Intended for healthcare professionals

Letters Face masks for the public during covid-19

Efficacy of face masks depends on spatial relation between host and recipient and who is being protected

BMJ 2020; 369 doi: (Published 22 May 2020) Cite this as: BMJ 2020;369:m2016
  1. Cliff Grover, retired chartered chemical engineer and specialist in gas/liquid separation processes
  1. Banchory, UK
  1. cliff.grover{at}

Discussion on whether face masks are effective often fails to differentiate between who is being protected, in what physical manner, and the spatial relation between host and recipient.1

When worn by the host, surgical masks or similar will reduce direct outward velocities of ejected droplets from a cough, sneeze, or ordinary out breathing and can help make velocities more uniform. Such a mask will also typically coalesce very small droplets into larger ones, 2 shifting the droplet size distribution upwards, enabling faster dropout in air before reaching a recipient. If the host’s mask is moist or even wet, velocity reduction and coalescence properties will be largely retained (though limits exist). These principles are used in industrial mesh pad mist eliminators.

The spatial relation between host and recipient is very important. Droplet separation calculations will show that a distance of 2 metres is inadequate for the smallest droplets, even in “still air.”3 In a wind, small and even large droplets might be blown considerable distances from the host. Less than 2 metres might be adequate to avoid infecting a recipient if the host is downwind of the recipient; much larger distances than 2 metres might be insufficient if the host is upwind. A crosswind is favourable. If a host coughs or sneezes in an environment with an air conditioning system, even non-perceived cross draughts will distribute small droplets in the room or building.4

To protect the recipient from droplets reaching their own mask requires an N95 or similar device (or full air breather filter), in agreement with established wisdom, as its primary function is to absorb or filter these out.

Such qualitative factors can be modelled and quantified using computational fluid dynamics.35


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