The radon risk-coefficient, determined from long-term epidemiology studies of 11 groups of uranium and hardrock miners, is given in terms of exposure to airborne radon progeny, not lung dose, which must be computed using breathing rates and the airborne radioactivity as a function of particle size in the mine atmosphere. Aerosol measurements relevant to this issue are reviewed: in diesel-powered mines the range of activity median diameters is between 75 and 175nm (GSD=2), and these aerosols are expected to be hydrophobic and not double in size in the respiratory tract as is often assumed. However, since much of the exposure occurred in mines powered electrically or with compressed air, and few activity size distribution measurements have been made in such mines, it appears that further experimental work is needed to characterize the properties of aerosols to be expected in the working environments of the mines under consideration.