Prevention Methods

[Publication date of latest article cited: November 15, 2023]

Now that most people have some partial immunity to SARS-CoV-2, because they had a natural infection and survived or received vaccinations, societies shifted from a pandemic situation to endemic. But, the vaccine-induced or infection-induced immunity wanes, and SARS-CoC-2 is mutating new variants. So even partially immune people could still get COVID-19 disease again, possibly causing long COVID or death. Consequently, people should take a “vaccines-plus approach” and get up-to-date vaccines, and use non-pharmaceutical prevention methods when in higher risk situations (Lazarus et al.).

Each of the prevention methods  described below has limited effect, so all of them can protect us in risky situations (Aho Glele, de Rougemont; Christakis; De la Torre, Oren; Morrow et al.; Spinelli et al.; Wang C, Prather K, et al.).  For example, some coronaviruses in aerosols can flow through a mask or float more than 6 feet, but could be dispersed by ventilation.  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.).

For some examples, during the pandemic phase when Wuhan households used several prevention methods, they decreased household transmission reproductive numbers 52% (Li F, Li Y-Y et al.).  Using multiple interventions decreased the numbers of people each person infected (effective reproduction number, Rt) in Osaka outbreaks (Nakajo, Nishiura “Assessing”).    In a controlled experiment in Paris, 43% of indoor concert attendees were already vaccinated.  All used pre-screening tests, masks, hand sanitizer, and ventilation, but did not physically distance, and some sang along.  Follow-up showed their COVID-19 incidence was not higher than similar controls not attending the concert (Delaugerre et al.).

Altogether, people using these methods made a real effect.  For example, health care providers are at higher risk than most people, because they usually do their services close to or touching their patients in small rooms.  But only about 0.9% of US dentists had been infected with SARS-CoV-2 as of June 2020, because they used many prevention techniques (CDC “Interim U.S. Guidance for … Healthcare”; Deana et al.; Estrich et al.; Lewis et al.; Razmara et al.; Versaci).  In comparison, 9% – 20% of the US general public had been infected by the same period (Anand et al.; Stadlbauer et al.).

It is also shown indirectly by the reduced transmission of other respiratory-spread diseases.  During the COVID-19 pandemic, rates of colds, respiratory syncytial virus (RSV), and influenza decreased below the rates in the same months in previous years in the southern and northern hemispheres.  This sets an example for how we can reduce respiratory diseases in the future, comparable to the way our ancestors greatly reduced water borne diseases by improving water systems in the 1800s – 1900s (Belza et al.; Burki “Double Threat”; Jones N “How COVID-19 is Changing”; Jones N “Why Easing COVID-19 Restrictions”; Karlsson et al.; Marr; Soucheray “Substantial decrease”; Spinelli et al.; Uyeki et al.; Wang J, Xiao, et al.).

Physical distancing

[Publication date of latest article cited: October 31, 2023]

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; Centers for Disease Control and Prevention “Scientific Brief: SARS-CoV-2 Transmission”; Christakis; Dhand, Li; Jones NR, Qureshi ZU et al.; Lerner et al.; Meselson; Nazaroff; Tang, Li et al.; Spinelli et al.; Stadnytskyi et al.; Wang C, Prather K, et al.; World Health Organization “Mask Use”; WHO “Coronavirus disease (COVID-19): How is it transmitted?”).  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 betacoronaviruses [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.). Another systematic review and meta-analysis found an association between physical distancing and less COVID-19 incidence (Talic 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.).   In an Ecuador village, having more bedrooms relative to the numbers of people per household was associated with lower percentages of people seroconverted against SARS-CoV-2 (Del Brutto et al.). On a national level, the people of Finland reduced close contacts by 72% in April 2020, which probably caused about 59% of the reduction of transmission (Auranen et al.).  A consensus of international experts included physical distancing on their list of prevention methods to continue in the future, so COVID-19 is no longer a public health threat (Lazarus et al.). Other reviews found physical distancing helped reduce transmission, and its cost-effectiveness varied with epidemic phase, duration, and compliance (Murphy C, Lim, et al.; Vardavas 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, participants could meet 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.).

Ventilating and Filtering

[Publication date of latest article cited: October 30, 2023]

Ventilation can reduce the numbers of SARS-CoV-2 viruses floating in the air, and thus reduce or prevent inhaling the viruses (EPA “Indoor Air and Coronavirus”; EPA “Ventilation and Coronavirus”;   Centers for Disease Control and Prevention “Scientific Brief: SARS-CoV-2 Transmission”; Marr; Morawska L, Allen J, et al.; Morrow et al.; Nazaroff; Olsiewski et al.; Spinelli et al.; Wang C, Prather K, et al.; WHO “Coronavirus disease (COVID-19): How is it transmitted?”; Xu C, Liu W, 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.  Others had no or low concentrations of SARS-CoV-2 RNA, possibly because they exchanged air at high rates or were using negative pressure ventilation (Liu Y, Ning et al.; Ong et al.). Increased ventilation reduced SARS-CoV-2 transmission in Italian schools (Buonanno, et al.), and increased ventilation and filtering reduced transmission in US schools (Gettings, et al.).

