Quantifying Uncertainty in Low Velocity Human Aspiration Studies: Effect of Secondary Aspiration and Thin-walled Reference Sampling in Low Velocity Conditions

When particles strike a surface they can either rebound or deposit on the surface. If particles rebound on human facial skin they can be re-entrained into the airflow and subsequently inhaled, increasing aspiration efficiency estimates. To accurately model particle bounce on human facial skin and the effects on aspiration efficiency estimates, a realistic estimate of human facial skin coefficient of restitution (CoR) is necessary, which is the ratio of the rebound velocity to the impacting velocity of the particle. The CoR for human skin has been shown to be highly variable, and information on how the CoR varies across the face with age, gender, temperature, humidity and BMI is limited. This work investigated the effects of temperature and humidity levels as well as age and gender on facial skin CoR. The CoR was found to vary across five specific locations on the face, with values ranging from 0.55 to 0.75. Site, gender, and sampling season were found to be significant, although temperature and humidity levels were not found to be significant. While gender, and sampling season were statistically significant, changes in values were relatively small (0.05 at most) and are not considered practically significant. CoR by sampling location on the face varied by as much as 0.25. The use of uniform CoR value as modeling input parameters or for mannequin facial surfaces in experimental wind tunnel studies may not be accurate due to the high variability in CoR between facial sampling location.