The activity size distribution of unattached as well as attached 212 Pb to aerosol particles was measured in the open air of El-Minia City, Egypt. The samples were collected using a wire screen diffusion battery technique and a low pressure cascade impactor. The mean activity median thermodynamic diameter (AMTD) of unattached 212 Pb was determined to be 1.4nm with a relative mean geometric standard deviation (σ g) of 1.55. A mean unattached fraction (f p ) of 0.03±0.007 was obtained at a mean aerosol particle concentration of 32×103 cm−3. Sometimes the f p values were less than the detection limit of 0.008. The mean concentration of activity of 212 Pb was found to be 9.6±1.1mBqm−3. The mean activity median aerodynamic diameter (AMAD) of the accumulation mode of attached 212 Pb was determined to be 360nm with a mean (σ g) of 2.7. The mean value of specific air activity concentration of 212 Pb associated with that mode was determined to be 303±12mBqm−3. With a dosimetric model calculation (International Commission on Radiological Protection. Human respiratory tract model for radiological protection. Oxford: Pergamon Press, ICRP Publication 66, 1994) the total deposition fractions as well as total equivalent and effective dose have been evaluated considering the obtained parameters of the activity size distributions. At a total deposition fraction of about 97% for unattached activities the total equivalent and effective doses to the lung were determined to be about 0.18 and 0.02μSv, respectively, while total equivalent and effective doses of about 0.45 and 0.05μSv, respectively, were determined at a total deposition fraction of about 23% for the attached activities.