiation-exposed workers) (Health Canada 2019), published literature (Navaranjan et al. 2016), and government reports/grey literature (Beaugrand and Sutton 2012; Brown et al. 2012a; Canadian Broadcast Corporation 2014; Kalinowski 2014; U.S. Department of Health and Human Services 1987). They then estimated the proportion exposed in each detailed occupation and industry intersection.
For the estimation of the number of indoor workers exposed to radon, workers in indoor working environments where radon has the potential to accumulate were identified by occupation and industry by two authors (CP, PD). Distributions of radon concentrations were calculated, by province, with radon measurements from the Canadian Federal Building Survey, where 12,865 indoor federal workplaces had been monitored at the time of estimation (Health Canada 2014; Whyte et al. 2019). The data from this survey of federal buildings were used to model lognormal exposure distributions for workplace radon. From each province-specific radon exposure distribution, the proportion of workers exposed to specific ranges of radon were calculated (> 50–100, > 100–150, > 150–200, > 200–400, > 400–800, and > 800 Bq/m3). For subsequent calculation of lung cancer relative risks, the midpoints of these exposure ranges were assigned as the average radon concentrations for the exposure groups. For the unbounded category of > 800 Bq/m3, the province-specific mean of the radon measurements exceeding 800 Bq/m3 in the federal building survey was assigned. For high-risk workers, we approximated the average of the unbounded exposure category (> 800 Bq/m3) to be 9/4 times the value of the lower bound; therefore, the average radon exposure level estimated for these high-risk workers is 800 Bq/m3 × 9/4 = 1800 Bq/m3.
Calculation of ever-exposed population
Methods for population modeling are described in detail elsewhere (Kim et al. 2018; Labreche et al. 2019). Briefly, historical workforce data were obtained from Canadian census data from 1961, 1971, 1981, 1991, and 2001. Workforce data for between-census years were linearly interpolated. To avoid double-counting workers, after 1961, only new hires were added to the working population model. New hire information was available from the Canadian Labour Force Survey by sex, industry, and age group (Statistics Canada 1976–2003). Annual workforce data in the REP (1961–2001) were merged, by industry and occupation, to aforementioned exposure assessment results to obtain the total number of workers ever exposed, by sex, province, industry, and occupation. The population was further adjusted for the probability of survival to the target year (2011), which was estimated using Canadian life tables (Statistics Canada Demography Division 1960–2002). The reference population was the population aged 25 years and older reported in the 2011 Canadian census, which included persons alive in 2011 who were ever of working age during the REP.
Relative risk of lung cancer
The BEIR VI exposure-age concentration model (National Research Council (US) Committee on Health Risks of Exposure to Radon 1999) was used to derive relative risks of lung cancer for radon exposure. When the rate of exposure is less than 0.5 working levels (0.5 working levels = 85 working level months (WLM) = 5 millisieverts) as is the case in occupational exposure scenarios, the BEIR VI model may be represented as RR = 1 + 0.0768w*Ф, where w is the cumulative exposure (in WLM) weighted based on time-since exposure factors (5–14 years = 1; 15–24 years = 0.78; 25 + years = 0.51) and Ф is a factor accounting for age at the time of risk estimation (< 55 years = 1; 55–64 years = 0.57; 65–74 years = 0.29; 75 + years = 0.09). Conversion from 800 Bq/m3 to WLM was performed based on an assumption of 2000 h of work per year, leading to a correspondence of 1 Bq/m3 to 0.00126 WLM (ICRP 1993). Total cumulative exposure and time since last exposure information was calculated from average job durations, which were estimated by occupation, sex, and age group using data from the lifetime job histories of controls in the National Enhanced Cancer Surveillance System (Johnson 1998).