Major articleQuantifying exposure risk: Surgical masks and respirators
Section snippets
Experimental setup
Figure 2 is a schematic representation of our experimental setup. To quantify exposure, we constructed a chamber measuring 135 ft3 (5 ft length × 4.5 ft width × 6 ft height). The chamber was placed next to a hood with a small connection with a fixed, defined flow providing 6 air exchanges/hr. Aerosols were created by an AeroTech II nebulizer (3 devices used in rotation; Biodex, Shirley, NY) connected to an air tank with a flow of 10 L per minute. The nebulizer was filled with 3 mL of 0.9%
Aerosol particle distribution
Complete particle distributions and mass median aerodynamic diameters (MMAD) are summarized in Fig 4. Data are plotted as log aerodynamic diameter versus probability with confidence intervals. Particles leaving the source were similar to exhaled droplets in vivo because approximately 95% of the particles were less than 2 μm, with an average MMAD of 1.046 μm (95% CI: 0.984-1.11).15, 16 With environmental flow (upper panel), particles in the vicinity of the receiver when no masks were worn were
Discussion
The goal of mask protection is to reduce exposure to the receiver independent of environmental engineering control systems presumably by filtration. Our data, however, indicate that mask protection can be more effective than that provided only by filtration if the interaction between mask deflection of particles and environmental airflow is utilized. For exhaled particles, our study demonstrates the value of manipulating the source rather than trying to simply protect the receiver. The most
Future studies
Our in vitro model cannot guarantee in vivo results. However, our observations quantify important contributions of multiple interactive mechanisms of action that affect exposure, namely dilution, deflection, and filtration to allow more structured design of modified masks and better define paths for future clinical trials. For example, our experiments suggest that more studies are needed to optimize mask deflection. Also, we used a tidal breathing pattern; further studies focused on either a
Conclusion
Mask filtration, applied either at the source or the receiver, does not play a significant role in reducing exposure to the recipient unless a respirator is physically sealed to the face of the source. Deflection of exhaled particles, such as can be achieved with a surgical mask worn at the source, achieves far greater levels of protection than an N95 respirator on the recipient. Our study defines the importance of respiratory source control in reducing exposure risk.
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Supported in part by Crosstex International Inc.
Conflicts of interest: G.C.S. serves as a consultant to Crosstex International Inc. The remaining author has no conflicts to report.