Alpha dose exposure

The lung cancer risk from radon daughter exposure is known only for occupationally exposed males. In order to determine the risk in environmental situations it is necessary to determine whether the bronchial alpha dose, which confers the risk, is similar to that in mines. 

Particle size is a major factor which determines the alpha dose conversion factor for radon daughters (mGy/WLM). Data on indoor environments are emerging and indicate that a variety of specific conditions exist. For example, a dose factor four times that for a nominal occupational or environmental exposure exists if kerosene heater particles dominate the indoor aerosol and four times smaller if a hygroscopic particle dominates. 

Table II shows the nominal alpha dose factors for occupational mining exposure. Table III shows the alpha dose factors for the nominal environmental situation. Table IV shows the bronchial dose factors for the smallest sized particles, that dominated by the kerosene heater or 0.03 pm. particles. The radon daughter equilibrium was shifted to a somewhat higher value in this calculation because this source of particles generally elevates the particle concentration markedly with consequent increase in the daughter equilibrium. Table V shows the alpha dose for a 0.12 pm particle, the same as the nominal indoor aerosol particle, but for a particle which is assumed to be hygroscopic and grows by a factor of 4, to 0.5 pm, once in the bronchial tree.

Investigations with Animals. A further support of the hypothesis described above can be found in investigations carried out with animals Leonard et al. (1979) found in an area with high natural radioactivity in France a small but significant increase of chromosome aberrations in the lymphocytes of rabbits. The rabbits were kept in a hut for 12 months, and received up to 0.7 Gy/year from gamma rays together with more than 6 Gy alpha doses from radon and daugthers. Further experiments with rabbits at radon exposure under controlled conditions have shown that the chromosome... 

Lundgren D, Gillett N, Hahn F, et al. 1987. Effects of protraction of the alpha dose to the lungs of mice by repeated inhalation exposure to aerosols of plutonium-239 oxide. Radiat Res 111 201- 224. 

It has been demonstrated that it is possible to lower the level of airborne radon progeny by filtering and/or expose the air to an electric field. If the radiological risk is measured by the potential alpha energy concentration (PAEC) or exposure rate, for instance expressed in J m 3 or WL, the level may be lowered to about 10-20 % of the value in untreated air, while the reduced level may only be about 40-50 % of the untreated one if the average dose to a certain part of the respiratory tract, for instance expressed in Gy year 1, is considered. 

Figure 5. Doses averaged over all epithelial cells in the bronchial and alveolar regions of the lung per unit exposure to potential alpha-energy as a function of aerosol size, compared with doses to basal cells for several models of airway size and clearance behaviour.

Table II. Reference Values of Mean Bronchial Dose from Exposure to 1 WLM Potential Alpha-energy in Homes...

Figure 6. Doses averaged over epithelial cells in segmental bronchi per unit exposure to potential alpha-energy.

The main problems of the study of chromosome aberrations, caused by radon and daughters at their most frequently existing dose levels, i. e. boardering the natural burdens, ares (i) to get statistical significance at very low doses, and (ii) to study their induction by internal exposure to alpha emitters only. 

Preliminary studies, already, resulted in a dose response curve which rises sharply up to about 0.5 mGy/month and then flattens into a plateau (Figure 1). In the dose range up to 0.3 mGy/month only a small part (about 0.014 mGy/month) was due to internal alpha exposure, whereas at higher doses more and more the exposure to alpha rays were dominant. 

This was added to lymphocyte-rich blood plasmas and then removed after about 2 hours exposure. They found dose varying RBEs of 119 to 45 for doses of 8.5 to 70 mGy. Their dose estimation, however, was critizised by Fisher and Harty (1982) according to microdosimetric considerations. Therefore also the over tenfold discrepancy of their RBEs for alpha particles to the other studies was attributed to erraneous dose estimations. 

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