Contribution of facial feature dimensions and velocity parameters on particle inhalability

Abstract

To examine whether the actual dimensions of human facial features are important to the development of a low-velocity inhalable particulate mass sampling criterion, this study evaluated the effect of facial feature dimensions (nose and lips) on estimates of aspiration efficiency of inhalable particles using computational fluid dynamics modeling over a range of indoor air and breathing velocities. Fluid flow and particle transport around four humanoid forms with different facial feature dimensions were simulated. All forms were facing the wind (0.2, 0.4 m s(-1)), and breathing was simulated with constant inhalation (1.81, 4.3, 12.11 m s(-1)). The fluid flow field was solved using standard k-epsilon turbulence equations, and laminar particle trajectories were used to determine critical areas defining inhaled particles. The critical areas were then used to compute the aspiration efficiency of the mouth-breathing humanoid. One-tailed t-tests indicated that models with larger nose and lip features resulted in significantly lower aspiration efficiencies than geometries with smaller features, but the shape of the orifice into the mouth (rounded rectangle versus elliptical) had no effect on aspiration efficiency. While statistically significant, the magnitudes of differences were small: on average, the large nose reduced aspiration efficiency by 6.5% and the large lips reduced aspiration efficiency by 3.2%. In comparison, a change in breathing velocity from at-rest to heavy increased aspiration efficiency by an average of 21% over all particle sizes, indicating a much greater impact of aspiration efficiency on breathing rate in the facing-the-wind orientation. Linear regression models confirmed that particle diameter and breathing velocity were significant predictors to the aspiration fraction, while the facial feature dimensions were not significant contributors to a unifying model. While these effects may be less pronounced as the orientation changes from facing-the-wind, their impact confirms the importance of breathing velocity and, to a lesser extent, facial feature dimensions on exposure estimates in low freestream velocities typical of occupational environments.

Fig. 1

Illustration of the four facial feature dimensions examined: (a) baseline small nose and large lips, (b) large nose and large lips, (c) small nose and small lips, and (d) baseline nose and lips but elliptical orifice.

Fig. 2

Illustration of computational domain. The humanoid is facing the wind, with the positional origin located at the center of the mouth on the orifice plane. This work uses the simplified cylindrical torso. Computational domain dimensions are indicated.

Fig. 3

Aspiration efficiency over all test conditions by particle size. Data are separated by inhalation velocity (U_m, m s⁻¹). The error bars on the mean data line represent 1 SD over all data combined for each particle size, regardless of geometry or velocity conditions.

Fig. 4

Simulated (markers) and fit (solid lines) estimates of aspiration efficiency by particle size and inhalation breathing velocity using (a) linear form (ID-4) and (b) linear form with d_ae² term (ID-6).