The extrathoracic (ET) airways filter, warm, and humidify the inspired air and provide olfactory function. These multiple functions are reflected in its complex anatomy and physiology. The ET airways form the first line of defense against inhaled pollutants, both gaseous as well as particulate. To accurately assess the risk posed by inhaled particulate matter to the lung, it is essential to understand the filtering efficiency of the ET airways. In this paper computational fluid dynamics is used to simulate the airflow patterns and the thermodynamics of the ET airways. We detail the procedure to develop a computer reconstruction of the ET airways and the computer model to simulate the flow variable. Using this information we compute the particle trajectories, for both hygroscopic and nonhygroscopic aerosols, and use this data to evaluate the particle deposition pattern in the ET airways. The model predicts high relative humidity conditions in the ET airways. The model also shows that the high relative humidity conditions are conducive for rapid growth of hygroscopic particles and dramatically alter the deposition characteristics of ambient (hygroscopic) aerosol.