To reduce indoor transmission, many people have been advocating opening windows and pumping more outside air into buildings (Marr; Parker-Pope “6 Questions”; Wang C, Prather K, et al.; Xu C, Liu W, et al.).  Some experts recommended using air flow and filtering in buildings and rooms so occupants can both work and prevent COVID-19 transmission, as part of an overall healthy buildings program (Allen, Macomber; Centers for Disease Control and Prevention “Ventilation in Schools”; Dowell, et al.; Harvard TH Chan School of Public Health “For Health, Healthy Buildings”; Jones E, Young et al.; McNeill et al.; Morawska L, Allen J, et al.; National Academies of Sciences “Management of Indoor Air”; Olsiewski et al.; Parker-Pope “6 Questions”; Wang C, Prather K, et al.). These systems should make several air changes per hour (ACH).  This will purge almost all of the indoor air in an hour, depending on calculations of the room sizes and numbers of people (ASHRAE “Core Recommendations”; ACGIH ASHRAE “Ventilation for Industrial Settings“). Both highly- and briefly-trained people can use chemical and particulate measuring devices, especially for carbon dioxide, to guide adjusting the ventilating and filtering of room to maintain air quality in different buildings and climates (ASHRAE “Position Document on Indoor Carbon Dioxide”; Li T, Katz, Siegel; McNeill et al.).  In addition to ventilating the whole building or room, sending clean air to and exhausting used air from individuals also reduces aerosol exposures (Liu W, Liu L, et al.).  In addition to ventilating the whole building or room, sending clean air to and exhausting used air from individuals also reduces aerosol exposures (Liu W, Liu L, et al.).  While ventilating to improve indoor air quality (IAQ), people should also reduce energy use (ACGIH ASHRAE “Ventilation for Industrial Settings“; Taylor).

Heating, ventilation and air conditioning (HVAC) systems can remove some aerosols by using an aerosol arrestor, filtering, or exhausting. The filters should be rated at least MERV 13 (Minimum Efficiency Reporting Value) or the highest that the air conditioning system can use. If the filters have too high of a MERV rating for the system, then the air pump motors can wear out from trying to push too much air against too much resistance. Especially good filters are rated as HEPA (High-Efficiency Particulate Air) because they can remove from the air passing through at least 99.95% of 0.3 μm (micrometer) diameter particles. (ACGIH ASHRAE “Ventilation for Industrial Settings“; ASHRAE Guidance for Residential Buildings”; Parker-Pope “6 Questions”).

Portable air filters also reduced SARS-CoV-2 aerosols in room air. They are also called air cleaners or air purifiers. These devices should have HEPA filters to remove SARS-CoV-2 well (ASHRAE “In-Room Air Cleaner Guidance”; Conway-Morris et al.; EPA ”Research on DIY Air Cleaners”; EPA “Guide to Air Cleaners”; Liu D, Phillips, et al.; McNeill et al.; Thompson T). (Ueki H, Ujie M, Komori, et al. “Effectiveness of HEPA Filters”). Commercial air purifiers cost little to operate (Eco Cost Savings “Cost To Run An Air Purifier”).

Several experiments demonstrated the effectiveness of commercially sold portable air filters:
• Scientists disseminated infectious SAR-CoV-2 laden aerosols into an air chamber with HEPA air cleaners, which removed 85% – 99% of the SARS-CoV-2 (Ueki H, Ujie M, Komori, et al. “Effectiveness of HEPA Filters”).
• In a room with diagnosed COVID-19 patients, HEPA filters in the room reduced numbers of SARS-CoV-2 in exhaled aerosols and surfaces (Parhizkar et al.)
• In a real classroom during class sessions, commercial air purifiers reduced aerosols by 90% in 30 minutes, reducing estimated risk of SARS-CoV-2 infection “by a factor of six” (Curtius et al.).
• In a hospital ward previously used for COVID-19 patients, scientists released artificial aerosols. Without portable air cleaners, the aerosols spread through the HVAC system to other rooms. Two commercial portable HEPA air cleaners in a room reduced simulated aerosols spread through the HVAC system by 99% in 5.5 minutes (Marshall, et al.).
• Scientists aeriolized bacteriophage virus in a room, which is similar to other viruses, but safer for humans to conduct experiments. Commercial room air cleaners reduced the viruses in the air to small fraction in minutes (Cadnum et al.).
• In a room, in a simulated meeting, scientists used three mechanical breathing simulators to mimic one speaker producing aerosols and two participants inhaling. Two commercial air cleaners reduced aerosols by 65%, and using the air cleaners plus masks on the three simulators reduced aerosols by 90% (Lindsley, Derk “Efficacy of Portable Air Cleaners”)

Some companies sell devices which ionize air, causing negative or positive charged particles and aerosols to attach to each other and fall onto surfaces. Some companies add them to air filters. Tests showed that this did not significantly reduce particles. Ionization may create toxins such as tiny amounts of ozone. It has not been proven that this prevents disease  (ACGIH ASHRAE “Ventilation for Industrial Settings“; ASHRAE “In-Room Air Cleaner Guidance”; National Academy of Medicine “Public Health Lessons”; Zeng et al.).

