If face masks are to be required in public places, what is the evidence for their effectiveness and how can we avoid diverting a scarce resource from front-line NHS and social care staff?
The question of whether or not to use face masks widely is important because, in the absence of a suitable pharmaceutical measure – be it vaccines to prevent or drugs to treat SARS-CoV-2 infection – non-pharmaceutical measures are currently the only means of controlling infection.
It has been suggested that the wearing of face masks might become the norm when the lockdown ends but before mass vaccination against SARS-CoV-2 can be implemented. The mayor of London, Sadiq Khan, has called for the wearing of ‘non-medical face masks’ in situations where social distancing is not possible. Trisha Greenhalgh, writing in the BMJ, argues that the use of face masks (type not specified) by the public is a sensible precaution.1 However, the evidence for the effectiveness of face masks in preventing infection is weak and in the UK there is concern that the use of medical masks would divert scarce supplies from front-line NHS and social care staff. Others have argued that the wearing of masks could create false sense of security and lead people to pay less attention to handwashing and social distancing.
Several different types of masks are available:
- Disposable medical masks (surgical masks, fluid-resistant surgical masks) – designed to prevent contamination of wounds by medical personnel and to protect wearer from splashes/sprays of body fluids.
- Respirators (FFP3 masks, N95, P2 masks) tight-fitting, protect wearer against fine particles; disposable
- Cloth masks – (non-medical masks) often DIY, can be one or more layers of cloth; or antimicrobial fabric. Suitable for repeated use if regularly decontaminated.
For masks to be effective at preventing transmission of infection in non-healthcare settings we have to assume that the predominant mode of transmission is through inhalation of droplets resulting from coughs or sneezes. Masks could then work either by preventing release of droplets from infected persons and/or by preventing inhalation of droplets by uninfected persons.
Two recent systematic reviews are of interest here. Xiao and colleagues2 found that trials of masks to reduce influenza infections were a mixed bag of trials that involved masking infected persons to prevent passing on infection, masking of uninfected people to prevent from becoming infected and general use of masks to reduce infection rates. Compliance was always an issue – masks don’t work if they are not worn – and many studies were small or underpowered. This review found no significant effect of face masks on transmission of laboratory-confirmed influenza.
Jefferson and colleagues (in the updated Cochrane review, preprint)3 concluded that masks alone had no significant effect in interrupting spread of influenza-like illness. They also made the point that further study is necessary and that benefits need to be balanced against harms. They noted that after the SARS epidemic in 2003, a large number of harms were identified including respiratory fatigue, poor work capability, increased nasal resistance, elevated levels of carbon dioxide, facial dermatitis, acne and potential self-contamination events. Although all the trials were conducted during outbreaks of seasonal influenza – a disease with similar modes of transmission to COVID-19 – they were undertaken during relatively low-transmission periods, and this limits generalisability to the COVID-19 situation.
If masks are in short supply then perhaps a different approach is needed, such as the use of re-usable masks. A number of institutions in the USA have looked at methods for sterilising N95 masks so that they can be re-used. For example, Duke University has established a method using hydrogen peroxide vapour – a standard technique for sterilisation in pharmaceutical isolators.
In Israel the Health Ministry has begun a project, in cooperation with textile experts, to produce high quality masks that are washable, re-usable and suitable for widespread public use. On the first day of operation, 300,000 masks were produced. Residents of areas where the infection rate is high will each receive a kit containing three masks and instructions for correct use.
A different approach has been adopted in Vietnam where all citizens are now required to wear masks. Disposable medical masks can be sterilised by microwaving at 800W for one minute if the masks are first moistened. Effectively this is steam sterilisation. This method is only suitable for standard disposable medical masks and cloth masks, other types of mask could contain components that are not suitable or safe for microwaving. Masks should be microwaved singly, preferably in a dedicated microwave oven (in hospitals and offices) rather than one that is also used for food.
Washable cloth masks were compared with surgical masks (in healthcare workers) in one study.4 This was a three-armed study that compared surgical masks, cloth masks, and ‘no-mask’ (control arm) The results showed that cloth masks were associated with significantly higher rates of influenza-like illness compared with the control (‘no mask’) arm. The authors concluded that cloth masks should not be recommended for health care workers. However, the control group followed ‘usual practice’ with regard to mask wearing. In fact only 1% of the control group wore ‘no mask’ (the majority wore cloth masks or a mixture of cloth and surgical masks. Thus the results do not tell us whether a cloth mask is better or worse than no mask.
In a recent addendum, the authors acknowledge that cloth masks used in the study could have become damp and contaminated during use. They therefore suggest that cloth masks, used a last resort, could be sprayed with sanitiser or subjected to UV disinfection during shift breaks. The cloth masks used were double-layer cotton fabric. Basic principles suggest that UV disinfection would be a poor option because UV light cannot disinfect what it cannot reach. This is one of the reasons why the Vietnamese authorities have recommended microwaving moistened disposable masks at 800W for one minute instead.
If the use of non-medical face masks by the public in the UK is recommended, there could be a proliferation of DIY masks. It may be useful to know that the Centers for Disease Control (CDC) has produced detailed guidance on cloth face coverings including instructions for sewn and ‘no-sew’ face protection masks. 5 Another emergency approach comes from the French Medicine Academy, which has instructions on how to make a disposable mask using a paper napkin, two elastic bands and a stapler.6
Those interested in the particle filtration efficiency of cloth masks will be interested to see a pre-print that shows how adding a layer of nylon stocking (or tights) can improve the filtration efficiency of a mask.7
Konda and colleagues provide a more detailed discussion of the mechanics of masks and note that gaps resulting from an improper fit of the mask can result in over a 60% decrease in the filtration efficiency.8
- Masks probably do not prevent infection in the wearer but could reduce the risk of infecting others especially if the wearer is infected but asymptomatic.
- Masks need to be put on and removed carefully so as not to transfer infection from via the fingers.
- Masks need to fit snuggly, with no gaps, and be worn to cover the nose and mouth.
- Wearing a mask does not make you invulnerable – handwashing and social distancing remain important.
- Greenhalgh T et al. Face masks for the public during the Covid-19 crisis. BMJ 2020;369:m1435.
- Xiao J et al . Nonpharmaceutical measures for pandemic influenza in nonhealthcare settings-personal protective and environmental measures. Emerg Infect Dis 2020;26. 10.3201/eid2605.190994 32027586
- Jefferson T et al. Physical interventions to interrupt or reduce the spread of respiratory viruses. Part 1: face masks, eye protection and person distancing: systematic review and meta-analysis. medRxiv 2020.03.30.20047217. [Preprint.] 10.1101/2020.03.30.20047217
- MacIntyre CR et al . A cluster randomised trial of cloth masks compared with medical masks in healthcare workers. BMJ Open 2015;5:e006577. 10.1136/bmjopen-2014-006577 25903751.
- Centers for Disease Control and Prevention. Use of cloth face coverings to help slow the spread of COVID-19. www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/diy-cloth-face-coverings.html (accessed April 2020).
- The Connexion. French Medicine Academy: How to make a Covid mask. www.connexionfrance.com/French-news/French-Medicine-Academy-How-to-make-Covid-19-mask (accessed April 2020).
- Mueller A, Fernandez L. Assessment of fabric masks as alternatives to standard surgical masks in terms of particle filtration efficiency. medRxiv preprint doi: https://doi.org/10.1101/2020.04.17.20069567.
- Konda A et al. Aerosol filtration efficiency of common fabrics used in respiratory cloth masks. ACS Nano Article ASAP DOI: 10.1021/acsnano.0c03252