The outbreak of the COVID-19 pandemic, in 2020, has accelerated the need for personal protective equipment (PPE) masks as one of the methods to reduce and/or eliminate transmission of the coronavirus across communities. Despite the availability of different coronavirus vaccines, it is still recommended by the Center of Disease Control and Prevention (CDC), World Health Organization (WHO), and local authorities to apply public safety measures including maintaining social distancing and wearing face masks. This includes individuals who have been fully vaccinated. Remarkable increase in scientific studies, along with manufacturing-related research and development investigations, have been performed in an attempt to provide better PPE solutions during the pandemic. Recent literature has estimated the filtration efficiency (FE) of face masks and respirators shedding the light on specific targeted parameters that investigators can measure, detect, evaluate, and provide reliable data with consistent results. This review showed the variability in testing protocols and FE evaluation methods of different face mask materials and/or brands. In addition to the safety requirements needed to perform aerosol viral filtration tests, one of the main challenges researchers currently face is the inability to simulate or mimic true aerosol filtration scenarios via laboratory experiments, field tests, and in vitro/in vivo investigations. Moreover, the FE through the mask can be influenced by different filtration mechanisms, environmental parameters, filtration material properties, number of layers used, packing density, fiber charge density, fiber diameter, aerosol type and particle size, aerosol face velocity and concentration loadings, and infectious concentrations generated due to different human activities. These parameters are not fully understood and constrain the design, production, efficacy, and efficiency of face masks.
Advanced Research and Development of Face Masks and Respirators Pre and Post the Coronavirus Disease 2019 (COVID-19) Pandemic: A Critical Review