[Importance of studying exposure of the population to radon and its daughters]

Radon and thoron are naturally-occurring radioactive gases, which are products of uranium and thorium decay series, respectively. Uranium and thorium occur widely in the environment, in rocks, soil, air, water, building materials, humans, etc. Radon daughters in the air are predominantly attached to aerosols. A minor part, normally less than 10%, occurs as unattached atoms or ions. The relative distribution of attached daughters in indoor air and equilibrium factor depend on many variables, such as the decay constant, the concentration and size distribution of aerosols and ventilation rates. Increased ventilation decreases the concentration of radon and daughters in the air.

Concentration of radon, thoron and their decay products in the air indoors, in mines or houses, is higher than outdoors. In houses, the level of radon daughters may be enhanced by radon from radium rich building materials, landfill, soil and bedrock under the house, radon rich water and by poor ventilation.

In recent years, several evaluations of human health risks and estimations have been made in regard to the dose-response relationship and lung cancer risk attributable to inhaled radon daughters. Inhalation of radon and thoron daughters leads to deposition in the human respiratory tract and consequent irradiation. Deposition depends on various factors, such as the size distribution of aerosols to which the daughter products of radon are attached, and fraction of unattached daughters. On average, the dose to the basal cell layer in the lung is about 5 to 8 times higher than the dose in the pulmonary region.