Importance of cloud motion on cigarette smoke deposition in lung airways

Deposition patterns of mainstream cigarette smoke were studied in casts of human extrathoracic and lung airways. The laboratory tests were designed to simulate smoking (i.e., the behavior of undiluted cigarette smoke in smokers' lungs), not secondary exposures to non-smokers. The experimental data revealed concentrated deposits at well-defined sites, particularly at bifurcations (most notably at inclusive carinal ridges) and certain segments of tubular airways. The measurements suggest the occurrence of cloud motion wherein particles are not deposited by their individual characteristics but behave as an entity. The observed behavior is consistent with the theory of Martonen (1992), where it was predicted that cigarette smoke could behave aerodynamically as a large cloud (e.g., 20 microns diameter) rather than as submicrometer constituent particles. The effects of cloud motion on deposition are pronounced. For example, an aerosol with a mass median aerodynamic diameter (MMAD) of 0.443 micron and geometric standard deviation (GSD) of 1.44 (i.e., published cigarette smoke values) will have the following deposition fractions: lung (TB + P) = 0.14, tracheobronchial (TB) = 0.03, and pulmonary (P) = 0.11. When cloud motion is simulated, total deposition increases to 0.99 and is concentrated in the TB compartment, especially the upper bronchi; pulmonary deposition is negligible. Cloud motion produces heterogeneous deposition resulting in increased exposures of underlying airway cells to toxic and carcinogenic substances. The deposition sites correlated with incidence of cancers in vivo. At present, cloud motion concentration effects per se are not addressed in federal regulatory standards. The experimental and theoretical data suggest that concentrations of particulate matter may be an important factor to be integrated into U.S. Environmental Protection Agency (EPA) risk assessment protocols.