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Friday, August 14, 2020

Face coverings – Why can’t scientists agree?

When Grant Shapps advised that face coverings would be compulsory for those using public transport, he noted that scientists aren’t in full agreement about face coverings. Yet it is difficult to understand why the transmission of the Covid-19 virus should be a matter of scientific debate. Furthermore, such disagreements weaken the essential requirement for a clear message.

Keeping two-metre social distancing as trains get busier will, to quote Network Rail chief executive Andrew Haines, “result in a very chaotic situation”, which has its own risks. If effective, face coverings offer a way to reduce this distance safely. This issue is therefore of crucial importance to the rail industry and the safety of its passengers and staff.

Asking the right question

Fundamental to this disagreement is what question is being asked. If it is “how can members of the public be protected?”, then home-made face coverings are not the answer. The fine aerosol carrying the virus can only be stopped by a high-filtration surgical grade mask.

A more meaningful question is “how to control the virus at source”, which requires constraining the virus-laden fluid particles from the mouth. A paper produced by the University of Edinburgh entitled “Face Coverings, Aerosol Dispersion and Mitigation of Virus Transmission Risk” explains that the optical technique of schlieren imaging was used to visualise airflows from the mouth. This showed that coughing ejects droplets at around 10 metres per second and concluded that all masks enable a reduction of at least 90 per cent of the distance of the front throughflow.

The paper by Trisha Greenhalgh, Professor of Primary Health Sciences at the University of Oxford, entitled “Face coverings for the public: Laying straw men to rest”, provides detailed evidence to show that face coverings constrain the virus and rebuts criticism of one of her earlier papers.

Follow the science

Professor Greenhalgh considers the basic science of Covid-19, which replicates in the upper respiratory tract and so is likely to be transmitted mainly by relatively large droplets emitted by coughing, sneezing, and speaking. They then quickly turn into aerosols which are much harder to block.

Although individuals suffering from the virus should not be on public transport, Greenhalgh demonstrates that there is significant virus transmission by those without symptoms. She also shows that face covering need not be 100 per cent effective, as mathematical modelling suggests face coverings that are only 60 per cent effective and only worn by 60 per cent of the population would reduce the R number to below 1.0.

She provides examples of the effectiveness of face coverings. In one, a mask-wearing virus carrier who flew from China to Toronto and did not infect anyone else on the plane. Another example is the Czech Republic and Austria, which both introduced social distancing on the same day, with the former mandating compulsory face coverings. New infections fell more quickly in the Czech Republic, and only began to fall in Austria after masks were made mandatory two weeks later.

In her rebuttal of the criticisms of her earlier paper, Greenhalgh considers the respective merits of “systematic” and “narrative” reviews. She considers that, whilst systematic reviews, with their controlled experiments, are suitable for narrowly defined biomedical questions, more complex problems require the insight of a narrative review. She notes that, in the face of a pandemic, the search for perfect evidence may be the enemy of good policy.

Thus, it seems that the scientific disagreement about face coverings is about answering the right question and academic differences over scientific method. Reaching the correct conclusion requires consideration of all relevant factors, including the risks associated with a large increase in road traffic if rail commuter services can only operate at 15 per cent of their capacity.

If all such factors are considered, the rationale for face coverings is irrefutable. Yet this needs to be clearly explained to the public if their use, and any associated reduction in social distancing on trains, is to be accepted.

All safety practitioners know that, in the hierarchy of risk controls, the priority should be to remove the hazard at source and that PPE should be the last line of defence. Trisha Greenhalgh presents a convincing argument to demonstrate that face-coverings can largely eliminate the hazard of Covid-19 at source.

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