The thoron gas (220Rn) activity concentration as well as activity size distribution of unattached and attached 212Pb to aerosol particles was measured in the open air of Jeddah City, Kingdom of Saudi Arabia. An electroprecipitation method was applied for measuring the 220Rn concentration. A mean activity concentration of 220Rn was determined to be 1.80±0.47Bqm−3. The unattached activities of 212Pb were collected using the wire screen diffusion battery technique while a low-pressure cascade impactor collected the attached activities. The mean activity median thermodynamic diameter (AMTD) of unattached 212Pb was determined to be 1.32nm with a relative mean geometric standard deviation (σ g) of 1.45. A mean concentration of unattached activity of 212Pb was found to be 9.48±1.12mBqm−3. A mean unattached fraction (f p) of 0.028±0.002 was obtained at a mean aerosol particle concentration of 29×103 cm−3. Sometimes, the f p values were less than the detection limit of 0.009. A mean activity median aerodynamic diameter (AMAD) of the accumulation mode of attached 212Pb was determined to be 352nm with a mean (σ g) of 2.6. The mean value of specific air activity concentration of 212Pb associated with that mode was determined to be 310±12mBqm−3. With a dosimetric model calculation (ICRP, 1994) the total and regional deposition fractions, total and regional equivalent doses could be evaluated considering the obtained parameters of the activity size distributions. At a total deposition fraction of about 97% of unattached activities the total equivalent dose to the human lung was determined to be 0.16µSv while a total equivalent dose of 0.44µSv was determined at a total deposition fraction of about 23% for the attached activities. It was found that an unattached fraction of f P≈3% yields to about 27% of the total equivalent dose.