The role of masks and respirators in preventing respiratory infections in healthcare and community settings

RCTs in healthcare settings

We identified seven RCTs comparing the efficacy of masks with that of respirators in healthcare settings. Six were conducted before the covid-19 pandemic and one during the pandemic.33454780818283 These trials used different interventions and some of them measured different outcomes. Table 1 presents details of study design, interventions, results, and risk of biases.

A key design issue was whether healthcare workers in the respirator arm of the trial wore their device continuously throughout a shift or whether they wore it only when undertaking what was assumed to be a high risk procedure (so-called targeted or intermittent use of respirators). A recent meta-analysis that separately analyzed continuous and targeted use of respirators found that respirators were considerably more effective than masks if worn continuously.32 Intermittent use of respirators or medical masks only when performing high risk procedures was not protective. Three North American trials found no difference in efficacy between masks and respirators when used intermittently for aerosol generating procedures or treatment of patients with known infection.478182 These findings are consistent with those of a trial in China that compared medical masks, intermittent use of N95 respirators, and continuous use of N95 respirators; this study showed no difference in efficacy between intermittent N95 respirators and medical masks, but considerably greater protection from continuous N95 use.3380

The superiority of N95 respirators over medical masks has also been shown against a range of viral and bacterial respiratory infections.85 These findings suggest that these infections are likely transmitted through inhalation of contaminated air86 because medical masks (designed to stop splash or spray of liquid) were not sufficiently protective. Airborne transmission of common respiratory infections means that the risk of transmission is likely to be widespread throughout health facilities owing to ventilation systems mixing and dispersing air,32 and calls into question many guidelines suggesting risk is only present during close contact or aerosol generating procedures.87

Healthcare facilities and many buildings use heating, ventilation, and air conditioning systems, which can draw in fresh air and might filter the air, are designed primarily for climate control. These systems use ducts to disperse air throughout a building, so human respiratory aerosols generated in one part of a building might be dispersed to another. In reality, infectious airborne particles accumulate and disperse widely in an indoor setting,88 posing a risk to people distant from the source of infection.89

When the covid-19 pandemic began, one RCT had been published comparing cloth masks with single use medical masks in a healthcare setting in Vietnam.45 The incidence of the primary outcome (influenza-like illness) was significantly higher in participants in the cloth mask arm (relative risk 13.00, 95% confidence interval 1.69 to 100.07), but there was no significant difference between arms for laboratory confirmed infection. The study was conducted in a setting where many participants washed their cloth masks by hand in cold water. A further post hoc analysis conducted in 2020 of data from that 2011 trial (after widespread concern about the safety of cloth masks) showed that if cloth masks were washed in a washing machine, their performance was similar to medical masks.90 WHO has since issued a recommendation for washing cloth masks after use at 60°C.91

Observational, case-control, cohort, and experimental studies in healthcare settings

A range of evidence exists from cohort,92 case-control,93949596979899 cross sectional,100101102103104105 laboratory experimental,106107108109110111112 and other epidemiological (including time series, modeling, and case series) studies.113114115116117118119120121122 Some of these studies were conducted during the SARS outbreak,949596979899103104105113116117118119123 others examined transmission of tuberculosis,121124125 respiratory syncytial virus,92 and pertussis.102 A systematic review and meta-analysis performed in early 2020 using observational data from SARS, Middle East respiratory syndrome (MERS), and early data on SARS-CoV-2 showed 85% protection from medical masks or respirators (analyzed together) in all settings combined, with greater protection in the healthcare setting, attributed to increased N95 use.69 In a subanalysis, respirators were 96% effective compared with masks, which were 67% effective.69

Respirators are recommended as source control for people with tuberculosis,121124125 but no studies could be found that measured their clinical efficacy in preventing tuberculosis.126 One study in South Africa found that use of masks as source control by patients with multidrug resistant tuberculosis greatly reduced airborne transmission to guinea pigs exposed to ward air.127 Additionally, an observational study found medical masks protect against nosocomial transmission of pertussis.102 In vivo studies have also reported increasing levels of filtration performance and protection factors (in ascending order) for cloth masks, medical masks, and respirators.108112128

There is a large body of experimental evidence supporting the effectiveness of masks and the superiority of respirators, summarized in a review.32 Laboratory based studies have shown low filtration efficiency of surgical masks compared with N95 respirators.32129 Additionally, aerosol studies have reported that the blocking of exhaled aerosols improves with the number of layers of a mask, and is better with a surgical mask than a cloth mask.35 The overall protection factor of N95 respirators was 8-12 times greater than that of surgical masks.130 Protection improves when two or more surgical masks are worn, but is still lower than protection from respirators.131

