Environmental Tobacco Smoke Deposition in the Human Respiratory Tract: Differences between Experimental and Theoretical Approaches

Total deposition of environmental tobacco smoke (ETS) particles was measured in a group of 15 nonsmokers who inhaled ETS of count median diameter of 0.2 μm and geometric standard deviation of 1.6. A total deposition of 56.0 ± 15.9% was observed for nasal breathing and 48.7 ± 11.5% for oral breathing. In contrast, our stochastic deposition model predicted a total deposition of only 17.9% (male) and 15.7% (female) for nose breathing, and 13.4% (male) and 10.7% (female) for mouth breathing, if based on standard breathing conditions. Consideration of individual lung volumes and breathing parameters for each volunteer resulted in total deposition values of 16.9 ± 2.2% for nose breathing and 12.1 ± 2.1% for mouth breathing. The apparent discrepancy between experiment and modeling suggests that either single ETS particles increase substantially in size upon inhalation (up to an order of magnitude) and/or additional physical mechanisms must be invoked that are acting specifically upon ETS particles: (1) hygroscopic growth of ETS particles does not exceed 20-30%; (2) number concentrations in the ETS experiments (3.8 × 10⁴, to 1.3 × 10⁵ cm^-3) are too low to increase particle size by coagulation; (3) cast experiments indicate that electrical charge (image forces) may play an important role, but theory predicts only an increase of 20-60%; and (4) cloud settling is unlikely to be a significant factor at such low number concentrations. In conclusion, estimates of the magnitudes of these potential effects demonstrate that none of these mechanisms alone can be responsible for the significantly higher total ETS deposition observed in the experiments. This suggests that a combination of all these mechanisms may be necessary to reconcile experimental and theoretical ETS deposition data, the most likely candidates being image forces and hygroscopic growth.