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Terry Maguire looks at the evidence for the usefulness of mask-wearing and the type of face-coverings that are most efficient

My daughter, who studied fashion at University, has found a lucrative niche in designer cloth face masks and they’re proving very popular among her fashion-conscious peers and their trendy mothers. My other daughter, teaching overseas, cannot leave her apartment without a surgical face mask and of course internationally, there is more and more pressure to make the wearing of face-masks mandatory when in public. The face mask fashion is becoming de rigueur and the modern equivalent of the late medieval Plague
Doctor’s attire.   

SARS-CoV-2 is most likely transmitted by inhalation of both droplets and aerosols near the infected individual. This virus is very small indeed. The virus size is as small as a party balloon, as a party balloon is as small as the world; pretty small, and there are lots and lots of them — each day a Covid-19 sufferer might emit 1,011 viruses. That 12 zeros is a big number. It is also likely that before they get symptoms, if even they do, people are spreading the virus. We are doing what we can to mitigate against this virus; washing our hands often and socially isolating, so what is the evidence of adding the wearing of face masks to the list of things that might save us?

Certainly, studies indicate that cloth masks are ineffective at preventing SARS-CoV-2 transmission, whether worn by the infected person or as personal protective equipment (PPE) in those trying to avoid contact. Surgical masks likely have some use in limiting virus  dispersal to another person in a healthcare setting by stopping the spread of large cough particles and limit their dispersion. 


The best evidence of a mask’s performance is its filter efficiency and its fit. Masks are supposed to collect viral particles through physical mechanisms, including diffusion (small particles) and interception and impaction (large particles). The gold standard is N95 filtering face-piece respirators (FFRs), which are constructed from electret filter material that allows electrostatic attraction of viral particles for additional collection of all
particle sizes. 

The fit should be a measure of how well the mask prevents leakage of viral particles around the face mask. In the US, the National Institute for Occupational Safety and Health (NIOSH) conducted a study of the filtering performance of clothing materials. All cloth masks had near zero efficiency at 0.3µm, a particle size that easily penetrates into the lungs.

Another study evaluated 44 masks and unsurprisingly, the N95 mask filter efficiency was greater than 95 per cent. Surgical masks exhibited 55 per cent efficiency, general masks 38 per cent, and handkerchiefs from 2 per cent (one layer) up to 13 per cent (four layers).

The N95 FFRs should achieve a fit so that the face-piece must lower the outside concentration of particles by 99 per cent. When fit is measured on a mask with inefficient filters, it is really a measure of the collection of particles by the filter, plus how well the mask prevents particles from leaking around the face-piece. Cloth masks exhibit very low filter efficiency. Thus, even masks that fit well against the face will not prevent inhalation of small particles by the wearer or emission of small particles from the wearer.

One study of surgical mask fit suggests that poor fit can be somewhat offset by good filter collection, but will not approach the level of protection required to reduce the risk of viral infection, and most surgical masks have very poor filter performance anyway. 

Cloth failure

Cloth face masks in clinical settings go back to the late 1800s, first as source control on patients and nurses, and later as PPE by nurses. Cloth face masks failed miserably in stopping the 1918 influenza pandemic because the number of cloth layers needed to achieve acceptable particle efficiency made them difficult to breathe through and caused leakage around the mask. So, cloth face masks are inefficient filters and a poor fit, and there is no evidence to support their use by the public or healthcare workers to control the emission of particles from the wearer. 

Household studies found very limited effectiveness of surgical masks at reducing respiratory illness in other household members and clinical trials in the surgery theatre have found no difference in wound infection rates with and without surgical masks. Despite these findings, it has been difficult for surgeons to give up a long-standing practice.

Wearing surgical masks in households appears to have very little impact on transmission of respiratory disease. One possible reason may be that masks are not likely worn continuously in households, suggesting that surgical masks worn by the public will have no or very low impact on disease transmission during a pandemic. A randomised trial comparing the effect of surgical and cloth masks on healthcare worker illness found that those wearing cloth masks were 13 times more likely to experience influenza-like illness than those wearing surgical masks.

Leaving aside the fact that cloth and surgical masks are ineffective, telling or requiring the public to wear them could be interpreted by some to mean that people are safe to stop washing their hands regularly or isolating at home. Masks may confuse that message and give people a false sense of security. 

If masks had been the solution in Asia, shouldn’t they have stopped the pandemic before it spread elsewhere?  But I wouldn’t want too many people to heed the science here, as my daughter has a budding little industry going.

Terry Maguire
Terry Maguire

Terry Maguire owns two pharmacies in Belfast. He is an honorary senior lecturer at the School of Pharmacy, Queen’s University of Belfast. His research interests include the contribution of community pharmacy to improving public health