Variability in coefficient of restitution in human facial skin

Background: If particles rebound on human facial skin, they can be re-entrained into the airflow and subsequently inhaled, increasing aspiration efficiency estimates. A realistic estimate of facial skin coefficient of restitution (CoR) is necessary to accurately model particle bounce. This study investigated the effects of sampling location, temperature, humidity levels, age, gender, and BMI on facial skin CoR. Methods: A torsional ballistometer was used to measure facial CoR for 30 participants divided into three age groups (18-30, 31-40, and 41-65 years), at three temperatures and three humidity levels. The study was repeated twice: once in the late winter and once in the early summer to capture the seasonal variability. Results: The CoR significantly varied across five facial locations, with values ranging from 0.55 to 0.75. Gender, sampling season and the interaction between sampling location and age were found to be significant, but changes in values were relatively small (0.05 at most) and are not considered practically significant. Conclusion: CoR was non-uniform across the face. 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 locations.