Some companies sell devices that create ozone, claiming that it removes pollutants and microorganisms. But ozone does not remove many of them when using ozone amounts that are less than health standards (Environmental Protection Agency. “Ozone Generators that are Sold as Air Cleaners”; EPA”Air Cleaners”; Ontario Society of Professional Engineers).
Many companies are selling portable air filtration devices that remove viruses and other microorganisms indoors, to use in homes and workplaces. An effective air purifier fits these criteria: (ASHRAE “In-Room Air Cleaner Guidance”; EPA”Air Cleaners”; Kimmerling et al.; Rahman “Reviews of Air Purifiers”).
• have a high-efficiency particulate air (HEPA) filter.
• remove 99.97 percent of airborne particles that are 0.3 micron (µm) in size, which is equivalent to MERV >17.
• have the Clean Air Delivery Rate (CADR) printed in its instructions or package so one can compare it to other devices.
• Print the square surface of the room it can filter, so one can compare that to the room where people will use the device.
• make a noise level that people in the room can tolerate.
• save energy, such as those rated as Energy Star.
• not produce ozone.
• not ionize the air, or it has a way to switch off the ionizer.

Many people made their own air purifiers from fans and filters. To make these do-it-yourself (DIY) air purifiers, most taped filters to rectangular “box fans,” either one filter on the intake side of the fan, or one on the intake side and one on the exit side, or four or five in a cube shape on the intake side. Scientists tested several designs and found they filter the air as well as commercially sold air purifiers (Cadnum et al.; Derk et al.; Ontario Agency for Health Protection and Promotion; Srikrishna “Can 10x Cheaper “; UC Davis WCEC). They cost a fraction as much as commercial air purifiers (Srikrishna “Can 10x Cheaper “; UC Davis WCEC), and use little electricity (Eco Cost Savings “Box Fan Wattage”). When made correctly, they did not overheat or catch fire (Davis A, Black M. “An Evaluation of DIY Air Filtration”; EPA “Research on DIY Air Cleaners”). Some experts do not recommend using DIY air cleaners permanently instead of commercial devices, because the DIY devices vary in methods people made them. Experts advise using fans made since 2012 that meet Underwriters Laboratory safety standards (EPA”Air Cleaners”).

One of the most safe and effective designs is called a “Corsi-Rosenthal box” after the engineering professors who invented them. They have one box fan and four or five filters in a cube shape taped to the fan intake side. Videos demonstrating the construction methods are on these web pages: (Clean Air Crew; Corsi).

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 numbers of people, time, ventilation, room dimensions, breathing rate, respiratory activity, face-mask use, and respiratory aerosols infectiousness (Bazant; Bazant, Bush).

People have been placing clear plastic between people to block exhaling viruses on each other.  This could prevent some aerosol exposures in the short run (Liu W, Liu L, et al.), while allowing droplet and aerosol flow to the sides (Li W, Chong A, et al.). The barriers block most aerosols if the barriers are 91 cm wide and at least 39 cm higher than cough height (Bartels, et al.). In the longer run, it could create pockets of still air which impede ventilation to disperse viruses (Parker-Pope “Those Anti-Covid Plastic Barriers”; Wang C, Prather K, et al.).

Face masks

[Publication date of latest article cited: October 31, 2023]

Experts and public health and medical organizations recommended that the general public wear masks during the pandemic (Brooks, Butler, et al. “Universal Masking”; Brooks, Butler, et al. “Effectiveness of Mask Wearing”; California; Centers for Disease Control and Prevention “Types of Masks”; Centers for Disease Control and Prevention “Guidance for Wearing Masks”; Centers for Disease Control and Prevention “Science Brief: Community Use of Cloth Masks”; Desai, Aronoff “Masks and Coronavirus”; Lerner et al; National Academies of Sciences “Effectiveness…”;Nazaroff;  San Diego County; Wang C, Prather K, et al.; World Health Organization “Mask Use”; WHO “Coronavirus disease (COVID-19): How is it transmitted?”) for several reasons:

Masks also protect humans in society and medical facilities from COVID-19 (Boulos et al.; Brooks, Butler, et al. “Universal Masking”; Centers for Disease Control and Prevention “Science Brief: Community Use of Cloth Masks”). For some examples:

