Modelling particle retention in the alveolar–interstitial region of the human lungs

Abstract

Better information is available now on long-term particle retention in the human lungsthan there was in 1994, when the human respiratory tract model (HRTM) was adopted bythe International Commission on Radiological Protection (ICRP). Three recent studies areespecially useful because they provide such information for groups of people who inhaledvery similar aerosols. For all three the HRTM significantly underestimates lung retention ofinsoluble material. The purpose of this work was to improve the modelling of long-termretention in the deep lung. A simple physiologically based model developed to predict lungand lymph node particle retention in coal miners was found to represent lungretention in these studies adequately. Instead of the three alveolar–interstitial (AI)compartments in the HRTM, it has an alveolar compartment which clears to thebronchial tree and to a second compartment, representing the interstitium, whichclears only to lymph nodes. The main difference from the HRTM AI model isthat a significant fraction of the AI deposit is sequestered in the interstitium. Toobtain default parameter values for general use, the model was fitted to data fromthe three recent studies, and also the experimental data used in developmentof the HRTM to define particle transport from the AI region for the first yearafter intake. The result of the analysis is that about 40% of the AI deposit ofinsoluble particles is sequestered in the interstitium and the remaining fractionis cleared to the ciliated airways with a half-time of about 300 days. For somelong-lived radionuclides in relatively insoluble form (type S), this increased retentionincreases the lung dose per unit intake by 50–100% compared to the HRTM value.