Wearing a mask can help reduce transmission in two ways:

See the below infographic which illustrates that masks are most effective in reducing transmission, if both people sick with COVID-19 and healthy people are wearing masks. Secondly, if only some people (sick or healthy) wear masks, there is much lower level of protection.

According to WHO recommendations, masks should be used together with a comprehensive package of preventive interventions to prevent infection with SARS-CoV-2. These other interventions include improved ventilation/air filtration/ultraviolet germicidal irradiation, physical distancing, hand hygiene, respiratory etiquette (e.g. coughing into a tissue or the inside of one’s elbow) and getting vaccinated.

There are several categories of face coverings that WHO and CDC recommend for protecting a person against SARS-CoV-2 infection: 1) respirators, 2) disposable medical masks, and 3) reusable, non-medical masks. Each of these categories includes multiple types of masks (see Table 1). Masks types are differentiated by the materials used to make them, and their filtration efficiency, breathability, and fit. There are standard methods for testing masks and respirators for assessing if face coverings meet national or international standards for filtration efficiency, breathability, and fit leakage (how well the mask fits the wearer’s face).

Filtration efficiency is expressed as the percent of particles that flow from one side of the mask through to the other side. A higher filtration efficiency means that fewer particles are able to go through the mask; hence, masks with higher filtration efficiency provide better protection against respiratory diseases. Generally, for particles >0.3 micrometres (um), a mask’s filtration efficiency is higher for larger particles. As a result, some masks can protect a wearer from bacteria, but not viruses, because bacteria are larger than viruses. Bacterial filtration efficiency is tested by spraying the mask with the bacterium Staphylococcus aureus and assessing what percent of the bacteria penetrates the mask. S. aureus are used for the test because S. aureus is a leading cause of infections acquired at hospitals. Since viruses are substantially smaller than bacteria, a mask’s ability to protect the filter viral particles is better represented by its particle filtration efficiency as opposed to its bacterial filtration efficiency. Particle filtration efficiency is tested using sodium chloride molecules.

Breathability is measured flowing air through the mask and assessing how the air pressure on one side of the mask compares to the air pressure on the other side of the mask. A low difference in pressure means that it is easy for air to flow through the mask (indicating high breathability). The difference in pressures measured on each side of the mask is reported as ‘pressure drop’ in millibars or pascals. People who engage in heavy labour while wearing a mask need to have masks that are more breathable.

Fit is based on how much air blowing at the mask flows through the mask compared to how much leaks around the edge of the mask. Fit can be qualitatively evaluated at home by placing one’s hands around the edges of a mask, breathing out forcefully, and feeling how much air is expelled around the edges of the mask.

The performance standards for filtration efficiency, breathability, and fit leakage are different for different types of masks and indicate different levels of protection.

Table 1: Summary of face covering types, characteristics, and recommended users.

^ Medical masks should be used by the general public only if healthcare worker’s need for medical masks meeting minimum quality standards has been satisfied.

* High risk individuals include those aged 60 and older and those with an underlying comorbidity such as cardiovascular disease, diabetes, chronic lung disease, cancer, cerebrovascular disease, immunosuppression, obesity, or asthma.

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WHO guidance on the use of respirators and masks to prevent and control SARS-CoV-2 infection is available here. To find out more about N95 respirators, medical masks and fabric masks see the other sections within our resources. In this document, “masks” refers to both filtering facepiece respirators and masks and “high-quality masks” refers to filtering facepiece respirators, KF94s, and medical masks.

The figure below highlights the differences between some of these face coverings that may be used by the general public:

Face shields may provide some protection from respiratory droplets, though large gaps between the shield and face may limit effectiveness, especially in particular head orientations. WHO recommends that face shields can be used as an inferior alternative to masks when masks are not available or a person cannot wear a mask due to cognitive or respiratory impairments, for example. If face shields are to be used, the following precautions and use instructions should be implemented:

N95 respirators are one type of filtering facepiece respirators (FFR). Respirators are recommended for frontline health staff treating patients with or suspected to have COVID-19. Without testing to ensure they fit tightly on the user’s face, FFRs do not provide their maximal protection. For example, N95 respirators can filter at least 95% of particles 0.3 um in diameter, as required by the 42 CFR Part 84 (the U.S. federal regulation that U.S. National Institute for Occupational Safety and Health [NIOSH] uses to certify air-purifying particulate respirators). However, without a special fitting procedure, they are not necessarily 95% effective. Since FFRs need to fit the face tightly, they cannot be used by individuals with facial hair. Since FFRs were designed for adult faces, they cannot be effectively used by children.

