HONG KONG — Scientists are exploring the possibility that the coronavirus that causes Covid-19 could be spread through the air over a much wider area than via coughs and sneezes, but the World Health Organisation has urged caution saying the available evidence has yet to support this.

“Airborne transmissions” are defined as tiny aerosol droplets — smaller than 5 micrometers in diameter — that can linger in the air for hours.

They can also spread the disease much further than the 2 metres covered by the respiratory droplets that are currently seen as the primary means of spreading the disease. Aerosols can also cause more damage when inhaled because they travel further into the lungs.

Dr Hanan Balkhy, assistant director general for antimicrobial resistance at the WHO told US National Public Radio on Monday that unlike cases where the virus has been spread through coughs and sneezes “if we were to have airborne transmission, we would see cases with no contact before getting ill with that disease. That’s just one example. And we are not seeing that”.

But Dr Anthony Fauci, the director of the US National Institute of Allergy and Infectious Diseases, has said he could not rule out the possibility and recent research has also found “limited evidence that some potential for airborne transmission exists”.

Researchers from the University of Nebraska Medical Centre and the National Strategic Research Institute at the University of Nebraska took air samples from 11 rooms where 13 confirmed cases were being treated.

As well as finding genetic material from the coronavirus on lavatories and on everyday items, 63.2 per cent of air samples taken inside the rooms and 66.7 per cent of those taken outside also showed traces.

They found the highest concentration of the genome of the virus 2 metres away from a patient receiving oxygen through a nasal tube and the positive in-room samples had higher traces than those taken in the corridors.

This study was published last Friday on the website medRxiv.org before it had been peer-reviewed and the researchers warned that the findings “do not confirm airborne spread”.

Scientists around the world are studying the transmission routes of the disease, which has already infected at least 900,000 people globally and killed more than 45,000.

Dr Fauci told a press conference last month: “Is it possible that there is aerosol transmission? Yeah, there certainly is.

“But clearly, what we have seen in the situations where people have gotten infected from the areas that we have experience – China, South Korea, now Europe – most of it is in the situation where people are close enough to each other that a symptomatic person will have a real droplet transmission. So, I’m not ruling out the possibility that it’s aerosol.”

The WHO has advised caution over studies looking at aerial transmission. Its statement on Friday followed the publication of a study in the New England Journal of Medicine in mid-March that concluded “people may acquire the virus through the air and after touching contaminated objects”.

The study conducted by scientists at Princeton University, University of California Los Angeles and the US National Institutes of Health said that the virus was detectable in aerosols for up to three hours.

The latest WHO statement noted that the study was modelled on an experimentally induced aerosol-generating procedure that did not reflect a clinical setting.

The global health agency said that the virus is still “primarily transmitted between people through respiratory droplets and contact routes”, such as coughing and sneezing.

Despite noting that the evidence did not yet support airborne transmission, it said it was possible “in specific circumstances and settings in which procedures that generate aerosols are performed,” such as when a patient is intubated in a hospital or being disconnected from the ventilator. SOUTH CHINA MORNING POST