Covid loses 90% of its infectivity within minutes

 From The Guardian

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].”

The study, which has not yet been peer-reviewed, suggested that as the viral particles leave the relatively moist and carbon dioxide-rich conditions of the lungs, they rapidly lose water and dry out, while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.

When this was lower than 50% – similar to the relatively dry air found in many offices – the virus had lost around half of its infectivity within five seconds, after which the decline was slower and more steady, with a further 19% loss over the next five minutes. At 90% humidity – roughly equivalent to a steam or shower room – the decline in infectivity was more gradual, with 52% of particles remaining infectious after five minutes, dropping to about 10% after 20 minutes, after which these was no difference between the two conditions.

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