Size distribution of radon progeny aerosol in the working area of a dry former uranium mine

To provide improved particle size data for reconstructing the radiation dose incurred by Colorado plateau uranium miners, sampling exercises were made in 1988 and 1994 at Twilight Mine in western Colorado, a former uranium mine that is now used as a research and training facility. The strategy was to simulate or approximate some of the conditions to which the miners were exposed, then sample the aerosol with modern instrumentation. By combining data from an impactor with data from a diffusion-based sampler, it was possible to cover the particle size range from 0.5 to 5000 nm. The major new finding of these sampling exercises was that, with or without diesel operation, 5 to 30% of the potential alpha energy was bound to particles larger than 1000 nm. These particles formed a well-defined mode (the coarse-particle mode) in the particle size distribution. Current lung models show that tracheobronchial deposition increases with increasing particle size above 1000 nm. Consequently, the existence of coarse particles implies that the lung dose incurred by uranium miners was larger than previously thought. An approximate calculation based on the new data indicates a 40% increase in dose.