In an open letter to the World Health Organization, 239 scientists from 32 countries have claimed that the Covid-19 coronavirus is airborne, The New York Times reported on Saturday. The scientists plan to publish their findings in a journal this week.

The claim contradicts previous evidence that suggested that it was transmitted from person to person through droplets from the nose or mouth, which are expelled when a person with the disease coughs, sneezes or speaks.

WHO has so far emphasised that the virus can spread through the air only in case of medical procedures that produce aerosols, or droplets smaller than five microns. The global health body has instead promoted frequent hand washing as a means to keep the virus away, though the probability of the contagion spreading through surfaces is very low.

But Dr Benedetta Allegranzi, WHO’s technical lead on infection control, said the evidence that coronavirus is airborne is unconvincing. “Especially in the last couple of months, we have been stating several times that we consider airborne transmission as possible but certainly not supported by solid or even clear evidence,” she said. “There is a strong debate on this.”

In April, a group of 36 experts on air quality and aerosols had urged WHO to consider growing evidence on airborne transmission of the virus. The health body called Lidia Morawska, the group’s leader and a WHO consultant, for a meeting. However, at the meeting, WHO experts continued to stress the importance of hand washing, and felt it should be emphasised over aerosols, The New York Times reported.

At the meeting, Morawska provided evidence of several incidents that indicate airborne transmission of the virus, contending that the WHO was making an artificial distinction between tiny aerosols and larger droplets, though infected individuals produce both.

Dr Linsey Marr, an expert in airborne transmission of viruses at Virginia Tech University in the United States, said the scientific community knows since 1946 that coughing and talking generate aerosols. She said that though scientists have been unable to grow the virus from aerosols in the laboratory, most of the samples taken are from hospital rooms with good air flow, which dilutes viral levels.

Dr Marr also contended that WHO is relying on an outdated definition of airborne transmission – a definition that states that an airborne pathogen has to be highly infectious and to travel long distances.

Concerns about airborne transmission of the coronavirus have intensified following a massive surge in cases in some countries, such as the United States, which has been reporting close to 50,000 new infections per day. Dr Marr said “superspreader” events can be explained by the hypothesis that the virus is airborne.

As of Monday morning, the coronavirus has infected 1,14,19,638 people globally, and killed 533,781. As many as 6,161,732 people have recovered, according to the Johns Hopkins University.

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