  • In rural Bangladesh, a randomized trial comparing villages with community mask promotion and villages without that intervention found intervention villagers increased their mask usage and reduced symptomatic SAS-CoV-2 infections (Abaluck et al.).
  • In the Nebraska hospital described above in the menu sections 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, Rivera, et al.).
  • In a case-control study, consistently using a respirator or mask was associated with not testing positive for SARS-CoV-2 infection.  Always using them, and using higher filtration N95 or KN95, were associated with even lower odds of infection (Andrejko 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 cloth or surgical 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.).
  • In Costa Rican households, mask wearing by the first person to get SARS-CoV-2 infection (index case) was associated with reducing the number of other household members getting infected by 67% (Sun, Loria, et al.).
  • • In Indian households, people who wore masks were less likely to get infected from the first person to get infected, a lower secondary infection rate (Yadav et al.).
  • Self-reported mask-wearing was associated with decreases in instantaneous reproductive number (Rt) of US states and zip code areas (Clapham, Cook; Rader et al.).
  • US counties’ mask mandates were associated with reductions in case growth rates 1 – 100 days after implementation (Guy et al.).
  • US states where higher percentages of people wore masks in public had lower rates of COVID-19 disease cases than states with low percentages wearing masks (Fischer CB, Adrien, et al.).
  • During a resurgence in Melbourne, Australia, the government announced a mask mandate, and then more people wore masks and fewer got COVID-19 (Scott et al.).
  • Reviews and meta-analyses of many studies of the effects of face mask use found that in most studies wearing face masks was statistically significantly associated with lower risk of COVID-19 (Boulos et al.; Candevir et al.; Chu et al.; Li Y, Liang, et al.; Talic 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, postpone exponential growth of transmission, and show support for community pandemic responses (Association of American Medical Colleges; Centers for Disease Control and Prevention “Science Brief: Community Use of Cloth Masks”; Cheng KK, Lam, et al.; Christakis; Gandhi, Marr; Howard, Huang, et al.; Mandavilli “Confused about Masks?”; Nazaroff; Royal Society; Wang C, Prather K, et al.).  In areas with mask wearing mandates, hospitalization rates rose more slowly than areas without mandates (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 numbers of SARS-CoV-2 without serious symptoms might even cause immunity (Gandhi, Beyrer et al “Masks do More”.; Gandhi, Rutherford “Facial Masking for Covid-19”; Spinelli et al.; Wu KJ “Masks may reduce viral dose”).  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”).

Some materials and designs filter more than others.  Nonwoven meltblown fabrics with electrostatic charge can filter more than some cloth fabrics.  The governments of China, South Korea, and the USA set standards for nonwoven respirators, including N95, KN95, and KF94.  N95 respirators filter more effectively and prevent transmission better than surgical masks and cloth masks, especially if the respirator fits the person’s face closely. These respirators can allow enough air flow that people can wear them in medical facilities, workplaces, and communities (Brosseau, Ulrich, et al. “What can masks do? Part 1”; Brosseau, Ulrich, et al. “What can masks do? Part 2”; Brosseau, MacIntyre, et al “Wear a respirator”; Centers for Disease Control “Types of Masks and Respirators”).

Some governments set standards for nonwoven masks, including N95, KN95, and KF94.  Some companies sold counterfeit masks claiming to meet these standards, so people should beware of poorly made masks having fake or no logos or manufacturing information on them (Centers for Disease Control “Types of Masks and Respirators”; Chen “12 Signs You Have a Fake”; Parker-Pope “How to Find a Quality Mask”).

Aerodynamics experiments found that cloth 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, shop towel, and medical masks (EN 14683 type II).  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 (Adenaiye et al.; Centers for Disease Control and Prevention “Science Brief: Community Use of Cloth Masks”; Clapp et al.; Clase et al.; Duncan et al.; Fischer et al.; Gandhi, Marr; Hao et al.; Hill et al.; Konda et al; Lewis T; Lindsley et al. “Efficacy of face masks”; Rodriguez-Palacios et al.; Sterr et al.; Ueki, Furusawa, Iwatsuki-Horimoto, et al. “Effectiveness of Face Masks”.; Wang C, Prather K, 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.).

Masks work better if they fit tightly to the skin around the nose and mouth.  For example, some people wear two masks, or tie the ear loops, to prevent fewer aerosols from escaping from the sides (Adenaiye et al.; Brooks, Beezhold, et al. “Maximizing Fit”; Centers for Disease Control “Types of Masks and Respirators”; Lewis T; Wang C, Prather K, 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.).  But when medical professionals wore an N95, surgical mask, and face shield (more than most people wear) and walked moderately for an hour, this increased carbon dioxide in their blood enough to cause fatigue (Iitani 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 clearly with their face toward the listeners, using more upper facial expressions and body language, and communicating online without masks (Brooks “Effectiveness of Mask Wearing”; Mheidly et al.; Sugar).

For all these reasons above, and because of the highly infectious Delta and Omicron coronavirus variants, the probability of getting infected increased in 2021-2022.  Experts recommended that people vaccinated against COVID-19, and people unvaccinated, should wear a mask when indoors, and near other people outdoors.  Vaccinated people should wear a mask when near unvaccinated people, but do not need to wear a mask when outdoors and over 6 feet (2 meters) from unvaccinated people or those not known to be vaccinated (Centers for Disease Control and Prevention “Science Brief: Community Use of Cloth Masks”;  Centers for Disease Control and Prevention “Choosing Safer Activities”; Centers for Disease Control and Prevention “Types of Masks”).

After the many simpler mechanistic or laboratory studies of masks summarized above, some scientific groups did large, randomized control trials of masks or respirators, or a systematic review and meta-analysis of them.  They found that fewer people wearing masks got infected than people not wearing masks, or fewer wearing respirators got infected than mask wearers, but the results were not statistically significant.

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 in both groups, might have biased the calculations toward finding little difference between groups (Frieden, Cass-Goldwasser).

A multicenter, randomized, noninferiority trial comparing use of respirators vs medical masks in 29 health care facilities in 2020 -2022 in Canada, Israel, Pakistan, and Egypt (Loeb et al.) found statistically insignificant differences. In the article’s comments, and elsewhere (Brosseau et al. “Wear a Respirator” ; Van Beusekom “Study on Masks), other researchers criticized it for the mixing together workers who did not always follow protocols, some of whom had COVID vaccines and some who did not, and different transmission situations in varied health facilities in four countries over two years of changing epidemic waves.