RCTs in community settings

We identified 12 heterogeneous RCTs of facemasks in community settings used for primary prevention in wearers without infection.4849132133134135136137138139140141 Of the primary prevention trials, some showed a protective effect of masks, while others did not. Table 2 summarizes the heterogeneity in settings, interventions, outcomes, and results of these trials. In general, there were low rates of compliance or low incidence of infection. The largest trial, a well designed community RCT in Bangladesh, showed statistically significant protection against symptomatic SARS-CoV-2 infection.49 The data suggest masks are protective in high transmission community settings, especially if used early during epidemics, if combined with hand hygiene, and if wearers are compliant.32 A recently published meta-analysis showed that community mask use compared with no mask use protected against influenza-like or covid-like illness, and against PCR confirmed influenza when masks were combined with hand hygiene.32 The most recent community RCT in Norway showed surgical masks were 29% protective against self-reported respiratory symptoms compared with no mask.141

Observational, case-control, cohort, and laboratory studies in community settings

Observational studies might be affected by confounding and bias and should be interpreted cautiously. However, there are several reasons why these biases would all tend to skew results towards the null.32 Cloth masks were used by the general public during the 1918 influenza pandemic142143 and found to be effective. During SARS in 2002-03, masks were reported to be effective in China, Hong Kong, and Canada where compliance was high.144145146 Several observational and natural experiments have shown masks to be protective in high risk settings.142143144145147 Other than aged care and long term care facilities, the highest risk community setting is households because close, prolonged exposure occurs when an infected family member is present. A household study early in the covid-19 pandemic showed that use of masks in the household reduced the risk of infection by 79% when they were worn before the index case became symptomatic, confirming the role of presymptomatic transmission in outbreaks.148

Mask use was mandatory in many countries during the peak of the covid-19 pandemic and various types of product were used.149 A large, well designed case-control study conducted in California, US during the covid-19 pandemic showed consistent use of any mask in indoor public settings reduced risk of infection, with the highest protection provided by a N95 respirator (83%), followed by a surgical mask (66%), and a cloth mask (56%).150 A range of modeling studies have also shown reduced population transmission.149151152153154 Studies that have used epidemiological approaches to analyzing the effect of mask use during universal masking also suggest protection.155156157

Randomized controlled trials

Four RCTs examined mask use as source control505152170 (table 3). In the first RCT in France, there was no difference between the two arms (medical masks v no masks; odds ratio 0.95, 95% confidence interval 0.44 to 2.05), but the trial finished early because of low recruitment and the subsequent influenza A (H1N1) pandemic in 2009.170 The second RCT was conducted among Hajj pilgrims. Lower rates of symptomatic illness (influenza-like illness) were reported among contacts of pilgrims who used masks (31%) compared with those who did not use masks (53%; P=0.04). However, compliance was low, and laboratory confirmed viral infection was not statistically significantly different between the two groups. The third RCT randomized 245 patients with influenza-like illness presenting to a fever clinic to mask or control arms and observed infection rates in their household contacts.171 No difference between the two groups was found in intention-to-treat analysis, however analysis by actual mask use showed low rates of infection in household contacts of patients who wore masks.171 A fourth RCT quantified the amount of influenza virus and seasonal coronavirus in the exhaled breath of participants with a range of respiratory infections, with and without masks. There was a significant reduction in influenza and coronavirus RNA in exhaled aerosols in the mask group.52 Both these viruses were especially found in small aerosols rather than large droplets, but viral load was low.52Table 3 presents further details of the trials.

Other source control studies

An experimental study showed that the spread of influenza virus from an infected patient, measured by coughing onto a Petri dish, might be reduced when the patient wears a facemask or a respirator.172 In this study, nine patients with influenza participated, and each one repeated the experiment with an N95 respirator, a medical mask, and no mask. No virus was detected when an N95 or medical mask was worn for any patient, while seven samples were positive when patients were unmasked. A study on volunteers with influenza reported an almost threefold reduction of viral particles in exhaled breath with the use of medical masks.173 This study also found an almost ninefold higher viral load in fine aerosols compared with larger exhaled particles, supporting evidence that influenza is an airborne virus. During the SARS outbreak, medical and cloth masks were used as source control and reported to be effective.105 A recent study showed that all types of face covering (cloth masks, surgical masks, and respirators) reduce SARS-CoV viral load in exhaled breath of volunteers with covid-19. However, N95 respirators were superior to other face coverings, even when used without training and fit testing.174 There is also evidence that use of facemasks reduces the risk of tuberculosis transmission in people with the disease.175 Despite the lack of human clinical trial data, medical masks are recommended by WHO, the CDC and the European Centre for Disease Prevention and Control to use as source control for patients with tuberculosis.176177178

