By Mark Waghorn via SWNS
Keeping six feet apart does not protect against catching Covid - even outdoors, according to new research.
The social distancing rule is a "random measurement" of safety in the absence of masks, say scientists.
It could have been set anywhere between 3 to 10 feet, or more - depending on the risk tolerance of a public health authority.
Infected individuals spread the virus through coughing, speaking or even breathing.
They can expel larger droplets that eventually settle or smaller aerosols that may float in the air.
The study used computer modeling to quantify how they travel. It also found coughs vary widely.
Lead author Professor Epaminondas Mastorakos said: "I remember hearing lots about how Covid-19 was spreading via door handles in early 2020.
"I thought to myself if that were the case, then the virus must leave an infected person and land on the surface or disperse in the air through fluid mechanical processes."
The results in Physics of Fluids underline the continued importance of vaccination, ventilation and masks as we head into winter.
Early in the pandemic health experts focused on hand-washing and surface cleaning.
They have developed various programs to investigate how it behaves in different environments.
First author Dr. Shrey Trivedi said: "One part of the way this disease spreads is virology: how much virus you have in your body, how many viral particles you expel when you speak or cough.
"But another part of it is fluid mechanics: what happens to the droplets once they are expelled, which is where we come in.
"As fluid mechanics specialists, we are like the bridge from virology of the emitter to the virology of the receiver and we can help with risk assessment."
Simulations showed how much would reach another person in the same room from a cough containing 1,000 droplets.
The researchers found there isn't a sharp cut-off once the droplets spread beyond six feet.
When a person coughs and isn’t wearing a mask, most of the larger droplets will fall on nearby surfaces. But smaller droplets suspended in the air can quickly and easily spread well beyond six feet.
How far and how quickly these aerosols spread will depend on the quality of ventilation in the room.
In addition to the variables surrounding mask-wearing and ventilation, there is also a high degree of variability.
Dr. Trivedi said: "Each time we cough, we may emit a different amount of liquid.
"So if a person is infected with Covid-19, they could be emitting lots of virus particles or very few, and because of the turbulence they spread differently for every cough."
Calculations took into account turbulent flow and detailed descriptions of droplet motion and evaporation.
Prof Mastorakos said: "Even if I expel the same number of droplets every time I cough, because the flow is turbulent, there are fluctuations.
"If I am coughing, fluctuations in velocity, temperature and humidity mean the amount someone gets at the two-meter mark can be very different each time.”
The six feet/two-metre rule is an effective and easy-to-remember message for the public.
But it isn’t a mark of safety given the large number of variables associated with an airborne virus, said the researchers.
Vaccination, ventilation and masks - while not 100% effective - are vital for containing Covid.
Added Prof Mastorakos: "We are all desperate to see the back of this pandemic, but we strongly recommend people keep wearing masks in indoor spaces such as offices, classrooms and shops.
"There is no good reason to expose yourself to this risk as long as the virus is with us."
The researchers are carrying out similar simulations for spaces such as lecture rooms to assess the risk as people spend more time indoors.
The World Health Organisation recommends a distance of at least one meter from others - even if they don't appear to be sick.
It also advises people to avoid crowds and close contact and to wear a properly fitted mask in poorly ventilated settings.
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