[Date of latest publication cited: October 30, 2020]
As described in this web site’s sections “Saliva and mucous droplets” and “Aerosols,” larger droplets often fall from the air in one or two meters, and small aerosol particles can float for many meters. So, the greater the distance you are from an infected person, the less likely that you will inhale many of the viruses they exhaled (Anfinrud et al.; Asadi, Wexler, et al.; Bourouiba; Brosseau; Dhand, Li; Lerner et al.; Meselson; Tang, Li et al.; Stadnytskyi et al.). A comparison of studies of the effects of physical distancing in household, community, and healthcare settings found that “at least 1 m physical distancing seem to be strongly associated with a large protective effect, and distances of 2 m could be more effective” (Chu et al.). A study comparing three levels of distancing among indoor concert attendees found that more distancing led to fewer infections (Noack). A survey in Maryland found physical distancing was more associated with lack of COVID-19 infection than other variables were (Clipman et al.).
Face masks and shields
[Date of latest publication cited: November 20, 2020]
Public health and medical organizations recommended mask wearing (California; Centers for Disease Control and Prevention “Recommendations…”; Centers for Disease Control and Prevention “Scientific Brief: Community Use of Cloth Masks”; Desai, Aronoff “Masks and Coronavirus”; Lerner et al; National Academies of Sciences “Effectiveness…”; San Diego County). Masks can probably prevent some COVID-19 transmission (National Academies “Airborne Transmission of SARS-CoV-2”; ), for both the wearers and the people around them (Centers for Disease Control and Prevention “Scientific Brief: SARS-CoV-2 and Potential Airborne Transmission”; Centers for Disease Control and Prevention “Scientific Brief: Community Use of Cloth Masks”; Mandavilli “Confused about Masks?” ). They reduce the numbers of aerosols and viruses going through them (Alsved et al.; Bandiera et al.; Makison Booth et al.; Milton et al.; O’Kelly et al.; Ueki et al.; Viola et al.). Homemade fabric face masks reduce the distance droplets and aerosols spread from mouths (Aydin et al.; Bandiera et al.; O’Kelly et al.; Rettner; Rodriguez-Palacios et al.; Verma et al. “Visualizing the effectiveness”; Viola et al.). Masks prevent some larger droplets and aerosols from being dispersed from the wearer to other people (Anfinrud et al.; Aydin et al.; Bandiera et al.; Mueller, Fernandez; Ueki et al.). Masks also prevent infected people from releasing respiratory viruses into the air (Leung et al.). When COVID-19 infected hamsters were near other uninfected hamsters, and surgical mask material was placed between them, the other hamsters were infected less often and less seriously than when there was no mask (Chan J, Yuan et al.; Medical Xpress).
Masks also protect humans in society from COVID-19 (Centers for Disease Control and Prevention “Scientific Brief: Community Use of Cloth Masks”). Comparing contacts of COVID-19 patients who got infected with those not infected found that the uninfected contacts wore a mask consistently, stayed more than 1 meter (3 feet) distance, and washed hands often (Doung-Ngern et al.). For example, two hair stylists with symptomatic COVID-19 infections worked with 139 clients in a salon, while both stylists and clients wore masks. None of the clients, their contacts, and other stylists in the salon reported symptoms. About half were tested, and all of those found negative (Hendrix et al.). Retrospective studies showed that when the first infected case in a family wore a mask, they transmitted to other family members at less than half the rates of families in which the first case did not wear a mask (Hong L, Lin, et al.; Wang Y, Tian, et al.). Masks were similarly effective in protecting people from SARS-CoV-2, other coronaviruses, and influenza in community and healthcare situations (MacIntyre, Chughtai). Some wondered if difficulty in breathing through a mask could reduce the oxygen in wearers (called “hypoxia”). An experiment showed that mask wearing did not reduce blood oxygen saturation in older people potentially sensitive to oxygen reduction (Chan N, Li, et al.).
