Influences of parameter uncertainties within the ICRP 66 respiratory tractmodel: Particle deposition

Risk assessment associated with the inhalation of radioactive aerosols requ ires as an initial step the determination of particle deposition within the various anatomic regions of the respiratory tract. The model outlined in I CRP Publication 66 represents to date one of the most complete overall desc riptions of not only particle deposition, but of particle clearance and loc al radiation dosimetry of lung tissues. In this study, a systematic review of the deposition component within the ICRP 66 respiratory tract model was conducted in which probability density functions were assigned to all input parameters. These distributions were subsequently incorporated within a co mputer code LUDUC (LUng Dose Uncertainty Code) in which Latin hypercube sam pling techniques are used to generate multiple (e.g., 1000) sets of input v ectors (i.e., trials) for all of the model parameters needed to assess part icle deposition within the extrathoracic (anterior and posterior), bronchia l, bronchiolar, and alveolar-interstitial regions of the ICRP 66 respirator y tract model. Particle deposition values for the various trial simulations were shown to be well described by lognormal probability distributions. Ge ometric mean deposition fractions from LUDUC were found to be within approx imately +/- 10% of the single-value estimates from the LUDEP computer code for each anatomic region and for particle diameters ranging from 0.001 to 5 0 mum. In all regions of the respiratory tract, LUDUC simulations for an ad ult male at light exertion show that uncertainties in particle deposition f ractions are distributed only over a range of about a factor of similar to2 -4 for particle sizes between 0.005 to 0.2 mum. Below 0.005 mum, uncertaint ies increase only for deposition within the alveolar region. At particle si zes exceeding 1 mum, uncertainties in the deposition fraction within the ex trathoracic regions are relatively small, but approach a factor of 20 for d eposition in the bronchial region. Deposition fractions for particles above I mum become very uncertain within the deeper regions of the lungs (bronch iolar and alveolar-interstitial).