Occupational health and safety factors

The occupational health and safety hierarchy of controls rates removal of the worker from the hazard or substitution of the hazard as more important than personal protective equipment (PPE).179 However, unlike construction workers and other workers where the hazard (such as a faulty ladder) is incidental to the job, for a health worker, the hazard is the job itself; they must treat patients with deadly infectious diseases, and removal of these patients or substitution of them is clearly not an option. This context and the importance of respirators for health workers has never been explicitly recognized. Infection prevention and control guidelines traditionally consider only assumed mode of transmission, but a risk based approach is recommended that considers the pathogen, the setting, the occupational health and safety requirements, availability of treatment or vaccines, and uncertainty.180Figure 2 presents a model for considering host, pathogen, and organization.181 Availability of masks and respirators is one of the most important organizational factors, particularly in low resource settings or during global PPE shortages, so organizations should ensure adequate supplies of respirators for frontline staff.182 The precautionary principle should be used if a disease has high morbidity and mortality, or for a newly emerging infection with unknown impact or high risk of nosocomial infection.183 Risk perception, cultural background, discomfort, immune status, and pre-existing illness of the wearer might also influence use of masks and respirators.184 When risk perception is high, compliance will be higher, so studies of masks conducted during low risk periods might not reflect actual compliance during a pandemic.185

Fig 2

Multifactorial approach to selection of masks and respirators in healthcare settings

Fit testing of respirators

Fit testing is important for respirators, which can be done using qualitative or quantitative methods.186 For maximum protection, a respirator should fit the face and there should be no gap between the face and the respirator. Facial fit depends on face shape, shape of respirators (dome shape v duckbill shape), and type of straps (head straps v ear loops). Ear loops have poorer fit than head straps. Fit factors for respirators with ear loops (KN95) are lower compared with respirators with head straps.187 However, respirators might interfere with breathing owing to a build up of carbon dioxide inside the facepiece, particularly in people with pre-existing lung conditions such as asthma and chronic obstructive pulmonary disease.188189 Therefore, medical evaluation is a part of a respiratory protection programme and is highly recommended before the use of respirators for people with chronic obstructive pulmonary disease or other relevant medical conditions.190 A physician or other licensed healthcare professional should perform a medical evaluation of employees and keep records for follow-up.

Mask mandates, universal masking, and situational considerations

Mask policies might vary over time during the same epidemic, or situationally. Guidelines might recommend masks only for people who are sick, or for healthy people (universal masking, such as during mask mandates), or for specific settings or specific times. One of the key determinants of the value of community masking during pandemics or serious epidemics is asymptomatic or presymptomatic transmission. Asymptomatic transmission of influenza has long been recognized, but SARS-CoV-2 has a much higher degree of asymptomatic transmission.191 It is estimated that 30-50% of SARS-CoV-2 transmission is asymptomatic.191192 In one study, around 44% (95% confidence interval 30% to 57%) of secondary infections occurred during the presymptomatic stage191; this means that, in a high risk setting, people who are sick or healthy cannot self-identify risk of exposure in a crowded public setting unless testing is done.193194195 This is one of the strongest justifications for mass masking during epidemic waves of SARS-CoV-2. Special consideration is needed for high risk settings such as hospitals and aged care facilities. Nosocomial covid-19 is a major, ongoing problem, with a high mortality rate.196 Masking of staff in clinical areas can reduce this risk. One study showed that during the covid-19 pandemic, facilities that started universal staff masking had smaller epidemics than those that did not.78 Nosocomial influenza risk was also reduced by 50% with mask mandates.197 The covid-19 pandemic also highlighted the importance of asymptomatic, presymptomatic, and early symptomatic transmission, which is a rationale for universal masking in high risk settings or during periods of high community transmission.

