Much of Covid-19's ability to infect people is lost within 20 minutes of the virus becoming airborne, research has found.
The world's first simulations of how coronavirus survives in exhaled breath have shown that most of the loss happens within the first five minutes of droplets being sent into the air.
Researchers from the University of Bristol developed equipment allowing them to generate minute particles containing the virus, gently levitating them between two electric rings for durations of between five seconds and 20 minutes.
During this process, the temperature, humidity and UV light intensity of the particles' surroundings is tightly controlled, The Guardian reports.
Three variants, including Alpha, have been tested by the team. Experiments with the Omicron variant are due to start soon.
The study, which has not been peer-reviewed yet, indicated that viral particles quickly lose water and dry out once outside the lungs, where conditions are rich in carbon dioxide.
This disrupts Covid's ability to infect human cells.
How rapidly particles dry out varies according to the relative humidity of the surrounding air. At levels lower than 50 per cent, the virus lost around half of its infectivity within five seconds, with another 19 per cent lost over the next five minutes.
The decline was more gradual at 90 per cent humidity, where 52 per cent of particles remained infectious after five minutes, dropping to about 10% after 20 minutes, after which there was no difference.
Prof Jonathan Reid, director of the University of Bristol’s Aerosol Research Centre and the study’s lead author, said: "This is the first time anyone has been able to actually simulate what happens to the aerosol during the exhalation process."
He added: "People have been focused on poorly ventilated spaces and thinking about airborne transmission over metres or across a room. I’m not saying that doesn’t happen, but I think still the greatest risk of exposure is when you’re close to someone.
"When you move further away, not only is the aerosol diluted down, there’s also less infectious virus because the virus has lost infectivity [as a result of time]."
The temperature of the air made no difference to Covid's infectiousness, however.
Prof Reid said: "It means that if I’m meeting friends for lunch in a pub today, the primary [risk] is likely to be me transmitting it to my friends, or my friends transmitting it to me, rather than it being transmitted from someone on the other side of the room."
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