A list of FFRs approved for use by the CDC is available here and an infographic outlining required labelling of CDC approved FFRs is available here. Legally, products can only use the N95 trademark if these products have received NIOSH certification. KN95 are supposed to meet Chinese standards. However, about 60% of KN95 respirators that NIOSH evaluated in 2020-2021 did not meet the requirements they intended to meet (see more details here). See here for advice on identifying masks that likely do not meet the standards they indicate meeting.

Exhalation valves: Some FFRs come with an exhalation valve. This valve is a small plastic device that can be seen on the outside of the respirator. This exhalation valve makes breathing more comfortable while wearing the respirator. The one-way valve means that the respirator allows exhalation of respiratory particles that could contain SARS-CoV-2 and will therefore be less effective (see this supporting video) in limiting onward transmission of SARS-CoV-2. The CDC recommends that respirators with exhalation valves should have the valve covered following the manufacturer’s instructions.

Repeated use: Respirators are designated for only a single use. However, when there are severe shortages of Personal Protective Equipment, WHO recommends that as a temporary strategy PPE can be used for longer than normal or for multiple patient encounters, PPE can be decontaminated and used again, or non-standardized PPE can be used. A summary of decontamination procedures can be found in the annex of this WHO document on the rational use of personal protective equipment during severe shortages; additional information can be found on the N95DECON website.

A KF94 respirators is a type of filtering facepiece respirators (FFR) that has a particle filtration efficiency of at least 94%. KF94s are intended to be worn by the general public. Unlike N95s and similar FFRS that meet U.S. standards, a KF94 does not require a special fitting procedure or a fit test.

Health care staff who are not working with patients with airborne diseases often wear medical masks. Medical masks trap particles produced by the wearer when coughing, sneezing, or speaking, and prevent these particles from entering the environment. They block aerosols, droplets, splashes, sprays, splatter from a patient so that these particles are less likely to reach mouth and nose of the person wearing the mask. While medical masks do not have to meet any specific standard before they are made commercially available, they can be certified as meeting the ASTHM F1862, ASTM F2299 of ASTM F2101, and EU EN14683:2019 Annex C standards. A quick way to qualitatively examine the quality of a medical mask is to hold it up to a light - generally, the more opaque the mask, the better the filtration efficiency.

Who should wear a medical mask?

The WHO living guideline on infection prevention and control in the context of COVID-19 describes who should wear a medical mask and when to wear them, including the provision of example scenarios. WHO currently recommends wearing a medical mask[1] in the following scenarios in which “there is community or cluster transmission of SARS-CoV-2, irrespective of vaccination status or history of prior infection” and the following settings:

A medical mask should also be worn by the following individuals:

Watch this video from the WHO for more information on who should use medical masks and how to put them on, wear them, and take them off safely. These guidelines on who should wear masks have been adopted by most national governments but there are some exceptions and variations. Please check mask-related guidelines with the national department/Ministry of health in your country.

Hygienic use of medical masks

For guidance on the safe use of medical masks, please watch this video or see the below infographic from the WHO.

Reusable, non-medical masks do not meet the performance standards of medical masks, but do meet international guidelines (e.g. Association Française de Normalisation, World Health Organization, European Committee for Standardization, Bangladesh Directorate General of Drug Administration) and international standard for non-medical masks (ASTM F3502-21)

World Health Organisation currently recommends that a reusable, non-medical mask can be worn in the same situations that the general public is recommended to wear a medical mask. Specifically, these are scenarios in which “there is community or cluster transmission of SARS-CoV-2, irrespective of vaccination status or history of prior infection” and the following settings:

Reusable, non-medical commercially produced that do NOT meet the essential parameters, and homemade masks (which it is assumed are not tested to assess whether or not they meet the essential parameters), should only be used as a last resort – when respirators, medical, and reusable non-medical masks that meet international guideline or standards are not available. For guidance on purchasing or making a fabric mask, see appendix 1.

