Can coronavirus be spread through the air?

Can the novel coronavirus be spread through the air by tiny airborne particles, called aerosols? Up to now the World Health Organisation has said no, arguing that it is spread through droplets which are expelled by coughs or sneezes from an infected person, although these droplets travel through the air, they quickly fall to the floor if they don't land on a target individual.

However, a group of 239 scientists have now written to the WHO saying there is clear evidence that Covid-19 can be spread by small, airborne particles, which can infect people. The open letter, reported by the New York Times, which the researchers plan to publish next week, shows their evidence that the smaller particles can infect people. They are calling on the WHO to change their guidelines and scientific advice, based on their findings.

Airborne particles pose significant risk

If the experts theory is correct the airborne particles would pose an additional risk over and above larger droplets, because they linger longer in the air. In particular, the risk of catching Covid-19 would be far higher in crowded spaces with poor ventilation and if the virus does spread by airborne transmission mask-wearing would need to be encouraged in all closed spaces even where people are correctly socially-distanced. Droplets are very unlikely to move over two metres, while airborne particles can easily glide that distance.

WHO does not agree with airborne transmission

Dr. Benedetta Allegranzi, who heads up the technical side of WHO infection control, is however still unconvinced there is evidence for airborne transmission of the coronavirus. “Especially in the last couple of months, we have been stating several times that we consider airborne transmission as possible but certainly not supported by solid or even clear evidence,” she said, admitting however that, "there is a strong debate on this.”

Some scientists say the WHO has made an artificial distinction between aerosols and droplets and that the are both airborne ways of transmitting a virus. Linsey Marr, a virus expert at Virginia Tech, told the New York Times: "We’ve known since 1946 that coughing and talking generate aerosols."

Mask wearing to avoid the spread of coronavirus

Whether the coronavirus spreads only by droplets, or by droplets and aerosols, experts say the best method of prevention (beyond having no contact with other people, which is not possible or desirable for many) is to wear a mask.

“There is no incontrovertible proof that SARS-CoV-2 travels or is transmitted significantly by aerosols, but there is absolutely no evidence that it’s not. So at the moment we have to make a decision in the face of uncertainty, and my goodness, it’s going to be a disastrous decision if we get it wrong. So why not just mask up for a few weeks, just in case?” said Dr. Trish Greenhalgh, a primary care doctor at the University of Oxford in England.

Spreading Covid-19

In general people infected with the coronavirus develop symptoms around five days after becoming infected, but the incubation period can be as long as 14 days. The symptoms can include a dry cough, fever, tiredness, loss of sense of taste and/or smell, runny nose, sore throat. In severe cases patients develop breathing difficulties and even organ failure, with some cases being fatal.

Once a person is infected, but even before they start to show symptoms they become infectious and are able to spread the disease. Generally, once symptoms develop their viral load starts to decrease and they become less infectious, but will keep shedding the virus for up to two weeks after recovery.

People with very mild or even no symptoms can have a very high amount of virus in their upper respiratory tracts, meaning they are highly infectious, either through talking, sneezing, coughing, spitting or touching their mouth or nose and then surfaces.

Mask wearing can help stop the spread of the disease by ensuring viral particles expelled from an infectious person are caught by the mask rather than entering the air.