Friday 4 September 2020

Why face masks can prevent coronavirus spread

How well do face masks work in terms of minimizing coronavirus spread? The evidence is compelling if a person is infected and the right type of mask is worn (and worn correctly). Scientific data challenges the opinions of some leading politicians.

There's been lots of research about face masks, looking into the efficiency of surgical and respiratory masks, and ways hat people can attempt to make their own masks. In this edition of Essential Science we briefly look at where face masks have come from, how well they work , and what are the concerns with home-made face coverings.

Where did face masks come from?
Face masks have a long history. There is documentary evidence, for example, indicating that surgeons in the American Civil War (1860-1865) acknowledged the environmental risks from field surgery. For instance, army physicians speculated that the spread of pus-formation from one patient to another was probably airborne, which led to rudimentary face coverings being worn as well as the use of tent cloth to create divisions between infected patients who were believed to be contagious from others.
Do face masks slow down coronavirus spread?
There is scientific evidence that face masks reduce coronavirus spread. This is based on airborne transmission via respiratory aerosols representing the dominant route for the spread of COVID-19. Researcher based at Texas A&M University calculated that over 66,000 infections were prevented by using a face mask in little over a month in New York City. This research is published in the Proceedings of the National Academy of Sciences and it is titled "Identifying airborne transmission as the dominant route for the spread of COVID-19."

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As well as with new York City, a global study found that regions with an early interest in face masks had milder COVID-19 epidemics compared with countries like the U.S. and U.K. that delayed asking their citizens to wear face masks.
Putting a mask on the infection source is up to 300 times more effective than putting a mask on the potential receiver of the infectious droplets. It stands that wearing a mask does not help all that much if a person is not infected (in terms of not contracting the virus), as masks do not filter out viruses from the air. However, the use of a mask does prevent viruses from being breathed out. In assessing this, the assumption that a person is not infected should not be a factor for consideration, since the majority of people with the virus show no symptoms (that this, they are asymptomatic).

In terms of how wearing masks slows down infection spread, University of Maryland researchers ran a series of laboratory experiments where it was shown that face masks significantly reduced the amounts of various airborne viruses coming from infected patients. This was as measured using the breath-capturing 'Gesundheit II machine.'

Time matters
How long a mask is worn for is an important consideration (masks tend to become less effective after four hours of wear). It also follows that the more time a person spends in an environment where the virus is present, the less effective a mask becomes. In other words, don't go to a bar for four hours and think you're risk free because you're wearing a mask.
The surrounding humidity is also a factor, according to scientists at the University of Arizona, as aerosol size can be affected by humidity,in that if the air is drier, then aerosols become smaller faster. If humidity is higher, then aerosols will stay larger for a longer period of time, dropping out faster onto surfaces, leading to high level of viral contamination.

These findings are set out in the Journal of Hospital Infection. See: "COVID-19 and non-traditional mask use: How do various materials compare in reducing the infection risk for mask wearers?"
Face coverings
Face coverings can help us protect each other and reduce the spread of the disease from someone who is suffering from coronavirus, but not showing symptoms. In many countries, people have to wear a face covering on public transport and in shops.This is what’s known as ‘source control’

However, the standard of face coverings isn’t all that high; most don’t fit properly; and people tend to re-use them. Aerosol visualization technology shows how loosely folded facemasks and bandanna-style coverings provide minimal stopping-capability for the smallest aerosolized respiratory droplets.

This means surgical masks are far more effective. However, if there is no other resort, then home-made cloth face masks can offer some protection. These need a minimum of two layers (and preferably three). This is necessary to prevent the dispersal of viral droplets from the nose and mouth.

The effectiveness of multi-layers with home-spun masks is outlined in a paper published in the journal Thorax, titled "ace coverings and mask to minimise droplet dispersion and aerosolisation: a video case study."
With the use of multi-layers, a separate research study finds that a combination of cotton with natural silk or chiffon can effectively filter out aerosol particles. This is presented in ACS Nano ("Aerosol Filtration Efficiency of Common Fabrics Used in Respiratory Cloth Masks").
Overall summary
The consensus of the science is where face masks are worn both in the home (particularly by the person showing symptoms) and outside the home in situations where meeting others is likely (for example, when shopping), masks do have an impact on transmission with a relatively small impact on a person's lifestyle. Evidence from mathematical models shows that routine facemask use by 50 percent or more of the population will reduce COVID-19 spread to an R-number less than 1.0, flattening future disease waves and allowing less-stringent lockdowns. The reproduction or 'R' number relates to the number of people an infected individual passes the virus onto. This needs to stay below 1.0 for the pandemic to slow.
It remains that face masks are the last line of defense. It still remains that do be safe, stay away; if you need to go, keep two meters apart from other people; and keep washing or sanitizing your hands.

Posted by Dr. Tim Sandle, Pharmaceutical Microbiology Resources (

1 comment:

  1. Very interesting article and associated links Tim, thanks for posting.


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