WHO underestimates the risk of airborne spread of Covid-19, scientists say.

The potential for Covid-19 to spread through airborne transmission by remaining airborne is being underestimated by the WHO (World Health Organization), said a group of scientists.

In an open letter to be published this week, 239 scientists from 32 countries call for greater recognition of the role of airborne spread of Covid-19 and the need for governments to implement control measures.

WHO guidance states that the virus is transmitted primarily between people through respiratory droplets and contact. Aerosol transmission involves much smaller particles that can remain in the air for long periods of time and can be transmitted to others over distances greater than one meter.

Members of the WHO infection prevention committee said that while aerosol transmission may play a role, there is overwhelming evidence that the main routes of transmission are through direct contact and respiratory droplets expelled during coughing, sneezing or talking. They said introducing new measures to prevent aerosol transmission is impractical and unlikely to make much difference in the spread of infection.

The letter to be published in the journal Clinical Infectious Diseases is authored by Lidia Morawska of the Queensland University of Technology in Brisbane and Donald Milton of the University of Maryland, and has been endorsed by more than 200 scientists, including some who were involved in the elaboration of the WHO council.

They say emerging evidence, including from places like meat processing plants where there have been outbreaks, suggests that airborne transmission may be more important than the WHO has acknowledged.

Linsey Marr, an airborne virus transmission expert at Virginia Tech and a co-signer of the letter, told the New York Times that the WHO relied on studies of hospitals that suggested low levels of virus in the air. This underestimated the risk, she said, because in most buildings "the air exchange rate is usually much lower, allowing viruses to accumulate in the air."

 WHO states that certain medical procedures, such as intubation, are known to increase the risk of aerosol transmission, but that outside this context the evidence is less clear. “This is an active research area,” says the WHO.

Paul Hunter, a professor of medicine at the University of East Anglia and a member of the WHO infection prevention committee, said the WHO had struck the right balance in its advice.

“Aerosol transmission can occur, but it's probably not that important in the grand scheme of things. It's all about droplets,” he said. “Controlling airborne transmission will not do much to control the spread of Covid-19. This imposes unnecessary burdens, particularly in countries where they do not yet have sufficient staff or trained resources. ”

If airborne transmission has been shown to be a major factor, some experts have suggested that it could be helpful to wear masks indoors, even in places where social distancing is being enforced; that stricter regulations may be necessary for ventilation and air conditioning in order to minimize air recirculation; and that it may even be appropriate to install UV lights in some buildings to guard against potentially infectious particles.