Coronavirus loses 90% of its potential to infect us within 20 minutes of being airborne, according to the world's first simulations of how the virus survives in exhaled air.
In order to prevent infection, physical distance and wearing a mask are likely to be the most effective measures. Ventilation, while still important, may have less influence.
“People have been thinking about airborne transmission over metres or across a room. But, while that does happen, I believe the greatest risk of exposure is when you're near to someone,” said main author Prof Jonathan Reid of the University of Bristol's Aerosol Research Centre.
“Moving away dilutes the aerosol, but also reduces the virus's infectivity.”
Until now, our estimates of virus survival in tiny airborne droplets were based on research that used virus sprayed inside rotating Goldberg drums. US researchers discovered that contagious virus may still be detected three hours later. But such trials do not adequately reproduce our coughing or breathing.
Instead, University of Bristol researchers devised a machine that could produce any number of microscopic virus-containing particles and gently hover them between two electric rings for up to 20 minutes while closely managing the temperature, humidity, and UV light intensity. For the first time, researchers can mimic the aerosol's fate during exhale.
In the unpublished study, virus particles lose water and dry up rapidly when they exit the lungs' wet and carbon dioxide-rich environment, whereas the change to lower carbon dioxide levels is accompanied by an increase in pH. Both of these variables prevent the virus from infecting human cells, but the rate at which the particles dry out depends on the relative humidity.
When this fell below 50%, the virus lost about half of its infectivity in five seconds, and then lost another 19% during the next five minutes. At 90% humidity, the reduction in infectivity was more slow, with 52% of particles remaining infectious after five minutes, and 10% after 20 minutes, with no difference between the two conditions.
The temperature of the air had no effect on viral infectivity, contrary to popular perception that high temperatures reduce viral transmission.
If I meet friends for lunch at a pub today, the risk is more likely to be me or my friends spreading it, rather than someone on the opposite side of the room, said Reid. This emphasises the need for a disguise when people cannot physically distance themselves.
“Masks are quite effective… as is social distancing,” said Dr. Julian Tang, a clinical virologist at the University of Leicester. Improved ventilation near the source will also help.”
“Aerosols will fill up interior rooms swiftly in the absence of sufficient ventilation, so assuming the infected individual remains within the room, the levels of virus will be replenished,” said Dr Stephen Griffin of the University of Leeds.
It happened with all three Sars-CoV-2 types tested thus far, including Alpha. They aim to start testing the Omicron variant soon.