The vertical equilibrium distributions of Rn222, Rn220, and their decay products in a horizontally isotropic atmosphere are calculated from diffusion theory with the help of a digital computer. The change of the turbulent diffusion coefficient with altitude and the removal of Pb210 and Po210 by fallout were taken into account. Vertical profiles of the natural radionuclides in the atmosphere are presented for several typical profiles of the turbulent diffusion coefficient. Agreement with available experimental data is rather good. The results indicate the importance of the natural radioactive tracers in the study of mixing processes in the troposphere and in the mechanism of transport from the troposphere into the lower stratosphere.

A simple model is described to estimate the activity balance of Rn-and Tn-decay products in a turbulent mixed air volume in the steady-state equilibrium, taking into account the attachment to aerosol particles and the removal of activity by ventilation and wall deposition. On the basis of this model the principal relationships between these parameters and the degree of radioactive disequilibrium, the fraction of free atoms or ions and the concentration of potential a-energy in the air are calculated and discussed, especially with regard to the conditions in uranium mines. The concept of potential a-energy is transferred also to Tn-daughters. The results will be a useful guidance to estimate the relation between the inhaled potential aenergy and the lung dose of workers in uranium and fluorspar mines.

Irradiation of the human body by external or internal sources leads mostly to a simultaneous exposure of several organs. However, so far no clear and consistent recommendations for the combination of organ doses and the assessment of an exposure limit under such irradiation conditions are available. Following a proposal described in ICRP-publication 14 one possible concept for the combination of organ doses is discussed in this paper. This concept is based on the assumption that at low doses the total radiation detriment to the exposed person is given by the sum of radiation detriments to the single organs. Taking into account a linear dose-risk relationship, the sum of weighted organ doses leads to the definition of an "Effective Dose". The applicability and consequences of this "Effective Dose Concept" are discussed especially with regard to the assessment of the maximum permissible intake of radionuclides into the human body and the combination of external and internal exposure.

Consideration of the epidemiological evidence on radiogenic lung cancer in uranium miners and of the incidence of the disease generally leads to an upper estimate for the lifetime risk of 10(-4) cases per working level month for members of the general population.

The deposition of Rn daughters in the different regions of the human respiratory tract is derived from the particle size of their carrier aerosol in ordinary, unfiltered air. The resulting mean distribution of the radioactivity in the respiratory tract is estimated, taking into account the clearance from the alveolar tissue and the ciliary transport in the bronchial tree. The radiation dose distribution in the epithelium of trachea and bronchi is calculated, taking into account the a-absorption in the mucus layer and the variation of stopping power with a-energy. The resulting mean absolute a-dose in different regions of the respiratory tract is given for each inhaled nuclide, especially at their natural level in atmospheric air and their maximum permissible level. The results indicate, that owing to the ciliary transport, a considerable fraction of the dose in the upper respiratory tract is due to the transfer of activity from the bronchioli and lower bronchi in the mucus stream. The highest dose should be expected in the lower bronchi.