The World Health Organization (WHO) is reviewing a report that suggested its advice on the novel coronavirus needs updating, after scientists wrote an open letter saying there was evidence the virus could be spread by tiny particles in the air.
The WHO says Covid-19 spreads primarily through small droplets, which are expelled from the nose and mouth when an infected person breaths them out in coughs, sneezes, speech or laughter and quickly sink to the ground.
In an open letter published in the journal Clinical Infectious Diseases, 239 scientists in 32 countries outlined the evidence they say shows that smaller exhaled particles can infect people who inhale them, the newspaper said on Saturday.
Because those smaller particles can linger in the air longer, the scientists are urging WHO to update its guidance.
They wrote: "Studies by the signatories and other scientists have demonstrated beyond any reasonable doubt that viruses are released during exhalation, talking, and coughing in microdroplets small enough to remain aloft in air and pose a risk of exposure at distances beyond one to two metres from an infected individual."
The WHO said in a statement: "We are aware of the article and are reviewing its contents with our technical experts."
The letter, whose lead author is is Linsey Marr, a professor of engineering at Virginia Tech in the US and a world renowned expert on the spread of airborne diseases, says WHO guidance focuses too heavily on handwashing and social distancing.
"Hand washing and social distancing are appropriate, but in our view, insufficient to provide protection from virus-carrying respiratory microdroplets released into the air by infected people.
"This problem is especially acute in indoor or enclosed environments, particularly those that are crowded and inadequately ventilated," they wrote.
The scientists made a number of recommendations including that public buildings are well ventilated and overcrowding on public transport is avoided.
Professor Babak Javid, consultant in infectious diseases at Cambridge University Hospitals and not one of the letter's signatories, said the precise method of coronavirus transmission was still not fully understood.
He added: "It seems increasingly likely that transmission from surfaces (fomites) is uncommon, whereas transmission by close and prolonged contact in indoor spaces is responsible for the majority of transmission events. Such transmission can either be via large or small droplets.
"Whilst this may seem like hair-splitting, it has important practical considerations, since mitigation measures differ between modes of transmission. For example, the emphasis on hand-washing is really only very effective if fomite transmission predominates. Similarly, physical distancing is more relevant for large droplet transmission."
Any change in the WHO's assessment of risk of transmission could affect its current advice on one-metre physical distancing. Governments, which also rely on the agency for guidance policy, may also have to adjust public health measures aimed at curbing the spread of the virus.
WHO guidance to health workers, dated June 29, says that SARS-CoV-2, the virus that causes Covid-19, is primarily transmitted between people through respiratory droplets and on surfaces. But airborne transmission via smaller particles is possible in some circumstances, such as when performing intubation and aerosol generating procedures, it says.
Medical workers performing such procedures should wear heavy duty N95 respiratory masks and other protective equipment in an adequately ventilated room, the WHO says.
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