Coronavirus is not behaving in the air the way scientists expect

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,some aerosol experts such as WHO advisor Professor Lidia Morawska 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 the COVID-19 virus was also viable as an airborne pathogen,he said.

"I think as COVID-19 continues we will see its infection or[reproduction number] go up. It's certainly spreading easier than SARS."

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 says there’s still not enough evidence the virus can spread by aerosol outside lab conditions or beyond hospitals where procedures such as intubation and ventilation are known to aerosolise it - and staff in COVID-19 wards already take airborne precautions.

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 efficiently 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.

Another studypublished in The New England Journal of Medicine found the life of the new virus in the air did not appear longer than that of the virus behind SARS,but both could last up there 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".

Other early studies have returned mixed results - some found virus in the air of hospitals treating infected patients,some did not. One paper 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. Smear samples from the air conditioner failed to turn up active virus"a finding less consistent with aerosol transmission",they said.

Still,America’s National Academies of Sciences,Engineering and Medicinewrote to the White House earlier in April saying the current research supports the possibility that COVID-19 can be spread by aerosols from breathing,as was observed to a small degree during SARS.

And,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.

In a country like Australia,without rampant community transmission,most experts agree with the sentiment though some have called for greater focus on natural ventilation indoors alongside hand hygiene in light of emerging evidence on aerosols. An Australian government spokeswoman said air circulation systems such as heating and cooling were widely considered safe,though more data was needed.