ATSDR Draft Toxicological Profile for Radon_September 2008.pdf

RADON 613. HEALTH EFFECTSaction of the cilia may remove as much as half of the bronchi- and bronchiole-deposited particles. Inhuman bronchi and bronchiole regions, mucus moves more slowly when it is closer to the alveoli. For thefaster compartment, it has been estimated that it takes about 2 days for particles to travel from thebronchioles to the bronchi and 10 days from the bronchi to the pharynx. The second (slower)compartment is assumed to have approximately equal fractions deposited between BB 2 and bb 2 , with bothfractions having clearance half-times estimated at 20 days. Particle size is a primary determinant of thefraction deposited in this slow thoracic compartment. A small fraction of particles deposited in the BBand bb regions is retained in the airway wall for even longer periods (BB seq and bb seq ).If particles reach and become deposited in the alveoli, they tend to become imbedded in the fluid on thealveolar surface or move into the lymph nodes. Coughing is the one mechanism by which particles arephysically resuspended and removed from the AI region. For modeling purposes, the AI region is dividedinto three subcompartments to represent different clearance rates, all of which are slow.In the alveolar-interstitial region, human lung clearance has been measured. The ICRP model uses 2 halftimesto represent clearance: about 30% of the particles have a 30-day half-time, and the remaining70% are assigned a half-time of several hundred days. Over time, AI particle transport falls, and somecompounds have been found in lungs 10–50 years after exposure.Absorption into Blood. The ICRP model assumes that absorption into blood occurs at equivalent rates inall parts of the respiratory tract, except in the anterior nasal passages (ET 1 ), where no absorption occurs.It is essentially a 2-stage process, as shown in Figure 3-4. First, there is a dissociation (dissolution) ofparticles; then the dissolved molecules or ions diffuse across capillary walls and are taken up by theblood. Immediately following dissolution, rapid absorption is observed. For some elements, rapidabsorption does not occur because of binding to respiratory-tract components. In the absence of specificdata for specific compounds, the model uses the following default absorption rate values for those specificcompounds that are classified as Types F (fast), M (medium), S (slow), and V (instantaneous):• For Type F, there is rapid 100% absorption within 10 minutes of the material deposited in the BB,bb, and AI regions, and 50% of material deposited in ET 2 . Thus, for nose breathing, there is rapidabsorption of approximately 25% of the deposit in ET; for mouth breathing, the value is 50%.• For Type M, about 70% of the deposit in AI reaches the blood eventually. There is rapidabsorption of about 10% of the deposit in BB and bb, and 5% of material deposited in ET 2 . Thus,there is rapid absorption of approximately 2.5% of the deposit in ET for nose breathing, and5% for mouth breathing.***DRAFT FOR PUBLIC COMMENT***