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The study, which has not yet been peer-reviewed, suggested that as the viral particles leave the relatively moist and carbon dioxide-rich conditions of the lungs, they rapidly lose water and dry out, while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.
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.
The Dynamics of SARS-CoV-2 Infectivity with Changes in Aerosol
Microenvironment
Abstract
Understanding the factors that influence the airborne survival of viruses such as SARS-CoV-2 in
aerosols is important for identifying routes of transmission and the value of various mitigation
strategies for preventing transmission. We present measurements of the stability of SARS-CoV-2 in
aerosol droplets (~5-10µm equilibrated radius) over timescales spanning from 5 seconds to 20
minutes using a novel instrument to probe survival in a small population of droplets (typically 5-10)
containing ~1 virus/droplet. Measurements of airborne infectivity change are coupled with a detailed
physicochemical analysis of the airborne droplets containing the virus. A decrease in infectivity to
~10 % of the starting value was observable for SARS-CoV-2 over 20 minutes, with a large proportion
of the loss occurring within the first 5 minutes after aerosolisation. The initial rate of infectivity loss
was found to correlate with physical transformation of the equilibrating droplet; salts within the
droplets crystallise at RHs below 50% leading to a near instant loss of infectivity in 50–60% of the
virus. However, at 90% RH the droplet remains homogenous and aqueous, and the viral stability is
sustained for the first 2 minutes, beyond which it decays to only 10% remaining infectious after 10
minutes. The loss of infectivity at high RH is consistent with an elevation in the pH of the droplets,
caused by volatilisation of CO2 from bicarbonate buffer within the droplet. Three different variants of
SARS-CoV-2 were compared and found to have a similar degree of airborne stability at both high
and low RH.
www.medrxiv.org/content/10.1101/2022.01.08.22268944v1.full.pdf