Long-term retention of particles in the human respiratory tract

Twelve healthy non-smoking male volunteers inhaled monodisperse 1 and 4 μm diameter fused aluminosilicate particles labelled with strontium-85 and yttrium-88, respectively. Retention was followed for at least a year (372-533 days). Approximately 7% of the initial lung deposit of 1 μm particles and 40% of that of the 4 μm particles were associated with a distinct rapid clearance phase. These figures correspond closely to the calculated tracheo-bronchial deposits, indicating insignificant rapid pulmonary clearance. Retention of the remaining material (R(t)) generally followed a two-component exponential function, the phases having half-times of the order of tens of days and several hundred days, respectively. At 350 days after inhalation, R(t) averaged 46 ± 11% (x ± SD) for the 1 μm particles and 55 ± 11% for the 4 μm particles. Retention of the 1 μm particles by each subject was correlated with that of 4 μm particles. Estimated lung dissolution rates based on urinary excretion were 7 × 10^-4 and 2 × 10^-4 per day for the 1 and 4 μm particles, respectively. The estimated rate of clearance of particles from the pulmonary region to the gastro-intestinal tract fell from an initial value of 4 × 10^-3 per day to about 1 × 10^-3 per day at and beyond 200 days after inhalation.