Since a large portion of infected people are asymptomatic or presymptomatic, if everyone wears a mask when near other people, they could prevent spreading these viruses to others. If most people wore face masks in public, that would prevent some droplets and aerosols from an infected person reaching uninfected people, which could help reduce transmission, and show support for community pandemic responses (Association of American Medical Colleges; Centers for Disease Control and Prevention “Scientific Brief: Community Use of Cloth Masks”; Cheng KK, Lam, et al.; Howard et al.; Mandavilli “Confused about Masks?”; Royal Society). A comparison of several studies of the effects of face mask use found that “…wearing face masks protects people (both health-care workers and the general public) against infection by these coronaviruses…” (Chu et al.). In areas with mask wearing mandates, hospitalization rates rose more slowly than areas without mandates (Durkee; Vanderbilt University).
Hypothetically, a person wearing a mask could inhale fewer viruses than if they do not wear a mask. Even if the mask wearer gets infected, they might develop less serious symptoms, or no symptoms. Inhaling small number of SARS-CoV-2 without serious symptoms might even cause immunity (Gandhi, Beyrer et al “Masks Reduce Viral Inoculum”.; Gandhi, Rutherford “Facial Masking for Covid-19”; Wu KJ “Masks may reduce viral load”). But these hypotheses have not been proven. The relationships between viral dose, viral replication, and disease severity of COVID-19 are complicated. Some people might misunderstand that they could wear a mask to develop immunity with low risk of severe symptoms (Rasmussen, Escandón, et al.; Brosseau, Roy, et al.). Testing these hypotheses on humans might not be feasible or ethical, but experiments on ferrets and hamsters supported some of the ideas in these hypotheses. Perhaps more experiments could test more aspects of the hypotheses (Gandhi, Rutherford “Response”).
Aerodynamics experiments found that the face mask materials with optimum balance of filtration and breathability were quilter’s cotton, silk, polyester chiffon, flannel, denim, bed sheets, paper towel, canvas, polyester satin, and shop towel. Higher density (thread count) fabrics filtered better. Using layers of different materials filtered over 80% of aerosols, probably because of both mechanical filtering and electrostatic attraction (Centers for Disease Control and Prevention “Scientific Brief: Community Use of Cloth Masks”; Fischer et al.; Konda et al; Lindsley et al.; Robertson ; Rodriguez-Palacios et al.; Ueki et al.).
In an experiment in Denmark, all participants did social distancing, and some wore masks outside their homes, and others did not. The difference in infections between the two groups was not statistically significant (Bundgaard et al.; Kolata “Study questions whether masks”). But they did the study after a lockdown at the start of the pandemic, when the numbers of new cases in the country per day ranged from 56 to 473 (McCrory; Michas). Also, they did not study if nearby people got infected from the study participants. So, the study shows that mask wearing makes little difference when incidence is low and people are using other prevention methods (Mandavilli “Confused about Masks?”; Prasad).
Experiments found that wearing clear plastic face shields protects the wearer from droplets, especially the eyes. For example, when a Texas hospital mandated face shield use, COVID-19 and other hospital acquired infections decreased (Walker “Face Shields”). When Indian Community Health Workers did home visit contact tracing while protecting themselves with physical separations and personal protective equipment (alcohol hand gel, masks, gloves, and shoe covers), 19% got infected. When they added face shields, none got infected (Bhaskar, Arun). Experts recommended that people start wearing simple face shields when out in the community.
Several companies sell them, and people can make and disinfect their own face shields (Bacher, Ianzito; Infectious Disease Society of America; Perencevich et al. including the comments). A comparison of several studies of the effects of using them found “that eye protection could confer additional benefit” (Chu et al.).
But experiments found that when people exhaled while wearing a face shield, the aerosols flowed around the sides into the open air. When they exhaled wearing masks with valves, aerosols flowed through the valve into the open air (Verma et al .“Visualizing droplet dispersal”).
If people used face masks, face shields, hygiene, and distancing, and changed their cultures to expect each other to maintain those norms, then they could greatly reduce community transmission (Gawande). For example, circumscribed areas, traffic restrictions, home confinement, social distancing, centralized quarantine, and universal symptom survey probably contributed to decreases in infections and severe case rates in Wuhan, which provided an example of epidemic control methods people elsewhere used (Hartley & Perencevich; Pan A, Liu, et al.).
People conducted natural, real-life experiments with the effectiveness of masks by wearing masks on some air flights and not wearing them on some others. A review of the scientific literature on this found much secondary transmission on flights when few people wore masks, and little or no transmission cases when almost all people wore masks (Duocleff; Freedman, Wilder-Smith.).