Abstract:
Background: Particulate matter (PM)-related health effects have been studied extensively, yet wildfire smoke-specific PM health effects and its dose-response relationship have not reached a consensus due to variations in exposure scenarios and health outcome measurements. Additionally, researchers have not tested the wildfire smoke PM intervention strategies, especially HEPA (high-efficiency particulate air)-filter portable air cleaners (PACs) and HEPA-installed powered air-purifying respirators (PAPRs), for their particle removal efficiency and effectiveness in a laboratory setting.
Objectives: This research was dedicated to designing and engineering an exposure system that can be generalized, repeated, adjustable, and easily accessed by future controlled wildfire smoke human exposure studies. Woodsmoke particle size distribution and composition from different sources were investigated and we also tested woodsmoke particle removal efficiency and effectiveness of HEPA-filter PACs and the HEPA-installed PAPR system.
Methods: A total of 15 experimental trials were conducted to test and trial-run the exposure system with different sources of woodsmoke generation. Woodsmoke particles were introduced from the generation sources to the mixing chamber, and eventually to the exposure chamber where we monitored the particle concentrations and size distribution with low-cost and research-grade instruments. We assessed the woodsmoke particle composition using an aethalometer and monitored the CO and CO2 levels using a gas monitor. In addition, HEPA-filter PAC woodsmoke particle removal efficiency and HEPA-installed PAPR particle removal effectiveness were tested in two of the experimental trials, separately, with woodsmoke particles introduced into the exposure chamber.
Results: With time series on the particle mass and count concentration plots, we demonstrated the ability to relatively tightly control the woodsmoke particle concentrations (50 μg/m3 and 90 μg/m3) inside the exposure chamber with both wood pellet stove and cooking smoke guns as smoke generation sources. We also observed an overall higher particle count concentration from the wood pellet stove (around 700 particles greater than 0.3 μm per cc) than the cooking smoke gun (around 500 particles greater than 0.3 μm per cc) despite the target woodsmoke particle mass concentration from the cooking smoke gun trial being higher than the wood pellet stove trial. The wood pellet stove (around 5000 particles smaller than 0.3 μm per cc) also was believed to generate more smaller particles by count than the cooking smoke gun (around 2000 particles smaller than 0.3 μm per cc) and the black carbon content was also higher in the wood pellet stove trial. In terms of the particle removal efficiency and effectiveness from PM mitigation strategies, we discovered that regardless of the HEPA filter age, when PACs were turned on at the highest speed, both new and used HEPA filters removed woodsmoke particles effectively at similar efficiency. HEPA-installed PAPR system also reduced the woodsmoke particle concentration from around 50 μg/m3 to close to 0 in less than 1 second.
Conclusions: The results further proved that with further refining and engineering of the exposure system, researchers would have the opportunity to standardize their wildfire smoke controlled human exposure study protocol and migrate to an exposure system that is similar to ours. Both HEPA-filter PAC and HEPA-installed PAPR systems are efficient and effective in woodsmoke particle removal to potentially improve indoor air quality and reduce occupational wildfire smoke exposure.