Condensational Growth May Contribute to the Enhanced Deposition of Cigarette Smoke Particles in the Upper Respiratory Tract

Abstract

Previous experimental studies have shown that concentrated cigarette smoke particles (CSPs) deposit in the upper airways like much larger 6 to 7 μ m aerosols. Based on the frequent assumption that relative humidity (RH) in the lungs does not exceed approximately 99.5%, the hygroscopic growth of initially submicrometer CSPs is expected to be a relatively minor factor. However, the inhalation of mainstream smoke may result in humidity values ranging from sub-saturated through supersaturated conditions. The objective of this study is to evaluate the effect of condensation particle growth on the transport and deposition of CSPs in the upper respiratory tract under various RH and temperature conditions. To achieve this objective, a computational model of transport in the continuous phase surrounding a CSP was developed for a multicomponent aerosol consisting of water soluble and insoluble species. To evaluate the transport and deposition of dilute hygroscopic CSPs in the upper airways, a model of the human mouth-throat (MT) through approximately respiratory generation G6 was considered with four steady inhalation conditions. These inhalation conditions were representative of inhaled ambient cigarette smoke as well as warm and hot saturated smoke. Results indicate that RH conditions above 100% are possible in the upper respiratory tract during the inhalation of a warm or hot saturated airstream. For sub-saturated inhalation conditions, initial evaporation of the CSPs was observed followed by hygroscopic growth and diameter increases less than approximately 50%. In contrast, the inhalation of warm or hot saturated air resulted in significant particle growth in the MT and tracheobronchial regions. For the inhalation of warm saturated air 3°C above body temperature, initially 200 and 400 nm particles were observed to increase in size to above 3 μ m near the trachea inlet. The upper boundary inhalation condition of saturated 47°C air resulted in 7 to 8 μ m droplets entering the trachea. These results do not prove that the enhanced deposition of CSPs in the upper airways is only a result of condensational growth. However, this study does highlight condensational growth as a potentially significant mechanism in the deposition of smoke particles under saturated inhalation conditions.

Acknowledgments

This work was sponsored by Philip Morris USA. The authors thank Dr. Michael Oldham for helpful discussions and for reviewing the manuscript. Dr. Beverly Cohen is gratefully acknowledged for providing the tracheobronchial lung cast. Dr. Karen A. Kurdziel and James McCumiskey, VCU Department of Radiology and Molecular Imaging Center, are gratefully acknowledged for providing initial CT template data and for scanning the lung cast.

Notes

*R (right); L (left); apx (apical); inf (inferior); ant (anterior); post (posterior); med (medial); mid (middle); lat (lateral); up (upper); low (lower).

^‡The distribution of flow rates are from measurements in a human tracheobronchial cast (Cohen et al., 1990) that was identical with the computational model.