Quantifying exposure risk: Surgical masks and respirators

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.

Key Words: Aerosol, tidal breathing, infection control, workplace protection factors