... continued from pg 22 [Review of Radon]
INTRODUCTION
Radon (222Rn) is a naturally occurring radioactive gas generated by the decay of uranium-bearing minerals in rocks and soils. Exposure to radon and its short-lived progenies in air has long been identified as the second leading cause of lung cancer after tobacco smoking (NAS/NRC 1988, 1999; WHO 2009; ICRP 1993, 2014; UNSCEAR 1982, 2000, 2020). While exposure to indoor radon is the main source of natural radiation exposure to the population, lung cancer caused by exposure to radon decay products is the most common type of radiation-induced injury among occupationally exposed workers. Underground atmospheres have increased potential for radon exposure, especially in mining of uranium and associated substances such as copper, phosphorous, calcium, arsenic, barium, vanadium, and lead. As indicated in several reports of United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR 1982, 1993, 2000, 2008), exposure to radon represents the most significant contribution to occupational radiation exposure in underground mining operations.
In most underground uranium mines, radon doses to miners are strictly controlled and determined by monitoring radon progeny concentrations directly in the units of working levels (WL) (1WL = 2.08 × 10−5 J m−3) and radon progeny exposure in working level month (WLM). Unlike in uranium mines, radon exposure in non-uranium mines is normally not under regulatory control. Continuous monitoring and control of the radiation exposure levels of workers is not undertaken in conventional mines in many countries since, as reported by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR 2022), exposure data for non-uranium miners are very limited. For the period 2005–2009, the UNSCEAR Global Survey of Occupational Radiation Exposure only received detailed exposure data for non-uranium mining operations from four countries out of 57 United Nations member states that expressed interest in participating in the survey.
The radon-induced lung cancer is not specific only for uranium miners, because radon is a naturally occurring radioactive gas generated by the decay of uranium-bearing minerals in all rocks and soils in varying concentrations. For example, radon and γ-ray exposures were measured in 26 non-uranium mines in Australia (Ralph et al. 2020a). The results showed that, on average, exposure to radon progeny in non-uranium mines contributed to 71% of the total annual effective dose, ranging from 43% to 93% in different mines. A more recent study by Ralph and Cattani (2022) in 13 non-uranium mines in Australia also included committed effective doses from inhalation of dusts containing long-lived alpha-emitting nuclides in total annual effective doses. In this case, exposure to radon progeny in non-uranium mines contributed to 29% of the total annual effective dose, ranging from 0.7% to 90% in different mines (Ralph and Cattani 2022). Radon exposure in uranium and non-uranium mines can result in occupational health concerns.
Historically, radon concentration was high in underground mines. Underground working conditions have been improved significantly in recent decades. For example, in Canada, radon progeny concentration in underground uranium mines has been kept at a historically low level for the past two decades (1998–2018) with an average annual radon exposure of 0.23 WLM, compared to the 5-y average annual radon exposure of 1.4 WLM from 1993 to 1997 (Chen et al. 2021). In Polish metal ore mines, the mean annual radon exposure has stabilized at a historically low level since the beginning of the 1980s (Kluszczynski et al. 2002). The average radon concentration in Finnish underground mines
has decreased with time, being approximately 1,800 Bq m−3 in the year 1972, 300 Bq m−3 in 1990, and 100 Bq m−3 in 2000 (Koja et al. 2021). Therefore, this paper aimed to provide updated information on radon exposure to non-uranium underground miners based on review of more recent publications on measurements of radon and radon progeny concentrations in active underground non- uranium mines (i.e., mines in operation with ventilation on) found in the literature in recent two decades (2000 to present).
In Canada, mining associated with the nuclear fuel cycle (i.e., uranium) falls under the regulatory authority of the Canadian Nuclear Safety Commission (CNSC) and is subject to requirements for monitoring and reporting information on radiation doses to workers. Other types of mining are regulated by the provincial and territorial authorities. For CNSC-regulated uranium mining activity, miners’ dose records (including radon doses) have been reported to the National Dose Registry (NDR) since 1955. However, exposure monitoring for non-uranium mining activities using a licensed dosimetry service and reporting doses to the NDR is not required. In the most recent “Report on occupational radiation exposures in Canada 2008–2018” (Health Canada 2021), dose records were only available for workers with uranium mining activities (there were 629 underground workers in uranium mines in 2018; they were uranium mine underground miners, underground workers for maintenance, and other underground personnel). To fill the data gaps for large numbers of workers employed in various non-uranium mines, radon exposures to Canadian non-uranium mine workers were estimated with radon exposure information from literature review, assuming Canadian non-uranium mines operating under similar conditions to the averages from many other non-uranium mines around the world.
REVIEW OF RADON CONCENTRATIONS IN UNDERGROUND NON-URANIUM MINES
Radon gas contributes relatively little to the dose to the lung. The inhalation of the short-lived solid radon decay products and subsequent deposition on the walls of the airway epithelium of the bronchial tree deliver most of the radiation dose to humans. The equilibrium factor, F, between radon and its short-lived progeny in underground mine atmospheres can be very unstable and vary in space and time in the range of 0.1–1.0 (Chen and Harley 2020). Therefore, some radon measurements in mines were direct measurements of radon progeny concentration in working level (WL) (1 WL = 2.08 × 10−5 J m−3) or potential alpha energy concentration (PAEC, in units of J m−3). For the purpose of comparison with residential radon gas measurements, measurement results of radon progeny concentrations were converted to radon gas concentration in the units of Bq × m−3 using the equilibrium factor F = 0.38 determined from multiple simultaneous radon gas and radon progeny measurements performed in a total of 173 underground mines of various mining types in 18 countries (Chen and Harley 2020). Therefore, 1 J m−3 of radon progeny concentration was converted to 4.76 × 108 Bq m−3 of radon gas concentration (1 μJ m−3 = 476 Bq m−3). Due to the importance of F factor in radon dose calculation, current review also collected information of measured F factor whenever available in the literature.