The Cochrane Database of Systematic Reviews published a systematic review of randomized controlled trials of “Physical Interventions to Interrupt or reduce the spread of respiratory viruses” (Jefferson et al.).  The authors stated:

“Adherence with interventions was low in many studies. The risk of bias for the RCTs and cluster‐RCTs was mostly high or unclear.”

“The high risk of bias in the trials, variation in outcome measurement, and relatively low adherence with the interventions during the studies hampers drawing firm conclusions. There were additional RCTs during the pandemic related to physical interventions but a relative paucity given the importance of the question of masking and its relative effectiveness and the concomitant measures of mask adherence which would be highly relevant to the measurement of effectiveness, especially in the elderly and in young children.”

“There is uncertainty about the effects of face masks. The low to moderate certainty of evidence means our confidence in the effect estimate is limited, and that the true effect may be different from the observed estimate of the effect. The pooled results of RCTs did not show a clear reduction in respiratory viral infection with the use of medical/surgical masks. There were no clear differences between the use of medical/surgical masks compared with N95/P2 respirators in healthcare workers when used in routine care to reduce respiratory viral infection. Hand hygiene is likely to modestly reduce the burden of respiratory illness…”

Several people debated the implications.  For example, some (Cash-Goldwasser et al.; MacIntyre, Chugthai, et al. and the comments in Jefferson‘s articles) said that Jefferson’s review pooled data from studies of influenza and COVID-19, and in which people wore masks sometimes, and in which people wore them all the time, and in healthcare and community settings, thereby blurring these distinctions. Jefferson’s review used information from one RCT by Abaluck et al. promoted using masks in communities during high COVID incidence, and it did find that people in mask promotion villages used masks more, got COVID less, and died less than in villages where they did not promote masks, which shows that mask use worked. They did this study in Bangladesh, so the results might differ in other societies. But, the many other studies summarized above demonstrated that mask use worked in those societies also.

The randomized controlled trials by Bundgaard et al. in Denmark, and Loeb et al. in health facilities, and the This Cochrane systematic review fit the explanation in this web site’s chapter Information Sources, about mechanistic and laboratory studies showing different steps in cause and effect sequences.  Studies of respirators and masks showed the materials can prevent most aerosols from going through.  Studies of people and simulators show that properly worn respirators and masks can reduce the amounts of aerosols exhaled out to other people, or inhaled from others.  Studies of some areas with and without mask mandates showed people in some areas with mask mandates having less COVID-19 infections.

As also discussed in the chapter above on Information Sources, in large, complicated studies of people in communities or workplaces, they cannot monitor or manage the actions of every person for every variable all the time. So, sometimes people are not following protocols in both directions, with intervention participants sometimes not doing the invention, and control group participants sometimes doing the intervention. Some people misrepresent and do not use COVID-19 public health measures, such as masks (Levy et al.; Oreskes; Soares-Weiser). Therefore, those kinds of studies have inherent biases toward statistical insignificance and null hypotheses (Cash-Goldwasser et al.; Fischhoff et al.; Oliver et al.).
Mathematicians designed inferential statistics to find small, subtle differences and correlations, when the researchers prevent flaws and biases. If two groups differed greatly, then even studies with moderate flaws and biases would show different statistical results from the groups, even if the flaws and biases leaned in different directions and partly cancelled each other. For example, if scientists did a partially flawed study comparing the weights of house cats and cattle, with several cat breeds, several cattle breeds, and different brands of scales, they would still find a statistically different difference between cats and cattle, because of the enormous differences between the two species.

Altogether, these many mechanistic and laboratory studies summarized above show that wearing respirators or masks can prevent some aerosol spread and COVID-19 infections. But, the smaller number of large randomized controlled trials apparently show that the differences between no mask, mask, and respirator use in communities and facilities are not actually enormous, because but many people do not wear respirators or masks consistently, or always comply with mask mandates. Therefore, people should not over-interpret these controversial RCTs either way, not saying that masks and respirators are useless, or that these RCTS are useless.

But other studies in communities and workplaces (in the paragraphs and bullet points above) could also have had similar biases and flaws with people in the intervention not always wearing masks, people in the control group sometimes wearing masks, etc. Yet these studies found statistically significant differences, perhaps there really were large differences that even the partially flawed studies could find. For example, in the randomized trial Bangladesh, the intervention villages really did use masks more, and have fewer COVID-19 cases than the villages without the intervention (Abaluck et al.). And in the retrospective case control study in Thailand, uninfected contacts of COVID cases really did use masks, stayed distant, and washed hands more than the infected contacts did (Doung-Ngern et al.).

Face Shields and Eye Glasses

[Publication date of latest article cited: December 1, 2022]

Wearing eye glasses or clear plastic face shields might sometimes protect the wearer’s eyes from droplets and aerosols (Coroneo, Collingnon; Maragakis; Matos et al.; Roshanshad et al.; Zeng W, Wang X, et al.).  Wearing a face shield decreases air flows with infectious particles, especially when an infected person wore the face shield (Tretiakow et al.). Face shields also prevent pathogens from reaching to the person by causing some of the pathogens stick to the shield.  So, wearers should decontaminate the shield after use (Breda-Mascarenhas et al.).

