[Publication date of latest article cited: February 22, 2021]

Each of the prevention methods below has limited effect, so we need to use all of them (Spinelli et al.).  For example, some coronaviruses in aerosols can flow through a mask or float more than 6 feet.  Some experts are calling this a “Swiss Cheese Respiratory Virus Pandemic Defence.”  Like slices of Swiss cheese standing up in a row, if some viruses go through a hole in the social distancing slice and a hole in the mask slice, they might get stopped by the ventilation slice (MacKay; Roberts S.).

Altogether, people using these methods are making a real effect, as shown indirectly by the reduced transmission of other respiratory-spread diseases.  The rates of colds and influenza decreased below the rates in the same months in previous years in the southern and northern hemispheres (Burki “Double Threat”; Jones; Spinelli et al.).


Physical distancing

[Publication date of latest article cited:February 22, 2021]

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.; Spinelli et al.; Stadnytskyi et al.; World Health Organization “Advice on the Use of Masks”).  For example, in an outbreak on a US Navy aircraft carrier, those who physically distanced from others, and avoided common areas were significantly less likely to get infected (Payne et al.).  A systematic review and meta-analysis of 38 studies of the effects of physical distancing in household, community, and healthcare settings of SARS-CoV-2 and related betacornaviruses [severe acute respiratory syndrome (SARS), and Middle East respiratory syndrome (MERS)] 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.).

Another method of social distancing is to form a pod or bubble, a closed, limited group with whom you can mingle face-to-face.  Theoretically, you could meet your needs for both safety and social interaction.  But, in actual practice, people’s pods vary, with some so open that they provide little protection (Gutman).  A model based on past events predicts that for indoor meetings, distancing reduces transmission, and in schools forming pods reduces transmission (Tupper et al.).


[Publication date of latest article cited: February 26, 2021]

Ventilation can reduce the numbers of SARS-CoV-2 floating in the air, and thus reduce or prevent inhaling the viruses (Spinelli et al.)  In the menu section “Aerosols,” in hospitals scientists found SARS-CoV-2 RNA in air samples in many rooms, with different kinds of patients, especially those having little ventilation no or low concentrations of SARS-CoV-2 RNA, possibly because they exchanged air at high rates or were using negative pressure ventilation.  To reduce indoor transmission, many people have been opening windows and pumping more outside air into buildings (de Man et al.;).  Increasing indoor humidity by using humidifiers could reduce COVID-19 transmission (Ahlawat; Bazant, Bush).  Some experts recommended methods for using air flow and filtering in school buildings so students and teachers can both learn and prevent COVID-19 transmission, as part of an overall healthy buildings program (Centers for Disease Control and Prevention “Ventilation in Schools”; Harvard TH Chan School of Public Health “For Health, Healthy Buildings”; Jones E, Young et al).

To help people understand and prevent airborne transmission risks, experts developed an online spreadsheet in which one can enter information on the characteristics of a place, and the spreadsheet will estimate numbers of people who could get infected (Jimenez).  Another model estimates safe limits on “cumulative exposure time” indoors, depending on the number people, time, ventilation, room dimensions, breathing rate, respiratory activity, face-mask use, and respiratory aerosols infectiousness (Bazant; Bazant, Bush).

Face masks

[Publication date of latest article cited: February 22, 2021]

Public health and medical organizations recommended that the general public wear masks (California; Centers for Disease Control and Prevention “Recommendations…”; Centers for Disease Control and Prevention “Scientific Brief: Community Use of Cloth Masks”; Centers for Disease Control and Prevention “Considerations for Wearing Masks”; Desai, Aronoff “Masks and Coronavirus”; Lerner et al; National Academies of Sciences “Effectiveness…”; San Diego County; World Health Organization “Advice on the Use of Masks”) for several reasons:

Masks also protect humans in society and medical facilities from COVID-19 (Centers for Disease Control and Prevention “Scientific Brief: Community Use of Cloth Masks”). For some examples:

