Bronchial dose factors

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.

Effective dose equivalent. If it is assumed that the weighting factor for bronchial dose equivalent is 0.06, the unattached fraction of potential alpha-energy in room air is typically about 570, and that the aerosol AMD is typically 0.12 pm (Reineking et al., 1985), the... 

Figure 11. Variation of unattached fraction of potential alpha-energy and equilibrium factor according to a model of room aerosol behaviour and the effect on bronchial dose rate per unit radon gas concentration.

Figure 2. Mean bronchial dose to basal cells, standardized for 1 WLM exposure to free radon 222 daughter atoms for different physical activities (assuming typical indoor exposure with equilibrium factor F = 0.4) ...

The conversion factor for the bronchial dose from both unattached and attached decay products is... 

Jacobi Paretzke (1985) estimated that the uranium miners in Colorado accumulated an average exposure of 820 WLM in the years 1950-77. Using the factor 16 mSv per WLM, the average bronchial dose would have been 13 Sv, giving a 20% chance of cancer on the basis of the ICRP (1981) estimate. The BEIR IV report of the US National Research Council (1988) recorded 256 deaths from lung cancer among the Colorado miners. The total exposure was 73 600 person-years, and 58 deaths would have been expected if there were no carcinogenic effects. 

Ihe dosimetric consequence of particle size determination is tiiat a realistic bronchial dose may be calculated (13). In tiiis case, essentially the same dose conversion factor applies at both Femald and at the residential home in New Jersey because the median diameter is identical. UNSCEAR 2000 (14) gives the... 

It is seen that the diameters of bronchioles (averaged over generations 11 - 15) vary little with age. The increase in bronchial size is greater, but still less than might be expected if airways are simply scaled for overall body dimensions (illustrated by the dashed curves in Figure 9, which are functions of body weight W). Since bronchiolar diameter does not change much with age it is likely that the thickness of bronchiolar epithelium is also relatively constant. However, in the case of the bronchi, it is reasonable to assume that epithelial thickness is proportional to bronchial diameter. Thus, it is necessary to use age dependent conversion factors between the surface density of alpha-decays and dose to cells. 

The biological effectiveness of dose depends on the type of radiation and also on the mass and sensitivity of the irradiated tissue. For alpha irradiation, a quality factor of 20 is assumed (ICRP, 1981), and the dose in Sieverts is 20 times the dose in Grays. In addition, ICRP recommends a weighting factor of 0.12 for irradiation of the whole lung and 0.06 for irradiation of bronchial epithelium only. Thus the effective dose equivalent , symbol HE, is defined as the dose to the whole body which carries the same risk as the given dose to the organ or tissue. This, for irradiation of bronchial tissue is 20 x 0.06 = 1.2 times the dose to the organ in Gy. 

The degree fo which radon daughfers attach fo aerosol particles is an important factor in determining the radiation dose to which bronchial cells are exposed. The unattached fraction (f) remain charged and are deposited in the airways more efficiently than the fraction attached to aerosol dusts. They therefore produce a higher resulfanf radiation dose to sensitive tissue. The particle size distribution of fhe aerosol-attached activity will also have a major effect on the airway deposition pattern and hence the radiation dose received in various regions of the respiratory system. 

Figures 5.2 and 5.3 show the effective dose per unit exposure of the tracheo-bronchial (bronchial and bronchiolar) and pulmonary or alveolar region as a function of aerosol particle diameter calculated for the radon decay products Po, " Pb, and " Po and the thoron decay products Pb, Bi and Po. A tissue weighting factor of the lung and the radiation weighting factor of 0.12 and 20, respectively, are taken into account. The effective dose from a radioactive mixture can be deduced by adding the effective doses of each decay product.

---