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  • Suzannah Lyons and Olivia Willis

Can coronavirus linger in the air, even if someone leaves the room?

Dry cough is one of the most common symptoms of COVID-19

By now, you (hopefully) know the drill: wash your hands, don't touch your face, and keep your distance from others. COVID-19 — the infectious disease caused by the new coronavirus — is thought to be mostly spread via respiratory droplets, the secretions we generate when we sneeze or cough. While we're still learning about the virus, it's these droplets that seem to be the main method of transmission, said epidemiologist Hassan Vally of La Trobe University. "When you cough and sneeze you produce droplets from your mouth that get projected forward," Dr Vally said. Usually this happens at a distance of about 1.5 metres, which fits in with physical distancing advice to stay at least 1.5m from others. But is it possible for coronavirus to linger in the air? And could you catch it (via the air) even after someone who is infected has left the room? The short answer: there's no evidence yet to show this is happening in the community. But there's a bit to unpack here, so let's step through it. How does COVID-19 spread? When an infected person coughs or sneezes they spray out a shower of potentially hundreds or thousands of particles of various sizes, said Euan Tovey, an aerobiologist at the University of Sydney. "Usually when you sneeze or cough, you generate more of the larger particles," Dr Tovey said. If you stand too close to an infected person, you could then breathe in the droplets they have coughed or sneezed into the air. The virus can also spread when these droplets from an infected person land on objects or surfaces around them. Other people then catch COVID-19 by touching surfaces infected with the virus, then touching their eyes, nose or mouth and letting the virus get into their respiratory system that way. Another possible route is faecal-oral transmission, because the virus can be found in faeces as well, said virologist Sacha Stelzer-Braid of the University of New South Wales Sydney. If an infected person goes to the toilet, but doesn't close the lid before they flush that's generating tiny droplets or aerosols, Dr Stelzer-Braid said. These droplets can then land on other bathroom surfaces, which others then get on their hands when they touch these surfaces. Then it's just a matter of the virus making its way from their hands to their face. How long does coronavirus last on surfaces? When it comes to respiratory droplets, we don't yet know how long the coronavirus last in the air after someone sneezes or coughs, Dr Tovey said, but it would be for less than five minutes. Research published last week found the virus can survive for hours and in some cases days outside a host, depending on the type of surface it's on. Viable virus particles — meaning they're still able to infect you — were detected for up to 72 hours on stainless steel and plastic surfaces, but no longer than 24 hours on cardboard, and four hours on copper. The research also found that when the virus was artificially turned into an aerosol, it remained viable for three hours. The virus becomes an aerosol when particles remain suspended in the air as tiny droplets — five microns in diameter or less — much smaller than the majority of the droplets you generate when you sneeze or cough. "Once it gets quite small like that then it basically stays aloft, it's like cigarette smoke," Dr Tovey said.

Could it stay alive long enough in the air to spread in the community? When a virus exists as an aerosol, airborne transmission is possible. But we're yet to see the coronavirus commonly spreading through aerosols outside the artificial conditions of the lab. "There is no credible evidence at this stage that proved airborne particles could spread the virus in the community," Dr Vally said. "Firstly, the virus has got to survive in the air. And then secondly, you've got to show that it's surviving in sufficient doses for people to become infected." Dr Vally said he would speculate that if airborne spread was a significant mode of transmission we'd see more people being infected more quickly, or people infecting more people than they do. Airborne transmission is what makes measles, another infectious disease, so contagious. It's able to survive in the air where an infected person has coughed or sneezed for up to two hours. Anyone passing through can catch it by breathing it in. Measles has a reproductive number of 12 to 18, meaning one infected person can transmit the disease to between 12 to 18 other people. So far coronavirus has a reproductive number of about 2.5, Dr Vally said. "I think the effectiveness of social distancing is showing that it's the respiratory droplets that are the main transmission route," he said. But, Dr Stelzer-Braid said we can't dismiss airborne transmission entirely. "We don't think it's an important route, but it's a possible route," she said. "I think the possibility's there we don't know enough right now. And that might not be comforting for people, but we are trying to work out the intricacies of it." There are, however, certain hospital procedures to help keep a patient's airway open that can make the virus airborne. That's why the World Health Organisation recommends the use of respirator masks (which filter airborne particles) for medical workers performing such procedures on COVID-19 patients.

What should you do? "I don't want people to panic and suddenly think that everyone needs to wear masks, because I don't think that's the case," Dr Stelzer-Braid said. "Someone who's infected should be wearing a surgical mask at least to prevent transmission to other members of the household and the community." And if you're in a house with someone who's infected you should really try to limit contact with that person, she said. If you can, really try and quarantine that person and limit contact with them, and if you're changing their bedding and washing their clothes, try and wear gloves… and try not to touch your face when you're doing that kind of work."

Social distancing is the most important thing you should be doing, in terms of being further away from people so that those respiratory droplets can't land on you, Dr Vally said. And finally, don't forget to keep washing your hands because the more you do, the more you're cleaning any infective particles off your hands. "It doesn't matter if you touch a contaminated surface if you haven't touched your face afterwards or you've washed your hands before you've touched your face, you shouldn't get sick."

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