Thoron concentration, aerosol characteristics of ²¹²Pb and estimation of equivalent dose

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±0.47 Bq m⁻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 1.32 nm 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±1.12 mBq m⁻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 352 nm 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±12 mBq m⁻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 0.16 µSv while a total equivalent dose of 0.44 µSv 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. - Highlights: • Size distribution of ²¹²Pb is determined in open air. • Thoron gas, unattached and attached concentrations of ²¹²Pb is measured. • Dosimetric model is applied to evaluate deposition fraction and dose equivalent. • 3% of unattached fraction yields to about 27% of the total equivalent dose