Radiation dose from inhalation of radon and decay products

15 Radiation dose from inhalation of radon and decay products 

If the rate of ventilation of the lungs is 0.75 m3 IT1, the rate of deposition of potential alpha energy in the alveolar region is then 0.15 Ep Jh-1. The mass of the lung in the standard man is 1 kg, and by definition 1J kg-1 is 1 Gy, so the dose rate is 0.15 Gy per Jh m-3. Curve A of Fig. 1.15, following James (1987b), shows the variation of the alveolar dose with dp. As dp decreases, the dose increases to a peak at 18 nm, and then declines because smaller particles are trapped in the upper respiratory tract and fewer reach the alveolar region. 

Unattached decay products have diffusivity appropriate to dp about 1 nm, and are almost completely retained in the respiratory tract 

The critical tissue is the bronchial epithelium, where radiation-induced lung cancers are believed to originate, and many calculations and experiments with models of the upper respiratory system have been done to estimate the dose to this region (James, 1987,a,b). 

Chamberlain Dyson (1956) measured deposition of unattached 212Pb in a rubber model of the trachea and main bronchi. At an inspiratory flow of 20 1 min-1, corresponding to a ventilation rate of about 0.6 m3 h-1, the activity deposited per cm2 of surface in the bronchi was 0.18% of that entering the trachea. No difference was found in the deposition of unattached 218Po (RaA) compared with unattached 212Pb (ThB). 

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