Studies of inspiratory airflow patterns in the nasal passages of the F344 rat and rhesus monkey using nasal molds: relevance to formaldehyde toxicity.

For highly water soluble and reactive gases, such as formaldehyde, the reported distribution of nasal lesions in rats and rhesus monkeys following inhalation exposure may be attributable, at least in part, to regional gas uptake patterns that are a consequence of nasal airflow characteristics. Inspiratory nasal airflow was studied at flow rates across the physiologic range using a unidirectional dynamically similar water-dye siphon system in clear acrylic molds of the nasal airways of F344 rats and rhesus monkeys. In both species there were complex and inspiratory flow streams, exhibiting regions of simple laminar, complex secondary (vortices, eddies, swirling), and turbulent flows, with only minor effects of the volumetric flow rates studied on these flow patterns. There was a precise association between points of dye intake at the nostril with complex but generally coherent streaklines throughout the nose, indicating the potential for sensitive dependence of nasal airflow on nostril geometry. On the basis of these studies, a classification for the major airways (meatuses) in the nasal passages of rats and rhesus monkeys was proposed. The spiral shape of the anterior nasal airway of the rat was considered to play an important role in local mixing of inspired airstreams. In the rhesus monkey, the complex geometry of the nasal vestibule contributed to the formation of secondary flows and turbulence in the anterior nose, which represents a potentially important difference between rheusus monkeys and humans. There was a good correlation between routes of flow, regional secondary flows, turbulence, and impaction of airstreams on the airway wall, with the reported distribution of formaldehyde-induced nasal lesions in rats and rhesus monkeys. These studies support the proposal that nasal airflow patterns play an important role in the distribution of lesions induced by formaldehyde.