"We're all running under a fire drill, there's still so much we don't know about this virus and it's all so urgent," he said. "Of course we need more research, but in science when you see a warning light blinking on like this, you need to pay attention."

Experts largely agree that the new virus is not airborne in the same way as notably infectious diseases such as measles. Its primary route of transmission, like the flu, is thought to be through large water droplets, which can shoot out from the nose or mouth for up to a metre when a patient coughs or sneezes. Sometimes they contaminate surfaces but they’re too heavy to survive long in the air.

But the droplet theory, and the 1.5 metre distance rule adopted by most health authorities around the world, is based on old medical dogma from the 1930s, aerosol experts say. Researchers have since found that some respiratory illnesses, including the flu, can also travel in aerosols just from breathing or speaking. If the new virus can too, some say it might explain recent outbreaks such as on locked down cruise ships or the phenomenon of 'superspreaders" also seen during SARS where one patient sheds much more virus than usual.

While large particles tend to carry more virus than smaller aerosols, Professor Roy said it had been assumed by health authorities that only big droplets could carry any live virus at all. This work now added to a growing body of evidence COVID-19 virus was also an airborne pathogen, he said.

But that still doesn't mean you'll catch it just from passing someone on the street. Like cigarette smoke, the virus will disperse in open spaces. And, just like smoke, it can build up in enclosed areas without ventilation.

The World Health Organisation still says there’s not enough evidence the virus can spread by aerosol outside lab conditions or beyond hospitals where procedures such as intubation and ventilator are already known to aerosolise it. Other experts have added that if aerosol were one of the main ways COVID-19 spread, the pandemic would likely have a different shape, infecting more people without a clear cause.

An Australian government spokeswoman said viruses do not always fall neatly into either aerosol or droplet but COVID-19 did not appear to spread in the air as well as other diseases considered to be airborne. A greater risk was posed by the "underestimated" spread of COVID-19 through touching contaminated surfaces, she said.

Of the new research, the government said such lab experiments were useful but at present physical distancing and hand hygiene had suppressed the spread of COVID-19 in Australia.

In the wake of more evidence that people without symptoms are shedding the virus, the US has called on the community to wear cloth masks in close quarters such as supermarkets. Both the WHO and the Australian government say masks are still only necessary for people with symptoms and those treating them, and must not be wasted by the general public as shortages of the product could put frontline workers at risk.

The WHO raised eyebrows in some corners of the scientific community last month when it emphatically "fact-checked" the idea that coronavirus might be airborne as "incorrect" over Twitter.

"I don't know all the data they have and this is not the only piece of the puzzle, but it's an important one," Professor Roy said. "I think as COVID-19 continues we will see its infection or [reproduction number] go up. It's certainly spreading easier than SARS."

Meanwhile other research published in The New England Journal of Medicine  has found the life of the new virus in the air is not dissimilar to that of the virus behind SARS but both could last for a number of hours. Of COVID-19, the scientists concluded: "aerosol transmission is plausible, since the virus can remain viable and infectious in aerosols for hours and on surfaces up to day".

Still other samples taken from the air of hospitals and public places have returned mixed results – some found the virus lingering but did not test if it was infectious. Another study tracked a cluster of infections at a restaurant in China back to one asymptomatic patient – those who had caught the virus were found to be sitting in the direct flow of air pushed by the air conditioner, suggesting, according to the researchers, that droplets were the main route of transmission.

The Australian government says there's not enough data yet on how the virus might spread in air conditioning or heating, but it is widely considered safe.