  • In the Nebraska hospital described above in the menu section on “Other routes” and “fomite surfaces”, staff used powered air purifying respirators, N95 filtering facepiece respirators, and other personal protective equipment, and no staff were infected (Santarpia et al.).
  • In an outbreak on a US Navy aircraft carrier, those who wore masks were significantly less likely to get infected (Payne et al.)
  • 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.).
  • 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.).
  • In a health care system, before all personnel wore masks SARS-CoV-2 positivity increased from 0% to 21.32%.  When all wore masks, positivity decreased from 14.65% to 11.46% (Wang X, Ferro, et al.).   Masks were similarly effective in protecting people from SARS-CoV-2, other coronaviruses, and influenza in community and healthcare situations (MacIntyre, Chughtai).
  • 14 patients were isolated with uninfected family members in South Korean hospitals.  One or both the infected and uninfected people wore masks, and were monitored and recorded by staff.  None of the uninfected family members became infected (Lee EJ, Kim DH, 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 few or no transmission cases when almost all people wore masks (Freedman, Wilder-Smith.).
  • US counties’ mask mandates were associated with reductions in case growth rates 1 – 100 days after implementation (Guy et al.).
  • Systematic reviews and meta-analyses of many studies of the effects of face mask use found that wearing face masks was statistically significantly associated with lower risk of COVID-19 (Chu et al.; Li Y, Liang, et al.).   But other reviews of many randomized trials and observational studies found that these did not have sufficient evidence and methodological rigor to meet clinical trial standards to establish that masks prevented SARS-CoV-2 (Chou et al. “Masks for Prevention“; Chou et al. “Update Alert 3: Masks for Prevention“; Eke, Eke; Qaseem et al.).

Altogether these studies do not completely prove that wearing masks prevent COVID-19 transmission.  But most of the evidence supports the hypothesis that masks prevent transmission, and little evidence supports that they do not prevent transmission.  So, wearing a mask is definitely worth the little effort required.

Since a large portion of infected people are asymptomatic or presymptomatic, if everyone wears a mask when near other people, those asymptomatic 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.; Gandhi, Marr; Howard, Huang, et al. ; Mandavilli “Confused about Masks?”; Royal Society).  In areas with mask wearing mandates, hospitalization rates rose more slowly than areas without mandates (Durkee; Vanderbilt University).  A model based on past events predicts that for many kinds of events, wearing masks reduces transmission (Tupper et al.).

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”; Spinelli et al.; 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 (World Health Organization “Key Criteria”), 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 or t-shirt 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”; Clapp et al.; Clase et al.; Fischer et al.; Gandhi, Marr; Hao et al.; Hill et al.; Konda et al; Lindsley et al.; Robertson ; Rodriguez-Palacios et al.; Ueki et al.).  Wet cotton masks filter more droplets than dry ones, because wet cotton fibers swell and reduce spaces between them (Akhtar J, Garcia, et al.).

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.).  A literature review found mask wearing had little effect on daily activities, exercise, and rehabilitation (Haraf et al.).

It is difficult to see a mask wearer’s lower facial expression, and hear their spoken words clearly, which reduces verbal and non-verbal communication.  Mask wearers can counteract this by speaking loudly and clearly with their face toward the listeners, using more upper facial expressions and body language, and communicating online without masks (Mheidly et al.; Sugar).

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 (Gandhi, Marr; Laine et al.; Mandavilli “Confused about Masks?”; Prasad “About the Danish Mask Study”).  Also, the low numbers of antigen-positive participants, and the possibility of false positives might have biased the calculations toward finding little difference between groups (Frieden, Cass-Goldwasser).

Face Shields and Eye Glasses

[Publication date of latest article cited: February 22, 2021]

Wearing eye glasses or clear plastic face shields might protects the wearer’s eyes from droplets and aerosols (Coroneo, Collington; Maragakis). 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).  A systematic review and meta-analysis of 15 studies of the effects of using them found that “eye protection was associated with lower risk of infection” against SARS-CoV-2 and related betacornaviruses [severe acute respiratory syndrome (SARS), and Middle East respiratory syndrome (MERS)] (Chu et al.).  Because of evidence like that, 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). 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.).

