Intended for healthcare professionals

Rapid response to:


Face masks for the public during the covid-19 crisis

BMJ 2020; 369 doi: (Published 09 April 2020) Cite this as: BMJ 2020;369:m1435

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Rapid Response:

COVID-19: how much can we depend on face mask as a long-term strategy?

Greenhalgh et al. [1] argued that the precautionary principle should be applied to support the use of face masks or coverings as we confront the COVID-19 pandemic. They argued that while there was lack of solid research evidence from the point of view of internal validity, the existing studies might not say much about the use of masks by real people trying to stay alive in a serious pandemic (i.e., external validity).

These recommendations are certainly no stranger to me who resides in Asia. In support of face masks, Javid et al. [2] rightly pointed out that some studies that did not find evidence in support of masks were partly due to nonadherence by the participants. They attributed nonadherence to the lack of apprehension about catching influenza. Similar to Greenhalgh et al. [1], they suggested that people may be more vigilant and motivated to wear masks to avoid the more deadly and contagious SARS-CoV-2.

If nonadherence is responsible for the lack of effectiveness of masks, then I suggest that another factor contributing to nonadherence is the fatigue from wearing masks. In my experience, the ordinary citizen often develops “mask fatigue”. Some could not tolerate it throughout the day, as was vividly portrayed by Carr who was in fact an allied health professional and used to wearing masks [3]. Others could but quickly develop resistance to using it further.

Typically, for those who do wear masks during daytime, they cannot wait for the moment when the mask can be taken off upon returning home. Yet, for those who share the household with others, their homes could be a place where they would be exposed to the virus. Familial transmission [4 5] constitutes a significant proportion of the cases. Thus, wearing masks at home, where prolonged close contacts take place on a daily basis, may be as important as wearing them in public.

It may be unwise to advise people to wear masks at home, as excessive demands will likely backfire. However, it may not be a bad idea if one is living in crowded households or with those at-risk such as older relatives. How do we balance usage so that the mask would be worn when it is most needed is a big question yet to be resolved.

It is possible that masks are especially useful in high-risk settings. A recent systematic review and meta-analysis [6] showed that only studies looking at visits to healthcare settings showed consistent effects in favor of masks. For schools/universities, only one of four studies showed a very small effect. For studies conducted in the general community, only one of three studies with a relatively large sample size reported a significant effect, which was unusually massive (a 73% reduction in risk), thus skewing the pooled result. No effects were observed in mass gatherings (Hajj pilgrimage), during air travel, or in households. Importantly, all studies that found positive results were done in Asia. Given Greenhalgh et al.’s [1] emphasis on external validity, more studies in Western communities are needed in the future.

A common argument for using masks is to prevent those infected from passing the virus to others. On this point, a small study by South Korean researchers [7] is revealing as they tested the efficacy of surgical and cotton masks in four actual COVID-19 patients. The patients were asked to cough five times each in the following sequence: without mask, with surgical mask on, with cotton mask on, and without mask again. Surprisingly, there were no noticeable differences in the viral loads collected across the conditions. Moreover, more contamination was found on the outer than on the inner surface of masks, with the former swabbed before the latter. They attributed the results to the turbulent jet from coughing and leaking on the edges of the mask. It was possible that in an experimental situation, the patients made conscious effort to produce the coughing action, leading to more forceful expulsion than usual and consequently reduced ability of the masks in trapping the virus. Moreover, this study does not provide a clue as to the efficacy of face masks in preventing presymptomatic transmission. More research is needed.

Before more research is available, let us not forget that if masks are introduced as a population-wide measure, mask fatigue in substantial numbers of the population will likely set in at some point, followed by nonadherence, as is already happening in Hong Kong. Keeping the mask on is going to be a greater challenge in the summer months, while the moisture from sweat may render the mask ineffective anyway and may also inadvertently create a favourable habitat for the virus [8]. The situation in Singapore suggests that the virus may not burn out in hot weather, which is not too far away from now. In any case, the utility of face masks is likely limited in the current outbreak, but promoting its use now may provide a “rehearsal” and increase readiness to adopt the recommendation if there is another outbreak in fall/winter. Meanwhile, more research about its efficacy in cutting transmission is needed to inform policy at the next outbreak.

1. Greenhalgh T, Schmid MB, Czypionka T, et al. Face masks for the public during the covid-19 crisis. BMJ 2020;:m1435. doi:10.1136/bmj.m1435
2. Javid B, Weekes, MP, Matheson, M. Covid-19: should the public wear face masks. BMJ 2020;369:m1442. doi:10.1136/bmj.m1442
3. Carr CV. Masks diminish the wearer and diminish quality of life. BMJ 2020. Retrieved from
4. Chan JF, Yuan S, Kok K, et al. A familial cluster of penumonia associated with the 2019 novel coronavirus indicating person-to-person transmission: a study of a family cluster. Lancet 2020;395:514-523. doi:/10.1016/ S0140-6736(20)30154-9
5. Qian G, Yang N, Ma AHY, et al. A COVID-19 Transmission within a family cluster by presymptomatic infectors in China. Clin Infect Dis 2020; Epub ahead of print. doi:10.1093/cid/ciaa316
6. Brainard J, Jones N, Lake L et al. Facemasks and similar barriers to prevent respiratory illness such as COVID-19: a rapid systematic review. MedRxiv 2020. doi:10.1101/2020.04.01.20049528
7. Bae S, Kim M, Kim JY, et al. Effectiveness of surgical and cotton masks in blocking SARS-CoV-2: A controlled comparison in 4 patients. Ann Intern Med 2020. Epub ahead of print. doi:10.7326/M20-1342
8. Lazzarino A. Covid-19: important potential side effects of wearing face masks that we should bear in mind. BMJ 2020. Retrieved from

Competing interests: No competing interests

30 April 2020
Sheung-Tak Cheng
University professor
Department of Health and Physical Education, The Education University of Hong Kong
10 Lo Ping Road, Tai Po, N.T., Hong Kong