There is "emerging evidence" of that transmission of coronavirus can be airborne, the World Health Organisation (WHO) have acknowledged.
It comes after a group of scientists urged the global body to update its guidance on how the respiratory disease passes between people.
WHO has previously said the virus that causes Covid-19 spreads primarily through small droplets expelled from the nose and mouth of an infected person, and that these quickly sink to the ground.
But more than 200 scientists in 32 countries wrote an open letter to the United Nations agency, outlining evidence that they say shows floating virus particles can infect people who breathe them in.
Because those smaller exhaled particles can linger in the air, the scientists in the group had been calling on WHO to change its guidance.
Professor Benedetta Allegranzi, technical lead for infection prevention and control, she said there was "emerging evidence" of airborne transmission of the coronavirus, but added that it was not definitive.
She said: "The possibility of airborne transmission in public settings - especially in very specific conditions, crowded, closed, poorly ventilated settings that have been described, cannot be ruled out.
"However, the evidence needs to be gathered and interpreted, and we continue to support this."
While WHO's recommended precautions of hand washing and social distancing are appropriate, the letter's signatories said these measures are “insufficient” in providing protection from “virus-carrying respiratory microdroplets” released into the air by those with Covid-19.
According to the group, measures that should be taken include providing sufficient ventilation, to minimise recirculating air, and avoiding overcrowding, particularly on public transport.
When asked about the letter during a press briefing on Tuesday, Prof Allegranzi said officials had been in discussions with the letter’s signatories.
“We acknowledge that there is emerging evidence in this field, as in all other fields, regarding the Covid-19 virus and pandemic,” she told reporters.
“And therefore, we believe that we have to be open to this evidence and understand its implications regarding the modes of transmission, and also regarding the precautions that need to be taken.”
She added that while airborne transmission could not be ruled out in certain conditions, the evidence was not “definitive” and needed to continue to be gathered.
Dr David Nabarro, a special envoy to the WHO on Covid-19, said that while airborne transmission was “certainly part of the puzzle”, it is not the main route of transmission.
“The major way in which that this virus is transmitted is through droplets which come out when you cough, sneeze or even shout,” he told the BBC Radio 4 Today programme
“But there does seem to be the possibility that very small particles containing virus can move further.
“It is certainly part of the puzzle. The key question is just how important is it.
“Right now we think it is not the major means of transmission. The major means of transmission is droplets, which is why we talk about one to two metres distance as being the key thing to avoid spread.”
Jose Jimenez, a chemist at the University of Colorado was one of the scientists who signed the paper, which was published on Monday in the Clinical Infectious Diseases journal.
He said: "We wanted them to acknowledge the evidence.
"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."
Mr Jimenez said historically, there has been a fierce opposition in the medical profession to the notion of aerosol transmission, and the bar for proof has been set very high. A key concern has been a fear of panic.
"If people hear airborne, healthcare workers will refuse to go to the hospital," he said.
He added that people might also buy up all the highly protective N95 respirator masks, "and there will be none left for developing countries."
Mr Jimenez said the WHO panel assessing the evidence on airborne transmission was not scientifically diverse and lacked representation from experts in aerosol transmission.
Any change in the WHO's assessment of risk of transmission could affect its current advice on keeping 1-metre (3.3 feet) of physical distancing.
Governments, which rely on the agency for guidance policy, may also have to adjust public health measures aimed at curbing the spread of the virus.
Maria Van Kerkhove, technical lead on the pandemic at the WHO said they would publish a scientific brief summarising the state of knowledge on modes of transmission of the virus in the coming days.
"A comprehensive package of interventions is required to be able to stop transmission," she said.
"This includes not only physical distancing, it includes the use of masks where appropriate in certain settings, specifically where you can't do physical distancing and especially for healthcare workers."