Cleaning and Chemical Disinfecting Surfaces

[Publication date of latest article cited:  February 22, 2021]

Hand, face, and surface hygiene can reduce the numbers of SARS-CoV-2 the person receives, and thus prevent SARS-CoV-2 transmission or reduce infection severity (Spinelli et al.).  In hospitals with many COVID-19 patients, cleaning removed viruses from surfaces.  For example, in Wuhan hospitals, scientists and medical staff found SARS-CoV-2 RNA on surfaces, cleaned them and their hands, sampled the surfaces again, and found no SARS-CoV-2 RNA (Ong et al.).

Many commercially available disinfectants and hand sanitizers can clean and inactivate this novel coronavirus SARS-CoV-2 (Center for Biocide Chemistries; Chin AWH, Chu,et al.; Department of Homeland Security “Evaluation of Disinfectant Efficacy” ;Kratzel et al.).  Early in the pandemic, the EPA made a list of 431 products they thought probably disinfected SARS-CoV-2 because they damaged viruses’ lipid envelopes (like coronaviruses) and protein capsids (like rhinoviruses and noroviruses).  Most of them contain as active ingredients quaternary ammonium compounds (QAC), hydrogen peroxide, sodium hypochlorite (bleach), ethanol (ethyl alcohol), or isopropanol (isopropyl alcohol) (Environmental Protection Agency “List N”).  Later experiments found that two Lysol sprays disinfected specifically SARS-CoV-2 (Environmental Protection Agency “EPA Approves…”).  Other coronaviruses infecting humans, including SARS, MERS, and HCoV (a cause of the common cold) can be disinfected by 62%–71% ethanol (such as alcohol gel), 0.5% hydrogen peroxide or 0.1% sodium hypochlorite (bleach) (Kampf et al.).  CDC recommended how to clean surfaces and disinfect SARS-CoV-2 (Centers for Disease Control “Cleaning…”).   Ordinary soap and water also destroy SARS-CoV-2 and disinfect hands and other surfaces (Chin, Poon; Dehbandi, Zazouli; World Health Organization “Water, sanitation, hygiene”).

Disinfecting surfaces reduced coronaviruses in the air.  In Wuhan hospitals, scientists and medical staff found SARS-CoV-2 RNA in the air.  Then they cleaned the surfaces, wore more personal protective equipment (PPE), and separated severe patients from moderately symptomatic patients in different wards or buildings.  Then they sampled the air again, and found less airborne SARS-CoV-2 RNA, which shows that these actions might prevent aerosol transmission (Liu Y, Ning, et al.).

Disinfecting surfaces probably also reduced infections in households.  A retrospective study found that households that had one infected person, and disinfected the home, transmitted to fewer other people in the household than households that id not disinfect (Wang Y, Tian, et al.)

Because of the many ways SARS-CoV-2 can stay on surfaces, if we are caring for a COVID-19 patient, we should wear a mask, and perhaps a clear face shield, not touch many surfaces in the patient’s room, wash our hands after each interaction with the patient, and wash or change clothes and shower after every few interactions with the patient.  But if we go out in public or to a store with no known infected people, then we do not need to do all those precautions each time.  We should, however, wash hands after going out, and wipe packages (Centers for Disease Control and Prevention “Households…”; Desai, Aronoff “Food Safety and COVID-19”; Food and Drug Administration; Herbst; Parker-Pope “Is the virus on my clothes?”; Stump and Nguyen).

After a year of research on this new pathogen, there is more evidence for droplet and aerosol transmission than fomite transmission, so perhaps we do not need to emphasize cleaning as much as some previously thought (Centers for Disease Control “CDC Updates”; Goldman; The Lancet Respiratory Medicine).

Mouthwashes and Nasal Rinses

[Publication date of latest article cited: November 13, 2020]

Mouthwashes damage SARS-CoV-2 lipid envelopes, which might prevent some people from getting infected or infecting others with COVID-19 (O’Donnell et al.).  Scientists tested common over-the-counter mouthwashes in artificial in-vitro laboratory conditions that mimic the nasopharyngeal secretions in people’s mouths.  They found that mouthwashes containing cetylpyridinium chloride, ethanol, methyl, thymol, povidine iodine, dequalinium chloride, and benzalkonium chloride inactivated almost all the SARS-CoV-2 (Meister et al.; Statkute et al.).  Another study showed that some over-the-counter nasal rinses and mouthwashes inactivated the coronaviruses that cause common colds (Meyers et al.).  Scientists should do more research on people using these methods.  But people should not over-interpret this to mean that these products will protect or cure most people from COVID-19 (Wu “No, Mouthwash Will Not Save You”).

