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Airborne transmission of Covid-19 “cannot be ruled out”, a World Health Organisation (WHO) official has said, after 239 scientists penned an open letter warning against underestimating the possibility of this mode of transmission.

There’s emerging evidence that suggests the virus might linger in the air after being expelled from a person’s nose and mouth – this can occur when a person breathes, talks or coughs. 

People release these micro-droplets – also known as aerosols – which are small enough to float around in the air, say scientists, and they pose a risk of exposure at distances beyond 1-2 metres from an infected individual.

Studies have found these small droplets could actually travel tens of metres in indoor settings. The worry is people could then breathe in these droplets and potentially fall ill.

“We are advocating for the use of preventive measures to mitigate this route of airborne transmission,” scientists wrote in the open letter.

Since the letter was published, the WHO has acknowledged that airborne transmission is a possibility. Something that, up until now, it said there was no evidence to prove.

During a news briefing on Tuesday, Benedetta Allegranzi, technical lead for infection prevention and control at the WHO, said the possibility of airborne transmission in public settings – particularly crowded, closed and poorly ventilated areas – “cannot be ruled out”.

“However, the evidence needs to be gathered and interpreted, and we continue to support this,” she added. 

What has been said previously about airborne transmission of Covid-19?

Up until now, it’s been widely agreed Covid-19 can spread from person to person through larger droplets expelled from the nose or mouth when a person coughs, sneezes, exhales or speaks. These droplets could be inhaled by an individual close by or land on surfaces around the person, and we can then catch the virus by touching these surfaces, then touching our eyes, nose or mouth.

But whether or not the virus is airborne has been fiercely contested – the World Health Organisation previously suggested there wasn’t enough evidence to show the disease is airborne – in an analysis of 75,465 Covid-19 cases in China, airborne transmission was not reported.

It did suggest, however, airborne transmission “might be possible” in healthcare settings, especially during certain procedures or treatments (for example, disconnecting someone from a ventilator). This is why personal protective equipment (PPE) is so vital for healthcare workers.

Paul Hunter, an infectious diseases expert and Professor in Medicine at University of East Anglia, previously told HuffPost UK: “Although droplets actually have to move through the air [to infect someone], they don’t tend to be airborne.”

Airborne, in this sense, would mean the viruses could stay in the air for a long period of time and travel long distances to infect somebody, he said. “Although it is possible that Covid-19 is in certain circumstances airborne, it probably doesn’t happen very often,” he stressed.

That said, scientists in Finland used modelling to show aerosol particles could remain in the air for several minutes after a person expels them – either through coughing, sneezing or speaking “then end up in the respiratory tract of others in the vicinity,” according to Ville Vuorinen, assistant professor at Aalto University.

What could be done to prevent airborne transmission?

Scientists who signed the open letter said certain measures should be taken to factor in the airborne transmission risk. One such measure is to provide “sufficient and effective ventilation” to indoor spaces – through supplying clean outdoor air and minimising recirculated air – particularly in public buildings, workplace environments, schools, hospitals, and care homes.

Other ideas include supplementing general ventilation with airborne infection controls such as using filters that can actually sift out the virus or ultraviolet lights that can kill the virus. And lastly, telling people to avoid overcrowding, particularly on public transport and in public buildings, they said.

Face masks and covers were not mentioned in the open letter. They wouldn’t necessarily help to protect against this mode of transmission.

According to the Canadian Centre for Occupational Health and Safety: “They [surgical masks] do not protect the wearer from inhaling small particles that can remain airborne for long periods of time. Surgical masks are effective barriers for retaining large droplets which can be released from the wearer through talking, coughing, or sneezing.”

N95 type respirators would be better equipped to filter out these particles, however due to global shortages and a great need to keep this PPE back for healthcare workers, members of the public are advised against buying them.