A group of 239 scientists from 32 countries have penned an open letter addressing the World Health Organisation's current Covid-19 advice.

The letter, due to be published this week, is calling for greater acknowledgement of the role of the airborne spread of Covid-19 and the need for governments to implement control measures.

The potential for Covid-19 to spread through airborne transmission by lingering in the air is being underplayed by the WHO, the scientists agreed.

WHO guidance so far states that the virus is transmitted primarily between people through respiratory droplets and contact.

However, aerosol transmission involves much smaller particles that can remain in the air for long periods of time.

Such transmissions can occur over distances greater than 1m, which vastly changes the understanding of the virus' spread.

What WHO has said?

Members of the WHO’s infection prevention committee have said that aerosol transmission may play some role.

But there is overwhelming evidence that the primary routes of transmission are through direct contact and respiratory droplets expelled during coughing, sneezing or speech.

They said introducing new measures to guard against aerosol transmission was unfeasible and unlikely to make much difference to the spread of infection.

Scientists have changed their view

The letter is due to be published in the journal Clinical Infectious Diseases.

Some of the scientists who have penned the letter have been involved in drawing up the WHO’s current advice.

But they say emerging evidence suggests that airborne transmission could be more important than the WHO has acknowledged.

This evidence has been derived from settings such as meat processing plants where there have been outbreaks.

In WHO's defence

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

“Aerosol transmission can occur but it probably isn’t that important in the grand scheme of things. It’s all about droplets,” he said.

“Controlling airborne transmission isn’t going to do that much to control the spread of Covid-19. It’s going to impose unnecessary burdens, particularly in countries where they don’t have enough trained staff or resources already.”

If airborne transmission were shown to be a major factor, some experts have suggested it could be helpful to wear masks indoors, even in settings where social distancing is being enforced.

Other measures could include tighter regulations for ventilation and air conditioning to minimise recirculating air, and that it may even be appropriate to install UV lights in some buildings to guard against potentially infectious particles.

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