Sunlight, Ultraviolet, and Heat

[Publication date of latest article cited: December 9, 2020]

Artificial ultraviolet light inactivates SARS-CoV-2 rapidly.  It damages the RNA, and the virus cannot replicate.  UVC has much more effect than UVA and UVB (Derraik et al.; Heilingloh et al.).  UVC used for 10 seconds reduced SARS-CoV-2 by 88% (Kitagawa et al.).  It reduced SARS-CoV-2 in wet droplets to undetectable levels in 4 seconds, and in dried droplets in 9 seconds (Storm et al.).  DUV-LED reduced SARS-CoV-2by 99%, so the viruses had no cytopathic effect on cells (Inagaki et al.).  Since ultraviolet lights are so easy to use effectively, companies are selling small hand-held and large wheeled devices for inactivating SARS-CoV-2 in homes and workplaces.

Experiments found that sunlight inactivates SARS-CoV-2 in minutes.  Simulated summer mid-day sunlight reduced SARS-CoV-2 in aerosols by 90% in 8 – 10 minutes.  Simulated winter sunlight reduced the viruses in aerosol by 90% in 19 minutes.  But indoors without sunlight, no detectable decrease occurred in 60 minutes (Department of Homeland Security “S & T’s Research”; Schuit et al.).  On surfaces, 3 – 6.8 minutes of simulated summer sunlight reduced SARS-CoV-2 in simulated saliva by 90%.  Simulated winter sunlight took 14.3 minutes to reduce them by 90%.  Without sunlight, this took 18 – 51 hours (Department of Homeland Security “S & T’s Research”; Department of Homeland Security “Factors Affecting the Stability”; Ratnesar-Schumate et al.).  Sunlight reduced SARS-CoV-2 faster than heat and humidity did (Department of Homeland Security “Predicting the Decay”).  So, the authors of these studies recommended doing activities outdoors in sunlight.

Experiments also found that heat inactivates SARS-CoV-2 (Department of Homeland Security “S & T’s Research”; Magurano et al.).  On surfaces, at 20° C they lasted 28 days (Riddell et al.).  Indoors, cool, and dry, these viruses reduce by 90% in 51 hours (Department of Homeland Security “Factors Affecting”).  At 55° C (130° F) that takes 0.6 hour (Department of Homeland Security “SARS-CoV-2 Surface Contamination”).  Airborne, they are most stable at 10° C, 20% relative humidity, without sunlight, so they do not reduce detectably in 1 hour (Department of Homeland Security “Predicting the Decay”).  Increasing humidity by itself decreased SAR-CoV-2 by little, but higher humidity increased the effects of sunlight (Department of Homeland Security “Predicting the Decay”) and heat (Biryukov et al.; Department of Homeland Security “S & T’s Research”; Schuit et al.).  Heating personal protective equipment (PPE) such as masks can destroy these viruses so people can reuse the PPE (Derraik et al.). Since the temperatures that inactivate these viruses (55° C) are hotter than humans can live in, summer weather does not reduce these viruses enough to reduce transmission greatly.  Other factors probably more determine transmission (Chin, Chu, et al.; National Academies of Science “SARS-CoV-2 survival…”).


Practicing the Recommendations

[Publication date of latest article cited: December 23, 2020]

People vary in willingness or opportunities to practice social distancing, mask wearing, or hand washing.  Surveys across the US found those more practicing these non-pharmaceutical interventions included: people with chronic cardiometabolic, respiratory, or immune diseases (Camacho-Rivera et al.); US Democrats and independents (Levanthal et al.; Pedersen, Favero); women and US Asians (Pedersen, Favero); US Blacks (Garneir et al.), people who perceive COVID-19 as a threat (Kasting et al.; Pedersen, Favero); non-essential workers, because essential workers must work in crowded or indoor places, and people in high population density areas (Garneir et al.).