To ascertain the threat to human health posed by the inhalation of the particulate phase for secondary cigarette smoke (SCS), it is necessary to determine the doses delivered to airway cells. The risk assessment of SCS particles is complicated by their hygroscopic properties. Inhaled particles that are hygroscopic may absorb the water vapor that is present in a warm, humid environment such as that found in the human respiratory system. As a direct result, the physicochemical characteristics of a particle such as size, shape, density, and composition may experience signi cant changes. Herein, the effects of hygroscopicity and deposition of SCS particles are examined in the nasopharyngeal region of the respiratory tract through which particles travel before entering the lung. To accomplish this goal, a computer model is de ned to describe the anatomical features of the airways within the human head and throat. Then the effects of water vapor uptake and deposition on inhaled particle size distributions are formulated. The results of the simulations indicate that hygroscopicity is a critical factor affecting the dynamics of inhaled SCS.

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