Face Masks and Prevention of Respiratory Viral Infections: An Overview

11 12 Airborne transmission of respiratory viruses consists of three sequential steps: (1) release of 13 respiratory fluids in the form of droplets from the nose and mouth of an infected person, (2) 14 transport of the droplets through air, and (3) entry of the droplets into the nose and mouth of an 15 uninfected individual. Talking, coughing, and sneezing emit droplets across a spectrum of sizes. 16 The water in exhaled droplets begins to evaporate in air and, as a result, the droplets are reduced 17 in size shortly after being emitted. Face masks are effective for capturing droplets just released 18 from the nose and mouth. Studies indicate that more than 50% of community transmission of 19 SARS-CoV-2 is from asymptomatic and pre-symptomatic cases. Use of face masks by the public 20 can effectively reduce the chance of infected individuals unknowingly spreading the virus. In 21 addition to being an effective device for source control, face masks can protect the wearers from 22 inhaling virus-laden droplets. Cloth masks and disposable masks provide reasonable protection for 23 the public, while surgical masks and N95 respirators give higher levels of protection as needed in 24 healthcare settings. Made with varied materials, these masks have different structural 25 characteristics. The collection efficiency of a face mask depends on droplet size, face velocity, and 26 the structural characteristics of the mask. For a given mask, capturing droplets is more effective 27 during exhalation than during inhalation. Pressure drop across the mask should be taken into 28 consideration when selecting a face mask. The best face mask is the one that gives the highest 29 collection efficiency with the least pressure drop. For an effective protection, a mask should fit the 30 face properly. While face masks have proven adequate in reducing airborne transmission of SARS- 31 CoV-2 infections, continuous improvement is needed to better prepare for future respiratory viral 32 threats.