WHO recommends that in scenarios with “known or suspected sporadic transmission, or no documented transmission” decision-makers should apply a risk-based approach to determine mask-wearing guidance. They should consider the following factors:

Most studies of the effectiveness of masks to interrupt disease transmission have focused on prevention of respiratory diseases other than COVID-19, the impact of mask wearing in healthcare settings. There is limited evidence that applies specifically to the question of whether mask use by the general public could reduce transmission of SARS-CoV-2.

A systematic review of mask use published in the Lancet found that wearing a mask in both healthcare and community settings reduced the risk of infection with coronaviruses that cause COVID-19, SARS, or MERS. However, few studies of COVID-19 in non-healthcare (community settings) were available and none were included in the final meta-analysis assessing mask use in community settings. The authors found that mask-wearing was more protective in healthcare facilities than in community settings. This could be because healthcare workers have increased exposure or because N95 respirators are more common in healthcare settings and were found to be more effective than medical masks or multi-layer fabric masks in preventing infection. It could also be because healthcare staff are more likely to adhere to correct procedures for the safe and effective use of masks. Further, N95 respirators, medical masks, and multi-layer fabric masks were found to be more protective than single-layer masks. However, the authors suggested that additional, higher quality evidence is needed as all studies included in this review were observational.

A 2023 Cochrane review of randomized controlled trials examining the impact of masks on respiratory disease transmission found that interventions intended to increase mask wearing had little impact on respiratory disease transmission. However, these interventions were not able to substantially increase consistent mask-wearing so the results cannot be used to understand the impact of large increases in mask-wearing on disease transmission.

However, these reviews combined multiple types of interventions in different types of settings. While masks may have been effective in certain settings, masks or other interventions may not have been effective in other settings, and combining them may lead to the conclusion that, on average, the category of interventions are not effective. Reviews that have specifically examined the impact of face masks on respiratory disease have found that masks wearing is helpful in protecting others from becoming infected (there was insufficient data to examine if masks protected the wearer from becoming infected): one review examined the impact of face masks and influenza transmission, another examined the impact of masks on respiratory infections specifically in healthcare workers.

Single, rigorous studies that have been specifically address the question of interest—the impact of mask-wearing by the general public on SARS-CoV-2 transmission in community settings—can also be informative. A large-scale cluster-randomized controlled trial that involved nearly 350,000 people in rural Bangladesh found that a mask promotion intervention aimed at increasing among adults was able to increase adult mask-wearing 29 percentage points (mask-wearing increased from 13% to 42%). The increase was approximately the same in village assigned to promotion of medical masks and villages assigned to promotion of three-layer fabric masks. This increase in mask-wearing caused an 12% reduction in COVID-19-like symptoms, and an 10% reduction in prevalence of people who had symptoms of COVID-19 and had anti-bodies to COVID-19 in their blood; among people over 60 years old in villages assigned to medical mask promotion, this reduction was 35%.

Modelling studies are studies that use evidence-based assumptions and statistics to predict what might happen to populations and disease transmission patterns given different levels of mask-wearing. Modelling studies have suggested that medical mask use is likely to be effective in reducing the rate of infection with COVID-19 (Study 1, Study 2, Study 3, Study 4, Study 5) and other respiratory diseases, particularly when combined with other non-pharmaceutical interventions like physical distancing and hand hygiene. In one modelling study, even low or moderately effective masks could have substantial impacts on transmission of COVID-19 in some settings. Another study showed that high adherence of medical mask use (80%) could effectively eliminate an outbreak of influenza. The Institute of Health Metrics and Evaluation’s COVID-19 transmission model dashboard illustrates how universal mask use may impact COVID-19 outcomes in different settings. Model results of models can be sensitive to the model assumptions and parameterization. For example, models that predict influenza transmission may not be directly applicable to the current COVID-19 pandemic.

Experimental laboratory studies are designed to simulate and test certain real-world conditions in a safe way.

