Investigation of the protection efficacy of face shields against aerosol cough droplets

Abstract

Simple plastic face shields have numerous practical advantages over regular surgical masks. In light of the spreading COVID-19 pandemic, the potential of face shields as a substitution for surgical masks was investigated. In order to determine the efficacy of the protective equipment we used a cough simulator. The protective equipment considered was placed on a manikin head that simulated human breathing. Concentration and size distribution of small particles that reached the manikin respiration pathways during the few tens of seconds following the cough event were monitored. Additionally, water sensitive papers were taped on the tested protective equipment and the manikin face. In the case of frontal exposure, for droplet diameter larger than 3 μm, the shield efficiency in blocking cough droplets was found to be comparable to that of regular surgical masks, with enhanced protection for portions of the face that the mask does not cover. Additionally, for finer particles, down to 0.3 µm diameter, a shield blocked about 10 times more fine particles than the surgical mask. When exposure from the side was considered, the performance of the shield was found to depend dramatically on its geometry. While a narrow shield allowed more droplets and aerosol to penetrate in comparison to a mask under the same configuration, a slightly wider shield significantly improved the performance. The source control potential of shields was also investigated. A shield, and alternatively, a surgical mask, were placed on the cough simulator, while the breathing simulator, situated 60 cm away in the jet direction, remained totally exposed. In both cases, no droplets or particles were found in the vicinity of the breathing simulator. Conducted experiments were limited to short time periods after expiratory events, and do not include longer time ranges associated with exposure to suspended aerosol. Thus, additional evidence regarding the risk posed by floating aerosol is needed to establish practical conclusions regarding actual transmittance reduction potential of face shields and surgical face masks.

Keywords:

• Aerosol monitoring
• cough simulator
• COVID-19
• image analysis
• personal protective equipment

Acknowledgments

This work was supported by the Israeli Ministry of Defense. The authors would like to thank Dr. Boaz Lev from the Israeli Ministry of Health for initiating the project, and Dr. Yehuda Alexander for his review and helpful comments.