Major article
Quantifying exposure risk: Surgical masks and respirators

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Background

The interaction between the source of respiratory infectious aerosols and the receiver has not been investigated. Using a bench model, we measured the effects of filtration and deflection achieved with surgical masks and N95 respirators.

Methods

We constructed a chamber designed to produce radiolabeled wet aerosols simulating contaminated particles exhaled during tidal breathing (source). Particles within the chamber were exposed to either 6 or 0 air exchanges/hr. Aerosols were defined by cascade impaction. Source aerosols were exhaled via a ventilated mannequin head suitable for mask protection. A similar ventilated head within the chamber assessed recipient exposure (receiver). A filter within the receiver quantified exposure. Two types of masks, an N95 respirator and surgical mask, were tested. Data were presented as percent of nebulized particles on the receiver filter (exposure) and simulated workplace protection factor (sWPF).

Results

In the presence of chamber air exchange, applying a mask on the source (primarily deflection) resulted in significant reduction in exposure to the receiver (sWPF170-320). Masks on receiver (filtration) did not significantly reduce exposure from that of no masks (sWPF1.37-2.21), except with a Vaseline seal (sWPF118). With 0 air exchanges/hr, only Vaseline seal was effective in reducing exposure (sWPF 16-101).

Conclusion

In a ventilated space, deflection of exhaled particles with a mask worn at the source achieved far greater levels of protection than any mask on the receiver. Mask filtration at source or receiver did not play a significant role in reducing exposure.

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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  • Cited by (0)

    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.

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