Crop researchers back use of face masks outdoors

Evidence backing tougher rules on the use of face masks in the battle against Covid19 has received support from an unlikely area of scientific endeavour – crop research.

With face covering guidance varying between nations within the UK, there has been some uncertainty over when and where to wear masks. But an expert in the transmission of crop diseases has warned that the chances of inhaling water droplets containing coronavirus could be greater than previously thought.

Dr Jon West, who studies the various ways crop diseases can spread at the Rothamstead research centre in Harpenden, Herts, believes that some water droplets produced from coughing, sneezing or even normal speech can persist in the air long enough to travel many metres.

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And his previous research into the spread of plant pathogens in water droplets has backed the wearing of face masks – perhaps even in crowded outdoor areas.

“The behaviour of a water droplet once in the air is the same, whether it holds coronavirus, a plant fungal spore or nothing harmful,” said West.

He said that while it was believed that any large droplets produced when sneezing or coughing could travel very quickly over a few metres they don’t remain airborne for long.

“On the other hand, the microscopic droplets we produce stay airborne for minutes and sometimes hours - but just like smoke they disperse rapidly, and any one individual droplet has a low likelihood of being infective.”

However, he said that studies of the spread of crop fungal diseases showed that a third size-category of medium sized water droplets - ranging in size from a hundredth of a millimetre to one millimetre - could hang around for more than 30 seconds in windy conditions and travel tens of metres - as could droplets containing Coronavirus.

“While it is difficult to study dispersal of human pathogens in real conditions, there is a wealth of literature on the dispersal of plant pathogen spores which can be used as a proxy to estimate droplet dispersal of human viruses.”

He said the wide size range could have seen the role of some particles neglected in previous medical studies.

“But we know many other biological particles in this size range disperse in air, such as spores from the cereal rust fungi which can disperse in air across continents and oceans, or moss spores up that have been collected in the arctic having dispersed from warmer production sites much further south,” said West.

Together with an infectivity which was believed to last several hours, he said that logic pointed towards a strong likelihood that there was considerable potential for an aerosol-based infection route for Covid 19, even in outdoor conditions.

“The fact that many thousands of microscopic droplets are produced by a cough and with the possibility that intermediate-sized droplets can also remain in air for many seconds, travelling many metres in outdoor moving air, greater importance should be placed on using face masks to prevent these droplets being inhaled.”