Non-standardized practices around mask use during epidemics or pandemics

PPE shortages might occur during pandemics, as seen during the H1N1 pandemic in 2009 and the covid-19 pandemic. Non-standardized practices, such as extended use and reuse of masks and respirators, double masking, and use of cloth masks in healthcare, were seen during the covid-19 pandemic.198199200 Knotting of the ear loops around the ear and tucking around the face, and double masking, might also improve fit and filtration effectiveness.201 Double masking can be done in various combinations, such as cloth covering the medical mask,201 cloth or medical mask over a N95 respirator, or using two medical masks. The aim of using an additional mask is to protect the underlying mask or improve filtration and fit, however this could reduce breathability, resulting in greater discomfort. A simulation study reported more than 85% reduction in the spread of particles emitted during a cough when a three ply cloth mask was used over a three ply medical procedure mask.201

Extended use and reuse of masks and respirators

In situations of shortages, or where cost is a limitation, or to reduce waste, extended use and reuse could be considered. The US CDC provides guidance on extended use and reuse, as well as conventional, contingency, and crisis capacity strategies.202 However, there is no standard definition of extended use and reuse of masks and respirators.203204 Disposable masks are worn for up to eight hours, or during a shift, unless soiled, wet, or damaged. Extended use of a respirator refers to wearing the same N95 respirator or mask longer than the recommended usage time, without removing it between patient encounters.203 Reuse is defined as using the same N95 respirator or mask for several patient encounters with removal and reapplication after encounters.203

There is limited evidence around the safety of these practices and guidelines are inconsistent.205 Pathogens might be present at the outer or inner mask surfaces206 and this could increase the risk of self-contamination. Moreover, extended use could lead to reduction in humid air filtration efficiency.207 Frequent hand washing is recommended for extended use and reuse, and products should be adequately stored for reuse. Finally, facial fit might be lost after prolonged use of respirators, especially trifold shape respirators compared with dome and duckbill shaped. A study in the US showed high rates of fit testing failure after reuse of N95 respirators by healthcare workers, increasing from 38.7% after the first shift of wear to 92.8% after five shifts.208 Various cleaning and disinfection methods for reusing disposable products have been studied, including soap and water, bleach, autoclave, isopropyl alcohol, vaporized hydrogen peroxide, ultraviolet germicidal radiation, ethylene oxide, microwave oven irradiation, steam, and dry heat.209210211212 However, most of these methods could degrade disposable masks. A systematic review found vaporized hydrogen peroxide and ultraviolet germicidal irradiation were the most effective decontamination methods.205

Use of cloth masks

Owing to shortages of disposable masks during the covid-19 pandemic, extensive research on improving the performance of cloth masks was conducted.90213214215216217 Cloth masks are commonly used, particularly in low resource countries, and until the covid-19 pandemic were ignored in policy documents, which all assumed adequate supplies of disposable masks.182 A range of options were used in early 2020, including single or double layer masks, neck gaiters, and bandanas. During covid-19, cloth masks, homemade face coverings, or a scarf or bandana were also recommended and widely used.36 Some observational studies showed they are effective, albeit less than medical masks and respirators.218 A large body of research was conducted in 2020 on last resort strategies for masking, including optimal design of cloth masks. One published RCT of cloth masks showed that they performed poorly.45 Cloth masks might be harmful if not washed daily in hot water owing to moisture retention and contamination accumulation. Analysis of data on washing from a cloth mask trial showed that if washed in a washing machine at high temperature, the cloth masks performed as well as surgical masks, but that poorly washed cloth masks are ineffective.90 Washing can also improve filtration of cloth masks by shrinking pore size.213219

In vivo studies show low filtration efficacy and high particle penetration for cloth masks, ranging from 40% to 90%.107220 A comprehensive study of fit, layers, fabric, washing, and water resistance showed that a high performing cloth mask could be designed.213219 However, cloth masks should not be used in healthcare settings; they could be used in community settings during shortages.

Emerging areas in mask design

The protection function of masks or respirators is related to design, material, and proper use, and more research is needed in all these areas. The design of masks should be improved for proper fit to the face, as well as for easy donning and doffing. A loosely fitted respirator could provide the same protection as a medical mask, while a tightly sealed medical mask or respirator will significantly improve the protection.221 Doffing is a high risk procedure with a risk of self-contamination during doffing, and mask design should enable safer doffing.222 New fabrics should be tested to improve comfort and to minimize adverse events associated with mask or respirator use. Many new designs and fabrics of masks were tested during the covid-19 pandemic to improve comfort, reduce contact transmission, and ease breathing.223224225 However, data are limited on the effectiveness of these methods.