Several experimental studies have simulated the effectiveness of various mask types to a) stop an infectious person spreading respiratory virus (‘source control’) and b) prevent an uninfected mask-wearer from becoming infected (‘personal protection’). One study found medical masks could effectively reduce the emission of coronaviruses and influenza viruses from infected patients, potentially limiting onward transmission. Another study found that N95 respirators, medical masks, and fabric masks (made from tea towels) all reduced exposure during various activities and that the effectiveness of masks did not decrease over a three-hour period. However, in simulated human breath experiments, fabric masks had minimal effect on limiting emission of respiratory particles. A study comparing medical masks, homemade cotton T-shirt masks, and no masks found that both medical masks and fabric masks reduced the number of microorganisms released during coughing. While both offered some protection, the medical masks were more effective in filtering microorganisms, potentially because the homemade masks did not achieve as close of a fit as the medical masks.

When interpreting lab-based studies, remember that experimental conditions often do not represent real-world conditions. For example, filtration efficiency is not typically assessed for individual pathogens like SARS-CoV-2. Instead, reported filtration efficiency is often based on the ability of a mask to filter out different sized particles (or pathogen surrogates in some studies). This is partly why there is little data on the efficacy of masks for SARS-CoV-2 specifically. Further, the generation of aerosols in the laboratory (often produced via equipment like a nebulizer) may not reflect the pattern or frequency of aerosol production during normal human speaking, singing, or coughing.

A study of fabric masks in a hospital setting found that medical masks performed better than fabric masks in preventing respiratory infection. This study also found that use of fabric masks did not reduce infection more than ‘standard practice’; standard practice is to wear a medical mask in many high-risk contexts. As such, the study’s finding does not mean that wearing fabric masks results in increased risk over wearing no mask at all; more likely it indicates that wearing a medical mask in certain high-risk situations and not at other times provides more effective protection than wearing a cloth mask during the entire hospital shift.

Summary of Mask Recommendations for Hygiene Promotion Staff

Below we describe the rationale behind each of these recommendations.

In some countries it is still safe for hygiene promotion staff to be going from household to household to share information about COVID-19 and promote hygiene behaviours. Hygiene promoters have a critical role to play in terms of interrupting transmission, but this type of hygiene promotion is also a risk. As hygiene promoters move around communities, they are putting themselves at a higher risk of getting infected. People infected with SARS-CoV-2 may be infectious (able to spread the virus) but not symptomatic (Study 1, Study 2, Study 3). This means that hygiene promoters who feel healthy may themselves be infected and at risk of spreading this to other people in the community.

Organisations should provide medical masks to their staff (rather than relying on staff to make or purchase masks), train staff on the safe use and disposal of masks and encourage their staff to wear a fabric mask over the medical masks to improve the fit of the medical mask and keep the medical mask clean, particularly if the medical mask will be reused.

If your organisation decides to promote or require medical masks among your hygiene promoters in a region where mask-wearing by the general public has not been recommended, you need to make sure that your staff are able to explain why they are wearing masks and why masks are not encouraged for the rest of the population. For tips on discussing COVID-19 and mask-wearing without using stigmatizing language, see this Oxfam guidance.

Experts agree that children (individuals less than 18 years old) can be infected with SARS-CoV-2 and they contribute to transmission of SARS-CoV-2 (see this viewpoint in Science). While one review found that COVID-19 among children is accounted for a small percentage of household transmission and transmission among children was less likely than transmission among adults for earlier variations (alpha and delta), children were more likely to be infected by and transmit the omicron variant compared to earlier variants.

WHO and UNICEF recommendations for mask-wearing among children are as follows:

Children <5 years old do not need to wear a mask because they would likely not be able to wear a mask properly without help or supervision.

In areas where SARS-CoV-2 is spreading, children 6-11 years old should wear a well-fitting mask in

Children 12 years and older should follow the same WHO recommendations for mask use as adults.

School mask policies should consider a) the age-related WHO recommendations for mask use among children, b) how the mask policy fits within a broader infection prevention and control strategy (e.g. hand hygiene, physical distancing), and c) the thoughts of educators who may have to balance the advantages of mask use against its potential burden on learning and other school activities. For additional information on the use of masks in schools please see this resource ‘Promoting good respiratory hygiene practices’.

