Simulations of how the coronaviruses survive in exhaled air suggest that it loses most of its ability to cause harm within five minutes of becoming airborne.
The importance of short-range Covid transmission was re-emphasised by the findings, with physical lysis and mask-wearing likely to be the most effective means of preventing infections. It is still worthwhile, but the impact is likely to be less.
People are thinking about airborne transmission over metres or across a room in poorly ventilated spaces. Prof Jonathan Reid, director of the University of Bristol's Aerosol Research Centre and the study's lead author, said that the greatest risk of exposure is when you're close to someone.
When you move further away, the aerosol is less infectious because the virus has lost its ability to transmit.
Our assumptions about how long the virus lasts have been based on studies that involved spraying the virus into sealed vessels called Goldberg drums. Researchers found that the infectious virus could still be detected after three hours. Experiments that replicate what happens when we cough or breathe are not accurate.
Researchers from the University of Bristol were able to create any number of tiny particles and gently levitate them between two electric rings for anywhere between five seconds to 20 minutes, while controlling the temperature, humidity and UV light intensity of their surroundings. This is the first time anyone has been able to do a realistic simulation of what happens during the exhalation process.
The study suggests that the viral particles leave the moist and carbon dioxide-rich conditions of the lungs and lose water and dry out, while the transition to lower levels of carbon dioxide is associated with a rapid increase in pH. The speed at which the particles dry out varies according to the relative humidity of the surrounding air.
When this was less than 50%, the virus lost half of its virulence in 10 seconds, but the decline was slower and more steady. After five minutes, half of particles remain infectious, but after 20 minutes, only 10% of particles remain infectious.
The temperature of the air didn't change the fact that viral transmission is higher during warm weather.
It means that if I meet my friends for lunch in a pub today, the primary risk is likely to be me passing it on to my friends, rather than it being transmitted from someone on the other side of the room. This shows the importance of wearing a mask in situations where people can't see each other.
The effects were the same across all three versions of the variant. They hope to start experimenting with the Omicron variant in the coming weeks.
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