Upper respiratory tract surface areas and volumes of laboratory animals and humans: considerations for dosimetry models.

To facilitate the development of regional respiratory tract dosimetry comparisons between laboratory animal species and humans, published surface area (SA) and volume (VOL) data for the upper respiratory tract (URT) were reviewed. The review of the literature revealed that (1) different studies used different techniques to prepare specimens and make measurements, (2) different areas of the URT were measured, and (3) URT surface areas and volumes have been reported for a limited number of individual subjects within a species but for a relatively wide range of species. The published data are summarized in tables in this article. New measurements made in an F344 rat and in a female human subject are also presented. Despite the differences in experimental protocols, it was possible to fit allometric scaling equations to the data: In(SA, cm2) = -0.34 + 0.52 In(body weight, g) and In(VOL, cm3) = 1.70 + 0.78 In(body weight, g). Separate scaling equations were also fitted for rats alone. To illustrate the use of these scaling equations in quantitative human health risk assessment, two dose metrics (fractional absorption/cm2 URT SA and fractional absorption/g body weight) for predicted URT uptake in laboratory animals and humans were calculated for acrolein and epichlorohydrin. Expressed as an animal-to-human ratio, the 95% confidence interval for URT SA could change the predicted dose ratio by up to a factor of 2. Additional studies are needed to describe the entire URT (from the nares through the larynx) quantitatively and to decrease variability in scaling equation predictions as well as to develop additional species-specific scaling equations. Three-dimensional imaging techniques provide a noninvasive method to obtain URT surface areas and volumes in humans and the larger laboratory animals. Comparisons of magnetic resonance image (MRI) and computed tomography (CT) scans made as part of this study suggest that the greater clarity of the mucosal-air interface in the CT image provides better resolution for the study of anatomic features. Because there is no radiation exposure associated with MRI imaging, however, it is more safely used than CT scans in making repeated measurements in a subject to elucidate changes in URT geometry associated with normal nasal cycling or other physiological changes.