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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:

Response to Greenhalgh et al.: Face masks, the precautionary principle, and evidence-informed policy

As the Covid-19 crisis deepens, Greenhalgh and colleagues argue that policymakers should encourage the public to wear face masks.(1) The evidence for such a measure is sparse. Two systematic reviews—both currently unreviewed preprints—conclude that “the evidence is not sufficiently strong to support widespread use of facemasks,”(2) and that “masks alone have no significant effect in interrupting spread of [influenza-like illnesses].”(3) Another preprint review, with more opaque methods, recommends “adoption of public cloth mask wearing, as an effective form of source control.”(4) With an author of this review, Greenhalgh has written a public-facing summary of the evidence, concluding that widespread uptake is warranted, and might even merit enforcement.(5)

In the absence of scientific consensus, Greenhalgh et al. invoke the ‘precautionary principle’. They suggest that enough evidence exists to suggest plausible, if not probable, benefit, and that the downsides are either negligible or improbable.(1) The precautionary principle is conventionally used to advise caution in the uptake of innovations with known benefits but uncertain or unmeasurable downsides.(6,7) Greenhalgh et al. take the opposite approach: that action is imperative because the risks are minimal and the potential benefits great. They identify four possible counter-arguments: the lack of evidence of effectiveness; the risk of poor adherence reducing effectiveness; the potential for ‘risk compensation’;(8) and the potential impact on mask supplies, particularly for healthcare workers. Given the unprecedented circumstances, they suggest that these arguments are either invalid, or outweighed by the likely upsides.

We agree that rapid translation of scientific knowledge into policy and practice is vital in a crisis. However, the potential downsides of a policy shift towards the mass adoption of face masks deserve more consideration. We offer five arguments to support a more conventional application of the precautionary principle.

First, the evidence for the effectiveness of face masks in reducing viral transmission is very weak. Few studies examine the use of face masks in community settings; those that do find no evidence of reduced transmission compared with no face masks.(2,3) Absence of evidence is not evidence of absence: both recent systematic reviews cautiously suggest that in some circumstances, wearing of face masks may be warranted.(2,3) They also note, however, the absence of systematic study of harms.(3)

Second, it is unclear whether the public is equipped to make proper use of face masks, and how readily good practice might be disseminated. Even healthcare workers can struggle with appropriate mask use;(9) poor use reduces effectiveness and poses an infection risk in itself. Used disposable face masks must be removed and discarded properly because they accumulate pathogens.(10) For non-disposable cloth-based masks, the evidence base is slim, although one hospital-based trial found worse infection outcomes from cloth masks than medical masks.(11) Greenhalgh and Howard advocate homemade masks fashioned from a “t-shirt, handkerchief, or paper towel,” ideally using “tightly woven fabric” and “including a layer of paper towel.”(5) The availability of time, space and materials to prepare, don, doff, and dispose of or properly disinfect a homemade face mask will vary markedly, with consequences for the efficacy of individuals’ masks and their effectiveness as a public health measure.

Third, at the microsocial level, encouraging uptake of face masks might lead to reduced compliance with other measures. This is the ‘risk compensation’ thesis, recognised in other areas of public health, for example bicycle helmets and vaccination against sexually transmitted infections.(12,13) The evidence base is inconsistent, with some studies finding evidence of risk compensation (including risks to self and risks to others), and others finding no effect.(8) Its transferability is not self-evident, given the unprecedented nature of the Covid-19 crisis. Nevertheless, there is a case that the use of face masks will reduce adherence to other, more effective, means of interrupting transmission.

Fourth, potential downsides exist at the macrosocial level. Greenhalgh et al. acknowledge that wider uptake of masks might result in greater pressure on supply chains for healthcare workers, but argue that the proper response is to produce more masks.(1) Homemade cloth masks might offer an interim solution, but if they prove ineffective or to be associated with negative outcomes, a rush to obtain equipment intended for use by healthcare workers is conceivable. This would have serious negative implications for healthcare systems and for health inequalities. As a highly visible symbol of virtuous behaviour, those who do not comply may be subject to stigmatisation or worse. Even a permissive policy risks ‘gold-plating’, where over-compliance effectively imposes a universal rule.(14) Businesses or states might see mask-wearing as a warrant for a premature return to ‘business as usual’, justifying unsafe workplaces or crowded commuting conditions.

Fifth, the consequences of such a public health intervention are inherently difficult to anticipate. Face masks are a complex intervention in a complex system: their impacts are emergent, unpredictable, and potentially counterintuitive.(15,16) Due consideration must be given to the harms that might accrue(17)—and to the ease of adaptation, should negative consequences emerge. Given the bluntness of policy as an intervention, adaptation may prove very challenging once a decision to encourage or legislate is made.

The global challenges from the Covid-19 pandemic are unprecedented. The need for urgent action is reflected in both demand for and supply of evidence-informed policy recommendations. However, engaged academia brings perils.(18) Public-facing communication about the scientific view on the balance of risks and benefits from widespread uptake of face masks has exceeded the evidence. In their lay summary, Greenhalgh and Howard declare that “the science says yes.”(5) Alongside the BMJ analysis piece, their principal source is Howard et al.’s literature review, which has not been subject to peer review and took, according to its brief methods section, “a community-driven approach […] for building the paper list used”(4)—an approach with clear potential biases.

Efforts to communicate a position so strongly in favour of widespread use of masks in the community, against current WHO advice and in the face of persistent evidence gaps, risk promoting policy based more on eminence than evidence. The unintended consequences of unequivocal advocacy of a contested position go beyond the downsides of policy implementation: they include the potential erosion of trust in science more generally, when the measures put forward fail to live up to their promise, or result in problems that could be, or had been, anticipated.

We endorse Greenhalgh et al.’s call for more research on the use of face masks. In the meantime, we urge adherence to the conventional understanding of the precautionary principle both in embracing new interventions and in science communication.

Graham P. Martin*, University of Cambridge

Esmée Hanna, De Montfort University

Robert Dingwall, Dingwall Enterprises Ltd / Nottingham Trent University

* Correspondence to: Graham Martin, The Healthcare Improvement Studies Institute, Department of Public Health and Primary Care, University of Cambridge. E-mail

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14. Voermans WJM. Gold-plating and double banking: an overrated problem? In: Snijders HJ, Vogenauer S, editors. Content and meaning of national law in the context of transnational law. Munich: Sellier European Law Publishers; 2009. pp. 79–88.
15. Braithwaite J, Churruca K, Long JC, Ellis LA, Herkes J. When complexity science meets implementation science: a theoretical and empirical analysis of systems change. BMC Med. 2018;16:63.
16. Plsek PE, Greenhalgh T. The challenge of complexity in health care. BMJ. 2001;323(7313):625–8.
17. Bonell C, Jamal F, Melendez-Torres GJ, Cummins S. ‘Dark logic’: theorising the harmful consequences of public health interventions. J Epidemiol Community Health. 2015;69(1):95–8.
18. Oliver K, Kothari A, Mays N. The dark side of coproduction: do the costs outweigh the benefits for health research? Health Res Policy Syst. 2019;17(1):33.

Competing interests: Robert Dingwall is a member of the New and Emerging Respiratory Virus Threats Advisory Group (NERVTAG), which advises the government on the threat posed by new and emerging respiratory viruses. The other authors declare no competing interests.

22 April 2020
Graham P. Martin
Health services researcher
Esmée Hanna; Robert Dingwall
University of Cambridge
THIS Institute, University of Cambridge, Clifford Allbutt Building, Cambridge Biomedical Campus, Cambridge CB2 0AH, UK