Considerations for mask use in specific circumstances and settings

Children with cognitive or respiratory impairments, disabilities, developmental disorders, or other health conditions that make wearing a mask difficult or impair mask-wearing: Mask use should not be required for children with cognitive or respiratory impairments, disabilities, developmental disorders, or health conditions that make wearing a mask difficult or impair mask-wearing. These children should be given alternative options such as wearing face shields. Adults working with children with disabilities who require close physical contact should wear a high-quality mask and adhere to other infection prevention and control measures such as frequent hand hygiene. Widespread mask use, coupled with physical distancing, may pose challenges for children with hearing problems who rely on lipreading or facial expressions to communicate. In these cases, families, educators, and others may consider using face shields or masks with clear panels to allow for lipreading. The below photo depicts a fabric mask with a clear panel in front.

Communication: Communication materials explaining the purpose and use of masks should be adapted to be context-specific and age-appropriate for children and should remain flexible to changes in evidence, community needs, and feedback or questions from children. Parents / guardians, educators, and trusted community members and leaders should communicate the importance and safe use of masks to children through role-modelling. Messaging to children should emphasize the role of masks as one part of a broader infection prevention and control strategy to ensure mask use does not result in children feeling a false sense of security and disregarding other preventative measures.

Mask design and use: Mask design should consider the quality, breathability, fit, and comfort of the child. As acceptability among children may be a barrier to use, masks should be child-friendly and be made in a variety of sizes, colours and designs.

Number of masks: Children may require use of several masks over the course of a day as they become soiled, wet, or lost.

Accessibility: Ensuring equitable access to masks among children is important and access must not be a barrier to use. Masks should be accessible free of charge to children living in limited resource settings.

Alternatives to masks: Some children may have difficulty wearing a mask due to disabilities, health issues, or other reasons. The WHO suggests the use of face shields in these circumstances.

Monitoring and evaluation: Given limited evidence of face mask use among children, the WHO recommends setting up monitoring and evaluation programs to assess the impact of masks on child physical and mental health. Key indicators should include impact on COVID-19 transmission, acceptability, use, and barriers of mask use, and potential effects on learning, development, and school attendance. The impact on children with special needs should be given specific attention. For more information on monitoring evaluation, please see this WHO document.

There are a range of reasons why mask wearing was previously not recommended by WHO for the general public; these are explained below.

Airborne transmission was not thought to be the primary route of transmission - Initially, WHO did not consider airborne transmission via aerosols to be the main route of transmission for SARS-CoV-2 among the general public. If the primary route was droplet transmission instead of aerosol transmission, handwashing could be more important for reducing transmission than masks. For more information on WHO’s understanding of transmission at the start of the pandemic, see this out-dated document from March 2020. There is now ample evidence that SARS-CoV-2 transmission is primarily airborne. For details, see this WHO scientific brief on transmission routes of SARS-CoV-2. Experimental studies have demonstrated that infectious SARS-CoV-2 can remain airborne for at least 3 hours and potentially up to 16 hours in laboratory-generated aerosols.

Prioritising masks for those who need them most - The demand for medical masks and N95 respirators increased dramatically with the spread of COVID-19. WHO and governments around the world struggled to meet this supply. It is critical that public use of these items does not prevent them being available to those who need them most.

Potential disregard of hygiene measures by mask-wearers – While masks are protective against respiratory disease, other behaviours, such as physical distancing, are also helpful. There was a concern that people wearing masks would overestimate the protection they received from the mask and disregard other key preventative behaviours such as physical distancing and handwashing with soap. Although one study in the U.S. found that counties with mandates to wear masks were associated with people spending 11-24 fewer minutes at home and increased visits to commercial locations. However, a large-scale cluster-randomized controlled trial in Bangladesh found that villages with mask-wearing promotion, which increased mask-wearing from 13% to 42%, also had an increase in physical distancing of 5 percentage points. A study in two Dutch cities also found no association between mask mandates and reduced physical distancing. Similarly, a study in Japan found increased mask wearing to be associated with improved hygiene practices.

Potential self-contamination - There was a fear that masks could become contaminated with SARS-CoV-2 virus then that contamination could transfer to the wearer’s hands and subsequently to the wearer’s mouth. However, CDC has concluded that the risk of infection from touching objects contaminated with SARS-CoV-2 is low.

