Covid loses 90% of ability to infect within minutes in air - study

Coronavirus loses 90% of its ability to infect us within 20 minutes of becoming airborne - with most of the loss occurring within the first five minutes, the world's first simulations of how the virus survives in exhaled air suggest.

The findings re-emphasise the importance of short-range Covid transmission, with physical distancing and mask-wearing likely to be the most effective means of preventing infection. Ventilation, though still worthwhile, is likely to have a lesser impact.

"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," said Prof Jonathan Reid, director of the University of Bristol's Aerosol Research Centre and the study's lead author.

"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]."

Until now, our assumptions about how long the virus survives in tiny airborne droplets have been based on studies that involved spraying virus into sealed vessels called Goldberg drums, which rotate to keep the droplets airborne. Using this method, US researchers found that infectious virus could still be detected after three hours. Yet such experiments do not accurately replicate what happens when we cough or breathe.

Instead, researchers from the University of Bristol developed apparatus that allowed them to generate any number of tiny, virus-containing particles and gently levitate them between two electric rings for anywhere between five seconds to 20 minutes, while tightly controlling the temperature, humidity and UV light intensity of their surroundings. "This is the first time anyone has been able to actually simulate what happens to the aerosol during the exhalation process," Reid said.

The study, which has not yet been peer-reviewed, suggested that as the viral particles leave the relatively moist and carbon dioxide-rich conditions of the lungs, they rapidly lose water and dry out, while the transition to lower levels of carbon dioxide is associated with a rapid increase in pH. Both of these factors disrupt the virus's ability to infect human cells, but the speed at which the particles dry out varies according to the relative humidity of the surrounding air.