Transport and uptake of MTBE and ethanol vapors in a human upper airway model

Potential human exposure to vapors of methyl tertiary-butyl ether (MTBE) and ethanol is of increasing concern because these materials are widely used as gasoline additives. In this study we analyzed numerically the transport and deposition of MTBE and ethanol vapors in a model of the human upper respiratory airway, consisting of an oral airway and the first four generations of the tracheobronchial tree. Airflow characteristics and mass transfer processes were analyzed at different inspiratory flow conditions using a three-dimensional computational fluid and particle dynamics method. The deposition data were analyzed in terms of regional deposition fractions (DF = regional uptake/mouth concentration) and deposition enhancement factors (DEF = local DF/average DF) at local micro surface areas. Results show that DF in the entire upper airway model is 21.9%, 12.4%, and 6.9% for MTBE and 67.5%, 51.5%, and 38.5% for ethanol at a flow rate of 15, 30, and 60 L/min, respectively. Of the total DF, 65-70% is deposited in the oral airway for both vapors. Deposition is localized at various sites within the upper airway structure, with a maximum DEF of 1.5 for MTBE and 7.8 for ethanol. Local deposition patterns did not change with inhalation conditions, but DF and the maximum DEF increased with diffusivity, solubility, and the degree of airway wall absorption of vapors, as shown by a greater deposition of ethanol than MTBE. The vapor deposition efficiency as expressed by the dimensionless mass transfer coefficient correlated well with a product of Reynolds (Re) and Schmidt (Sc) numbers. In conclusion, MTBE and ethanol vapors deposit substantially in the upper airway structure with a marked enhancement of dose at local sites, and the deposition dose may be reasonably estimated by a functional relationship with dimensionless fluid flow and diffusion parameters.