The use of respiratory protective equipment redefines breathing zone and increases inhalation exposure of self-exhaled pollutants

Abstract

The size of the breathing zone influenced by respiratory protective equipment (RPE) plays an important role in evaluating inhalation exposure (self-exhaled and external pollutants). The objective of this study was to define this size and evaluate the inhalation exposure of self-exhaled pollutants. The flow characteristics of ten types of RPE were experimentally investigated to obtain boundary conditions for Computational Fluid Dynamics (CFD) simulations. The CFD modeling framework of RPE was established and validated scientifically with experimental data, which was used to investigate the time-series characteristics of the thermo-fluid parameters inside and outside the RPE, the size of the breathing zone, and the influence of RPE on inhalation exposure. The results show that the range of viscous resistance and inertial resistance coefficients were 5.13 × 108–4.35 × 1010 and 1.42 × 105–7.62 × 105, respectively, for different types of RPE. The time-area averaged temperature and relative humidity inside the N95 mask would be up to 29.8 °C and 63.3 %, respectively, causing discomfort and possibly health problems. The breathing zone of an RPE wearer can be roughly defined as a hemisphere region with a radius of 11 cm around the mouth. Wearing RPE would lead to excessive inhalation exposure, with 36.5–59.8 % of the exhaled pollutants being re-inhaled. The time-averaged oxygen content in inhaled air would decrease from 21 % to 18.0–19.2 %, implying that long-term wearing RPE may impair the wearer's well-being. The data obtained provide boundary conditions for CFD simulation of different types of RPE, and give guidance for monitoring inhalation exposure and improving RPE.