Understanding aerosols in uranium mines in order to compute the lung dose for uranium miners

Abstract

The radon risk coefficient is determined from long term epidemiology studies of lung cancer incidence among uranium miners. The current estimate of 350 lung cancer deaths per million person Working Level Month was proposed by the Committee on the Biological Effects of Ionizing Radiation (BEIR IV, 1988), based on these studies.The unit of working level month is a measure of exposure to airborne radon progeny. To convert this exposure to lung deposition and then to lung dose requires a knowledge of or assumptions about the aerosol to which the radon decay products are attached. This issue is important since the aerosol and breathing patterns, and thus the dose per unit working level, can be quite different in mines where the risk coefficient is derived, and in homes, where the greatest population exposure occurs. In the Report (NRC, 1991) that made a detailed investigation of the conversation of home and mine exposure to dose, several assumptions about the activity weighted particle size distribution in these mines were made. Based on a review of the literature several of these critical assumptions have been found to be incorrect.Current miner lung dose estimates are based on an assumed ambient particle size distribution with an activity median diameter (AMD) in the range of 150 to 250 nm (NRC, 1991) and a doubling of the AMD in the respiratory tract; the aerosol characteristics assumed for the NRC evaluation were derived from measurements made in diesel powered mines. It was also assumed that this size distribution was typical of all uranium mines in the epidemiology studies.The assumption that article size doubles upon inhalation into the hot, humid breathing passages of the respiratory system is based on experimental measurements by Sinclair et al. (1974) made during the summer and winter on the ambient aerosol in New York City. It was hypothesized that the aerosol consisted of a mixture of salts and acids typical of a maritime industrial city. However, in many mines, the most important aerosol was diesel exhaust, which is soot coated with unburned fuel and lubricating oil. Such particles are hydrophobic, and will not increase in size as assumed. Even in mines without diesel equipment, the doubling assumption is probably invalid since there is no source of hydrophilic aerosol.The Report also assumed one size distribution for all mines; however, particle size distributions for diesel and non-diesel mines are known to be completely different; some mines in the epidemiology studies were diesel powered, others were not. In addition, some of the Canadian mines in the epidemiology study directly heated the ventilation air in the winter using propane or natural gas combustion, with the combustion products injected into the mine along with the heated air. This combustion generates an aerosol of carbon soot in the 20 nm size range, which could have been a dominant component of the aerosol in these mines during the winter months. Finally, the Report used an aerosol size distribution based on very few measurements made in mines under difficult conditions. Laborator measurements of diesel exhaust (Sawyer et al. 1995) and other diesel mine measurements (Cantrell and Rubow, 1988) indicate that the standard activity median diameter assumed (150–250 nm) (1) may have been too large for diesel mines.A crude estimate based on computed lung deposition (Table 1) indicates that because of these erroneous assumptions the dose to the miners in diesel powered mines has been underestimated by at least a factor of two. More generally, this implies that a dose reconstruction project is needed to complete the efforts of those groups that have estimated miner exposure to radon progeny. ▪