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When someone coughs, sneezes or talks, they send droplets flying into the air. After six months living with coronavirus, we know this is particularly dangerous indoors where crowds of people struggle to keep a distance. There have been significant outbreaks in restaurants, bars, churches, workplaces and cruise ships. Besides being places where people gather for hours on end, they often lack proper ventilation.
Since the beginning of the pandemic, the World Health Organisation (WHO) and infectious disease experts have insisted that coronavirus is primarily transmitted by coughing or sneezing large droplets into someone’s face. But the number of cases linked to indoor areas has sparked a debate about whether another form of transmission may be at play: aerosols.
Unlike droplets, which tend to fall to the ground or onto other surfaces within a few metres due to gravity, aerosols are bits of mucus or saliva smaller than five micrometres in diameter. They can be emitted in the same way as droplets, but also by breathing and are so light they can float before dropping to the ground. This means coronavirus could linger in the air for hours and build up in large concentrations.
It took an open letter from 239 researchers in 32 countries, spread across many different fields of science (from engineering to air quality to virology), for the WHO to soften its position after months of debate and to consider the mounting evidence that suggests aerosols pose a serious threat. In that case, keeping one or two metres apart, hand washing and cleaning surfaces over and over again may not be enough to prevent infection. Avoiding crowds altogether would make sense and so would wearing masks indoors. But to fight a virus that spreads in the air, we might need to dig deeper into our pockets.
Whether the novel coronavirus is indeed airborne isn’t a simple scientific question to answer. A number of lab experiments have demonstrated that Sars-Cov-2 can be aerosolised and survive in this form – up to 16 hours according to one study. Outside a carefully controlled lab, it is much more difficult to detect aerosols and study how they can transmit the virus. Researchers found genetic material from coronavirus in aerosols collected at two hospitals in Wuhan, the Chinese city where the virus first emerged, but pieces of viral RNA are not live virus and not infectious on their own.
As the pandemic unfolded, data from contact tracing started to point to the risk of airborne transmission in real-life scenarios. Researchers believe an air conditioner pushed virus-laden air across a windowless restaurant in Guangzhou, China, infecting nine people from three families whose tables were more than a metre apart from each other. “Air conditioning could recirculate the virus and keep it in the room longer than if the windows were open. Most air conditioning systems use a large fraction of recirculated air, to save on energy costs, and standard filters would remove maybe half of the virus in the air,” says Linsey Marr, a professor of engineering and aerosol expert at Virginia Tech, who contributed to the open letter that was published on July 6 in the journal Clinical Infectious Diseases.
However, there were other diners in the Guangzhou restaurant who were sitting away from the airflow and did not develop symptoms of disease. This could be an argument against airborne transmission – if aerosols lingered and concentrated in the stagnant air, one could expect more infections among the 83 customers and eight staff members that were present that day.
Another notable case study was from a choir rehearsal in the US state of Washington, where one person infected at least 33 of the other participants even though they maintained physical distance. Singing and loud speech in general can produce large volumes of aerosols, and evidence suggests so-called “speech super-emitters” emit more aerosols than most people, even during normal speech. Even if some of the choir singers came into direct contact with large droplets or contaminated surfaces, the fact that 53 of the 61 members developed symptoms after the practice (with 33 testing positive for the virus) suggests that aerosols may have been a driving force in the spread.
If people can indeed transmit coronavirus through exhaled air, social distancing may not always be effective. The group of 239 scientists called for precautionary measures to slow the spread of Covid-19 as businesses are reopening and people are returning to workplaces, schools and universities. These include proper ventilation and air filtration systems inside buildings to dilute the virus-laden droplets and avoid an outbreak as happened recently in Michigan, where 170 new infections were linked to a single bar.
“The improvements in indoor air quality from enhanced ventilation will have other benefits besides that of limiting the transmission of respiratory virus aerosols,” says Julian Tang, an associate professor of respiratory sciences at the University of Leicester and one of the authors of the letter. It would be a welcome improvement for anyone suffering hay fever, asthma, and other chronic allergic and respiratory conditions. Tang adds that wearing masks indoors could also protect against other seasonal respiratory viruses such as those causing the flu or cold-like symptoms. People who have a common cold tend to be less productive at work and will need to take time off to recover or care for their sick children. Missed hours due to the common cold are estimated to amount to a $25 billion (£20bn) loss in productivity.
The use of germicidal ultraviolet lights could also become common in places where natural sunlight cannot reach. One type of ultraviolet light, called far-UVC, is known to effectively inactivate human coronaviruses and safe to use around people, and UV robots are already being used to disinfect hospitals, airports, hotels and prisons.
It’s easy to get fresh air into restaurants, workplaces and classrooms by opening both doors and windows, but installing UV lights and air filtration systems are not a cheap and quick solution, says Julii Brainard, a senior researcher in health protection at the University of East Anglia. “We don’t know if the best air filtration systems could quickly enough stop a person coughing two cubicles over from you spreading their germs, assuming the germs they were coughing out were airborne,” she says. “It’s even harder to make air filtration or germicidal lights cost-effective in private homes or care homes, where a great deal of transmission has happened and will tend to be important transmission places in future.”
Aerosols and airborne transmission of viruses may finally be getting the attention they deserve, but the WHO insists the available evidence is not yet definite proof. The key question remains how big a role aerosols play in the spread of Covid-19. A scenario where most transmission of coronavirus occurs via large droplets and on rare occasions aerosols has very different implications to a scenario where aerosols are a major route of transmission. The latter would require the public health advice for businesses and individuals to change, again.
But for Tang at the University of Leicester, it would make sense to embrace the precautionary principle and act on what we already know about airborne transmission. “Every additional infection control measure offers an incremental protection against the virus,” he says. “Why not try to prevent more people from becoming infected?”
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