Interindividual Variability in Nasal Filtration as a Function of Nasal Cavity Geometry

Background: Interindividual variability in nasal filtration is significant due to interindividual differences in nasal anatomy and breathing rate. Two important consequences arise from this variation among humans. First, devices for nasal drug delivery may furnish quite different doses in the nasal passages of different individuals, leading to different responses to therapeutic treatment. Second, people with poor nasal filtration may be more susceptible to adverse health effects when exposed to airborne particulate matter (PM) due to greater lung deposition. Although interindividual variability of nasal filtration has been reported by several authors, a relationship for predicting filtration efficiency from nasal anatomy and ventilation is still lacking. Such a relationship is needed to (1) devise nasal drug delivery systems and (2) define limits of exposure to PM that are effective for the human population at large. Methods: Anatomically correct nasal replicas of five adults (four healthy individuals and one atrophic rhinitis patient) were used in aerosol experiments to measure nasal deposition of 1-12-mu m particles. The dependence of nasal filtration on nasal anatomy and breathing rate was investigated using various definitions of the Stokes number as well as phenomenological Impaction Parameters proposed in the literature. Results: Interindividual variability among the healthy adults was nearly eliminated when nasal filtration was plotted against a specific definition of the Stokes number or against a pressure-based Impaction Parameter. Nasal filtration in the atrophic rhinitis patient was lower than in the healthy subjects. Conclusions: The new definition of the Stokes number introduced in this study, which is based on a new definition of the characteristic diameter of the nasal passages, nearly eliminated interindividual differences in nasal filtration. Our results suggest that it is possible to estimate nasal filtering efficiency using measurements of transnasal pressure drop.