The World Health Organization (WHO) has acknowledged there is emerging evidence that the coronavirus disease 2019 (COVID-19) can be spread by tiny particles suspended in the air.

With this, the WHO has released an updated guidance on  the role of airborne droplets in transmission of COVID-19.

The airborne transmission could not be ruled out in crowded, closed or poorly ventilated settings, a WHO official said.

If the evidence is confirmed, it may affect guidelines for indoor spaces.

However, WHO officials have cautioned the evidence is preliminary and requires further assessment.

Benedetta Allegranzi, the WHO’s technical lead for infection prevention and control, said that evidence emerging of airborne transmission of the coronavirus in “crowded, closed, poorly ventilated settings that have been described, cannot be ruled out”.

An open letter from more than 200 scientists had accused the WHO of underestimating the possibility of airborne transmission.

The WHO has so far said that the virus is transmitted through droplets when people cough or sneeze.

“We wanted them to acknowledge the evidence,” Jose Jimenez, a chemist at the University of Colorado who signed the paper, told the Reuters news agency.

“This is definitely not an attack on the WHO. It’s a scientific debate, but we felt we needed to go public because they were refusing to hear the evidence after many conversations with them,” he said.

Another signatory – Professor Benjamin Cowling of Hong Kong University – told the BBC the finding had “important implications”.

“In healthcare settings, if aerosol transmission poses a risk then we understand healthcare workers should really be wearing the best possible preventive equipment… and actually the World Health Organization said that one of the reasons they were not keen to talk about aerosol transmission of Covid-19 is because there’s not a sufficient number of these kind of specialised masks for many parts of the world,” he said.

“And in the community, if we’re thinking about aerosol transmission being a particular risk, then we need to think about how to prevent larger super spreading events, larger outbreaks and those occur in indoor environments with poor ventilation, with crowding and with prolonged close contact.”

For months, the WHO has insisted that Covid-19 is transmitted via droplets emitted when people cough or sneeze.

Droplets that do not linger in the air, but fall onto surfaces – that’s why handwashing has been identified as a key prevention measure.

But 239 scientists from 32 countries don’t agree: they say there is also strong evidence to suggest the virus can also spread in the air: through much tinier particles that float around for hours after people talk, or breathe out.

On the other hand, today,  the WHO admitted there was evidence to suggest this was possible in specific settings, such as enclosed and crowded spaces.

That evidence will have to be thoroughly evaluated, but if it is confirmed, the advice on how to prevent the virus spreading may have to change, and could lead to more widespread use of masks, and more rigorous distancing, especially in bars, restaurants, and on public transport.

The World Health Organization (WHO) has released updated guidance on the role of airborne droplets in transmission of COVID-19, but the institution’s stance on the topic remains largely unchanged.

Airborne transmission means that the illness might have the potential to spread through particles known as aerosols, which can hover in the air after a person has left the area.

In the new guidance, released Thursday (July 9), WHO said that airborne transmission of COVID-19 in crowded, indoor locations with poor ventilation “cannot be ruled out.”

Previously, WHO said that airborne transmission of the new coronavirus was a concern only in hospital settings during certain medical procedures, such as when a patient is being intubated with a breathing tube.

Some reports of outbreaks occurring in restaurants,  Choir practice and fitness classes have suggested the possibility of airborne transmission, the WHO conceded.

But critically, WHO said that other forms of transmission, such as transmission through larger droplets released in coughs and sneezes and via contaminated surfaces, could still explain these clusters.

The guidance also reviews evidence for airborne transmission in laboratory settings, but notes that these lab conditions don’t necessarily reflect the real word.  /Stacy Ang with reports from BBC