Efficacy of face shields to ambient aerosols in local indoor and outdoor setting

Abstract

The vacillating pronouncements on the policy of wearing face shields of government officials in June 21, 2021, especially in outdoor settings, as a control measure to lower COVID-19 cases has led to public confusion and questions on its efficacy. A review of literature shows face shields are effective in reducing the transmission of pathogens from cough-induced aerosol in close proximity. However, previous findings show the effectiveness is limited to large particles or fomites. As more evidence shows transmission of SARS-Cov-2 through aerosols, there is limited experimental data on its effectiveness on smaller particle especially in the local setting. In this study, the roles of face shields and face masks in the reduction of inhalable ambient aerosols in an indoor and outdoor setting are quantified. Using a bimodal log-normal size distribution of ambient aerosols, changes in aerosol number concentration were compared. Results show wearing of mask and face shield significantly drops aerosol inhalation, wearing only a mask is almost the same (p = 0.186 difference) as wearing a mask and face shield tandem, and a face shield without masks is no different from having no facial covering/protection at all.