Wearing face shields protected health care professionals in most studies (Byambasuren et al.).  For example, 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).

People wearing eyeglasses tended to have fewer SARS-CoV-2 infections than people without glasses.  For example, in China, among COVID-19 hospital patients, 5.8% wore glasses.  But in the local general population 31.5% wore glasses (Zeng W, Wang X, et al.).

Lab experiments with manikins simulating a dental patient contaminated with alphacoronavirus 229E (a cause of common colds) and a dental operator found that when the operator did not wear a face shield, viruses went inside the operator’s mouth.  After wearing a shield no viruses were detected there (Ionescu et al.).

A randomized clinical trial among people of whom 77% were vaccinated against COVID-19, after the main pandemic phase, found that those assigned to wearing glasses did not have a statistically significant different number of COVID-19 infections, but had lower incidence of other respiratory infections (Fretheim et al.). This shows that wearing glasses had little effect on COVID-19 vaccinated people.

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 betacoronaviruses [severe acute respiratory syndrome (SARS), and Middle East respiratory syndrome (MERS)] (Chu et al.).   Another systematic review concluded that most studies showed that face shields and eyeglasses were associated with less SARS-CoV-2 infection, but the studies did not adjust for confounding factors such as other protection methods, so the results are not very certain (Byambasuren et al.).

Several companies sell face shields, and people can make and disinfect their own (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.  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, and Handwashing

[Publication date of latest article cited: August 24, 2023]

As described in the section above on Fomites, SARS-CoV-2 can remain viable and transmissible on surfaces for days.  To eliminate them, chemical disinfectants destroy them, and ultraviolet light inactivates their RNA genome.  Hand, face, face mask, 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 (Morrow et al.; Rowan et al.; 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.).  A systematic review and meta-analysis found an association between hand washing and less COVID-19 incidence (Talic et al.). A systematic review found inconclusive evidence from low quality studies for disinfecting reducing transmission (Madhusudanan et al.). Another systematic review and meta-analysis of randomized controlled trials found hand hygiene interventions associated with 14% fewer people with acute respiratory infections than in control groups, but little to no difference for influenza-like illnesses (Jefferson et al.). This might indirectly imply that handwashing could reduce SARS-CoV-2 infections. If one wanted to conduct handwashing programs, there are many models available from many countries, such as school lessons, community trainings, or providing handwashing stations and soap (Global Handwashing Partnership “Handwashing Handbook”; Global Handwashing Partnership “Annual Report”).

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” ; Ijaz et al.; Kratzel et al.; Rowan et al.).  The Environmental Protection Agency (EPA) listed products that disinfect SARS-CoV-2 by damaging their lipid envelopes and biochemical processes.  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 “About List N: Disinfectants…”; Environmental Protection Agency “Which Disinfectants Kill COVID-19?”; Environmental Protection Agency “List N Tool”; Ijaz et al.; Rowan et al.).  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…”).   Several kinds of soap and water also destroy SARS-CoV-2 and disinfect hands and other surfaces (Bell et al.; 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, we should disinfect appropriately for each situation.  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 could reduce exposure to coronaviruses by often washing hands after going out, and sometimes wiping packages if we suspect contamination (Centers for Disease Control and Prevention “Households…”; Desai, Aronoff “Food Safety and COVID-19”; Food and Drug Administration; Parker-Pope “Is the virus on my clothes?”).  We should prevent the disinfectants from harming people, surfaces, or the environment (Rowan et al.).

After a year of research on this new pathogen, there was 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; Lewis D, “COVID-19 rarely spreads through surfaces”; The Lancet Respiratory Medicine).

Mouthwashes and Nasal Rinses

[Publication date of latest article cited: October 11, 2023]

Mouthwashes damage SARS-CoV-2 lipid envelopes, which can reduce the viral load, and might prevent some people from getting infected or infecting others with COVID-19 (Chumpitaz-Cerrate et al.; O’Donnell et al.; Ziaeefar et al.).  SARS-CoV-2 can infect epithelial cells in the mouth and salivary glands, and comes out in saliva (Huang N, Pérez, et al.).  Scientists tested common over-the-counter mouthwashes in artificial in-vitro laboratory conditions that mimic the nasopharyngeal secretions in people’s mouths.  A systematic review found that only chlorhexidine made a significant difference in PCR-detected viral load compared to controls (Sbricoli et al.) Other studies found that mouthwashes containing  ethanol,  polyvidone iodine, dequalinium chloride, and benzalkonium chloride inactivated almost all the SARS-CoV-2 (Meister et al.).  Other studies showed that some over-the-counter nasal rinses and mouthwashes containing cetylpyridinium chloride (Filippo et al.; Ziaeefar et al.), methyl, thymol, and povidone iodine inactivated the coronaviruses that cause common colds (Meyers et al.).  A controlled clinical trial in COVID-19 patients found that ß-cyclodextrin and citrox (bioflavonoids) (CDCM) mouthwashes reduced SARS-CoV-2 viral load modestly compared to a placebo (Carrouel et al.).  Chlorhexidine reduced SARS-CoV-2 also, in some experiments (Rahman GS et al.; Ziaeefar et al.), as well as cetylpyridinium chloride, methyl, thymol, and povidone iodine, and in other experiments not as well (Damãio Costa et al.; Fernandez et al.).

