NSW Premier Gladys Berejiklian described certain encounters that have led to transmission of Covid-19 as "scarily fleeting".
Weeks earlier, Victoria's Covid-19 testing chief Jeroen Weimar expressed to the public his deep concern for strangers simply passing each other in the street.
Their grave concern? The highly-infectious Delta variant.
The variant fuelled one of a handful of clusters in Melbourne's Covid outbreak earlier this month and has been the variant responsible for Sydney's ever-growing Bondi cluster which ballooned to 31 cases on Wednesday.
"This Delta virus has shown itself to be very capable of spreading extremely quickly and in circumstances that NSW residents have not experienced in the same way before," Health Minister Brad Hazzard said.
Addressing her concerns over the variant earlier on Wednesday, Queensland's Chief Health Officer Dr Jeannette Young said while 15 minutes of close contact with an infectious person was deemed concerning at the start of the pandemic, it was now a mere five to 10 seconds.
How risky is the Delta variant?
So what exactly is the risk facing Sydney residents as the virus continues to spread in the community?
"There seems to be evidence the Delta variant is more prone to aerosol transmission than some of the other variants," Associate Professor Ben Mullins, an aerosol deposition expert at Curtin University’s School of Public Health, told Yahoo News Australia.
"That coupled with the weather conditions at this time of year means aerosolised viruses will stay longer in the air."
Explaining the term 'fleeting contact', something that has quickly become household terminology at daily Covid-19 press conferences, Professor Mullins said people were contracting the virus by simply walking through someone's exhaled breath.
"There's basically a cloud of aerosols in the air that's been exhaled by someone with the Delta variant and then someone else will walk through that and inhale a sufficient viral load to be infected."
He said it was critical Sydney residents strictly followed the latest mask mandate implemented by Ms Berejiklian on Wednesday as well as ensuring social distancing to reduce the risk of being infected this way.
Public transport should be avoided as cases increase, expert warns
NSW's Chief Health Officer Dr Kerry Chant said it was "particularly concerning" to see a retail environment like the Bondi Junction Westfield become a place of transmission, which had previously been deemed a low risk setting.
"I don't want to enhance [the idea] this virus jumped across a room or anything... but [after viewing CCTV footage] it was inadvertent contact you would have in retail settings that we would not have suspected transmission could happen," she said.
While Prof Mullins noted it is advisable to avoid shopping precincts in hotspot areas, most shopping centres are designed with minimal air exchange rates and large spaces meaning the risk is far less than confined spaces such as public transport.
He said transport routes, even if not declared hotspots, are of concern and Sydneysiders should reconsider using public transport even with the mask mandate – noting masks being used are far from 100 per cent effective.
"We've all seen the pictures and experienced how difficult it is to have adequate social distancing in public transport," Prof Mullins told Yahoo News Australia.
"So as good as it is for the environment, it's best to be avoiding it if possible at this time."
He said it may get to the stage where uninfected people, such as people who have not visited any of the exposure sites, wore better masks than surgical or fabric ones, with N95 masks offering better protection.
However infected people, or those who have symptoms or have been to exposure sites, must not wear these as they have an exhalation valve that does not filter aerosols, Prof Mullins noted.
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