Does wearing masks offer protection against viruses?

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There seems little doubt that the dispersion of Covid-19 droplets is reduced when people wear masks or face coverings. However, what is controversial is whether masks, of which there are many types, will protect wearers against the inhalation of the virus. Experiments performed at the Animal Virus Research Institute, Pirbright, Surrey (now The Pirbright Institute), many years ago are relevant to the debate.

The effectiveness of masks was tested by allowing a group of volunteers, either wearing or not wearing a mask, to spend time in an isolation room containing animals with foot-and-mouth disease (FMD). The air was contaminated with FMD virus because FMD-infected animals excrete the virus in their breath, with particles ranging in size from droplet nuclei to large droplets.¹

After exposure to the animals, the nostrils of the volunteers were swabbed and tested for the presence of FMD virus. The amount of virus recovered when a mask was worn was compared to when it was not. It was found that a surgical mask or an industrial gauze and cotton wool mask reduced the amount of virus inhaled by 9.5-fold or 8-fold, respectively, but paper masks had no effect.² In terms of infectivity this reduction of virus is very little.

The results demonstrate that industrial, surgical or paper masks provided very little or no protection against the inhalation of airborne FMD virus. The deposition of airborne particles in the human respiratory tract is complex, but it is generally agreed that large particles (>6 μm) are trapped in the nasopharyngeal region.³ Therefore, it can be concluded that the wearing of masks of the type tested would provide very little or no protection against the inhalation of virus associated with smaller particles, such as those in aerosols (<6 μm).

The wearing of masks ... would provide very little or no protection against the inhalation of virus associated with smaller particles

The size of the particles with which airborne Covid-19 is associated is not known, but there is mounting circumstantial evidence that aerosols are primarily involved.⁴ If this is the case then the masks mentioned above are unlikely to protect their wearers against the inhalation of airborne Covid-19.

References

¹ Sellers RF, Parker J. Airborne excretion of foot-and-mouth disease virus. J Hyg 1969; 67: 671–7

² Sellers RF, Donaldson AI, Herniman KAJ. Inhalation, persistence and dispersal of foot-and-mouth disease virus by man. J Hyg 1970; 68: 565–73

³ Cox CS. Inhalation and deposition. In: The Aerobiological Pathway of Microorganisms. Wiley-Blackwell, 1987: 164–7

⁴ Morawska L, Milton DK. It is time to address airborne transmission of Covid-19. Clin Infect Dis 2020; doi: 10.1093/cid/ciaa939