Since some mouthwashes destroy coronaviruses in the mouth, people should not use them for hours before using saliva in a PCR or antigen test for SARS-CoV-2. The mouthwash could cause a false negative test (Lippi et al.).

Nasal rinses and sprays could also prevent some transmission.  A nasal rinse of diluted baby shampoo inactivated 99% of common cold coronaviruses (Meyers et al.).  Prophylactic intra-nasal administration of a toll like receptor, which stimulates immune defense against microbes, prevented SARS-CoV-2 infection in ferrets (Boiardi, Stebbing; Proud et al.).

Scientists should do more research on people using these methods (Chumpitaz-Cerrate et al.; Ziaeefar et al.).  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”).  If someone frequently gargled those mouthwashes, and aerosols with coronaviruses floated into their mouth and landed on a surface with mouthwash on it, that could destroy most of those coronaviruses. But many other coronaviruses could still float through their nose and mouth into their respiratory system and infect the mouthwash user.

Sunlight, Ultraviolet, Heat, and Humidity

[Publication date of latest article cited: September 14, 2023]

Artificial ultraviolet light inactivates SARS-CoV-2 rapidly for disinfection (Morrow et al.).  It damages the RNA, and the virus cannot replicate.   But the virus structure remains intact (Lo C, Matsuura et al.).  UVC inactivates more viruses than UVA and UVB (Ashokumar et al.; Derraik et al.; Heilingloh et al.).  UVC used for 10 seconds reduced SARS-CoV-2 by 88% (Kitagawa et al.).  In 30 seconds it decreased virus titer to almost zero  (Lo C, Matsuura 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.).  Shorter wavelengths (254-268 nm) inactivated more quickly (5 seconds) and at lower doses (Mariita, Peterson).  Far UV (222 nm) inactivates viruses with less skin and eye damage on people (Bender; Nardell) Three UVC intensities on three SARS-CoV-2 concentrations stopped viral replication in each combination (Biasin et al.).

Since ultraviolet lights are so easy to use effectively for inactivating SARS-CoV-2 in homes and workplaces, companies are selling devices in different shapes: small hand-held, large wheeled, mounted on walls, and inside air ducts (Li T, Katz, Siegel).  To use UV in the upper part of a room, it should be attached well above people, and use a fan, so it does not harm people, while reducing viruses (Bueno de Mesquita, et al.; Centers for Disease Control and Prevention “Upper-Room Ultraviolet”; Nardell).  Many buildings now use germicidal ultraviolet light (GUV), ventilation, and filtration to reduce viruses, so people can stay safely indoors without masks.  This can cost less than ventilation and filtration for both initial installation and operation, using less energy, so it can be more climate-friendly (Milton).

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) they reduce by 90% in 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”). Increased relative humidity in room reduced numbers of SARS-CoV-2 from infected patients in exhaled aerosols (Parhizkar et al.). 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.).  Heat and humidity together reduced it: SARS-CoV-2 half-life in cell culture on plastic was over 24 hours at 40% relative humidity (RH) and 10° C, but about 1.5 hours at 65% RH and 27° C (Morris, et al.).  So, increasing humidity by using humidifiers could help reduce transmission  (Ahlawat; Allen et al.; Bazant, Bush; Science Daily).   SARS-CoV-2 in many bodily fluids had shorter half-lives in summer conditions than in fall, winter, and spring conditions (Kwon T, Gaudreault, 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 quickly (55° C) are hotter than humans can live in, summer weather does not eliminate them.  Other factors probably more determine transmission (Chin, Chu, et al.; National Academies of Science “SARS-CoV-2 survival…”).

Indoor Air Quality  

[Publication date of latest article cited: October 30, 2023]

In recent decades, more people are increasingly aware that buildings have many microorganisms and pollutants, and that people should study these and improve indoor air quality to maintain health (National Academies of Sciences, “Microbiomes of the Built Environment”).  These include particulates, carbon dioxide, humidity, volatile organic compounds, heat, cold, and noise, causing many kinds of health problems.  Many of these increased in the last century as people constructed buildings with more artificial materials which emit toxins.  The COVID-19 pandemic made many more aware of indoor air pollutants.  Maintaining cleaner air requires more ventilating and filtering, which use more energy to move, heat, cool and monitor the air, which costs more.  To improve air quality while saving energy and money, experts developed healthy building certification and a green building movement (Allen, Macomber; Landman; Lewis “Indoor air”).