Difficulties related to wearing masks - People with hearing impairments may struggle to communicate if they rely on lip reading. Mask wearing may be difficult for other sub-groups of the population including children, people with cognitive or intellectual disabilities, people with respiratory problems (unrelated to COVID-19) or those living in hot and humid environments.

Mixed evidence on effectiveness for public protection – In 2020, evidence on the effectiveness mask wearing to reduce transmission of respiratory diseases was largely drawn from studies on influenza, studies in healthcare facilities, and studies of medical masks or respirators. Given that some studies found mask-wearing was inconsistent even among people at risk of infection of respiratory diseases, it was difficult to assess if masks would be worn consistently enough but enough people to stem transmission of SARS-CoV-2.

Improper disposal - Littering of masks or respirators could pose contamination risks for waste management workers or others who come into contact with them.

Fabric mask design varies widely in material, shape, and number of layers, which can affect performance characteristics. Both fabric choice and mask design can impact the effectiveness and wearability of masks. When selecting or making a fabric mask, it is important to consider the following characteristics which will impact filtration efficiency and breathability:

Number of layers: Fabrics have pores and when fabrics are layered to make a mask, it is likely that the pores of one layer will do not align with the pores of another layer, and thus a mask with more layers can block particles better than a mask of fewer layers of the same type of material. The WHO recommends a minimum of three layers for most fabric masks - described in this video. Ideally, the mask should consist of an inner layer made of an absorbent material (e.g. cotton or cotton blend), an outer layer made of water-resistant material (polyester blends, polypropylene), and a middle layer either made of natural or synthetics materials. The following graph illustrates how filtration efficiency increases with the number of layers.

This video, based on a single case study, illustrates how 1- and 2-layer fabric masks block the release of respiratory droplets and aerosols during speaking, coughing, and sneezing compared with wearing no mask or a medical mask.

The gains in filtration efficiency achieved by using masks with multiple layers will depend on the type of fabric used as well as the number of layers used. For example, the filtration efficiency of a mask made of loose weave cotton (80 TPI) increased by four times when using 2 layers instead of one. In contrast, when using a tightly woven cotton fabric (600 TPI), the gain in filtration efficiency was negligible when using two versus one layer of fabric (Study 1).

Choice of material: Most studies that have attempted to compare materials for fabric masks have used non-standard and inconsistent methods, making it very difficult to directly compare which materials are best. There are a few high-quality datasets that indicate materials that have a tighter weave and do not stretch have higher filtration efficiencies (Kwong 2020). Note that materials with a very tight weave may be difficult to breathe through. Since non-woven, meltblown polypropylene has a relatively high filtration efficiency, hybrid masks that combine non-woven meltblown materials and fabrics generally have higher filtration efficiencies that masks made without a meltblown layer. If the pores of the fabric can be seen when the mask is held up to the light, this indicates an especially low-quality mask.

Fit: Masks come in a variety of acceptable shapes (flat-fold, duckbilled, moulded, etc). Fit is more important than shape. The mask should fit closely around the nose, cheeks, and chin. Poor fit can reduce the performance characteristics of the mask. Unfiltered air may enter or exit the mask at gaps created by poorly fitted masks. If you feel lots of air exiting around the edges of the mask, then this is an indication that the fit is poor.

Exhalation valves: Some commercially available fabric masks include a one-way exhalation valve meant to make it easier to breathe. Masks with such valves are not recommended as they allow the user to expel respiratory secretions and limit the function of the mask for source control.

Several online resources provide simple step-by-step instructions for making your own fabric face mask. The WHO has made a video on sewing tips for making fabric face masks. Fabric face masks are also widely available for purchase from online retailers but do consider the mask characteristics described above before purchasing them.

Masks with clear windows: Some masks have a clear plastic window allowing people with hearing impairments who rely on lip reading to communicate more easily. Medical masks with similar adaptations are approved for use in healthcare settings in the USA.

Hygienic use of fabric face masks

CDC recommends that fabric masks are washed once a day or as soon as they become wet or dirty and suggests that fabric masks are washed by hand or in a washing machine. To store your mask between uses, CDC recommends storing a clean and dry mask in a paper bag to keep it clean.

Author: India Hotopf

Date published: 01.06.2023