Since people transmitted SARS-CoV-2 mainly through the air inside rooms, inhaling clean air there could prevent much transmission.  One method is for people not to enter a room when no work session or meeting is scheduled.  SARS-CoV-2 in air decay by themselves, so if people stay out of a room for several hours, then people entering the room next should have a low risk of getting infected (Department of Homeland Security “Estimated Airborne Decay of SARS-CoV-2”).  Other methods include cleaning the air and improving indoor air quality (IAQ), with ventilation, filters, and UV.  Using these methods, many people could continue working and studying together during the pandemic, reducing the numbers getting infected.  People are improving buildings, setting measurable performance standards for building operations and maintenance, helping building owners improve IAQ, and writing legislation or rules incentivizing IAQ improvements.  It should also include individual actions, such as wearing masks, cleaning hands, physical distancing, staying home when sick, and getting immunized (Allen, Ibrahim “Indoor Air Changes”; California Department of Health “COVID-19 and Improving Indoor Air Quality”; Centers for Disease Control and Prevention “Ventilation in Buildings”; “Improving Ventilation in Your Home”; Dowell et al.; Haines C, Olsiewski P, et al.; Lazarus et al.; Lewis, Bose; Nardell; Ontario Society of Professional Engineers).

For decades before the SARS-CoV-2 pandemic, scientists and engineers studied and tested techniques and equipment for heating, ventilation, and air conditioning (HVAC) systems for buildings. These included the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) and American Conference of Governmental Industrial Hygienists (ACGIH).  In response to the pandemic, they changed the recommended techniques to emphasize ventilation, filtering, and ultraviolet germicidal irradiation (UVGI).  In response to climate change, they adjusted the techniques to save energy.  (ACGIH and ASHRAE “Ventilation for Industrial Settings”; ASHRAE “Handbook”; ASHRAE “Position Document on Infectious Aerosols”; ASHRAE “Coronavirus (COVID-19) Response Resources”; ASHRAE Epidemic Task Force “Core Recommendations for Reducing Airborne Infectious Aerosol Exposure”; ASHRAE “COVID-19: One Page Guidance Documents”; ASHRAE Epidemic Task Force “Building Readiness”; ASHRAE “Guidance for Residential Buildings”; Simmonds et al.; Zhang Y, Hopke P, et al.). The US Centers for Disease Control and Prevention (CDC) made similar recommendations (CDC “Ventilation in Buildings”; “Improving Ventilation”).   For example, these organizations recommended:

  • Use only outside air supply. Reduce recirculation as much as feasible.
  • Continue 6-12 air changes per hour (ACH), This will purge 99% in 30-60 minutes, even when no one is in the building.
  • Utilize filters rated at least Minimum Efficiency Reporting Value (MERV) 13 or as high the system can tolerate.
  • Use local exhaust ventilation (LEV) in many rooms, especially restroom fans, expelling air outdoors away from air intakes.
  • In hot weather, people cool themselves with personal fans. Adjust these so they do not blow from person to person.
  • Use Ultraviolet Germicidal Irradiation (UVGI), such as in air ducts or upper parts of rooms.

The US government increasingly emphasized indoor air quality in government buildings and for the public.  They are urging all building operators and owners to assess and improve using several methods.  The “Clean Air in Buildings Challenge” is a part of the National COVID-19 Preparedness Plan.  Owners and operators should develop clean indoor air action plans that evaluate indoor air quality, prepares improvements, schedules maintenance and inspections, and invites those using the buildings to learn and participate (EPA “Clean Air in Buildings Challenge Document”; EPA “Clean Air in Buildings”; EPA “Coronavirus”; EPA “Indoor Air and Coronavirus”; EPA “Ventilation and Coronavirus”; EPA”Air Cleaners”; Kimmerling et al.; National Academies Press “Public Health Lessons”; National Academies of Sciences “Management of Indoor Air”; OSHA “Improving Workplace Ventilation”; White House “Clean Air in Buildings Pledge Opportunity”; White House Summit on Indoor Air Quality; White House “Fact Sheet”; White House OSTP “Fact Sheet: Departments”; White House “Resources and Materials”).

Since SARS-CoV-2 transmission changed from pandemic to endemic, people are not transmitting to non-immune people in every place, and they do not need to use maximum prevention methods in every situation.  In the endemic phase, some people are transmitting in some places.  They should assess the risks in each room based on one’s own vaccination status, age, health, and on the characteristics of the room and people, and the importance of the event or meeting  (Srikrishna “How to Make Smart Decisions“).  Endemic does not mean that COVID is finished.  People together in rooms, including workplaces, homes, and schools, should choose which methods to use: ventilation, UV, filtration, vaccination, hand sanitizer, etc. (Vipond, Hu).

Practicing the Recommendations

[Publication date of latest article cited: October 2022]

People used both medical technologies and social behaviors to control this pandemic (Oren). But people vary in willingness or opportunities to practice social distancing, mask wearing, hand washing, etc.  Many believe anti-science and anti-vaccine ideas (Hotez).  Surveys across the US found those people more practicing these non-pharmaceutical interventions included:

In Singapore, those more practicing included: women; younger age; and married people (Long VJE, Liu, et al.).

In a survey of Chile, Colombia, Cuba, Guatemala, and Mexico, the Cubans reported the highest use of masks.  Colombians reported more use of confinement methods, such as restricting interaction at work.  Guatemalans reported more use of hand washing and social distancing, and less use of confinement (Meda-Lara et al.).

In a survey of adults’ views of children’s hand hygiene and surface cleaning in seven countries, Indian respondents had the lowest capability, opportunity, and motivation.  In the United Kingdom, capability and motivation predicted children’s handwashing.  In Australia, Indonesia, and South Africa, capability predicted it, and in Saudi Arabia all three components predicted (Schmidtke, Drinkwater).