It may seem surprising, but more than a year after the coronavirus pandemic began, we still don鈥檛 know the relative importance of the different ways of catching covid-19: by touching contaminated surfaces or by breathing it in.
In March 2020, UK government advice was that face coverings weren鈥檛 generally needed and that people should focus on frequent handwashing, the use of hand sanitiser and on wiping down surfaces.
Since then, the that the virus can spread through the air, not only through larger droplets, but also through smaller ones called aerosols that linger in the air for hours and spread for many metres 鈥� estimates for the cut-off range from 5 to 100 micrometres.
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Though most coronavirus safety guidelines now recognise that the virus can spread through the air, they tend to focus on the risks from catching it via surfaces or large droplets that fall from the air in seconds and rarely spread more than 2 metres.
Studies in places where covid-19 is rare, which helps narrow possible transmission routes, have turned up incidents where aerosols seem to be the culprit. For instance, a person in South Korea was found to have caught the virus from someone sitting more than 6 metres away at an air-conditioned restaurant for .
A study of a superspreading event during a found that people sitting 5 metres away caught the virus. But community infections were going unreported at the time, so the bus riders could have been infected by other people, says Didier Pittet at the University of Geneva Hospitals in Switzerland, who wasn鈥檛 involved in the study.
There have also been cases within apartment blocks, where the virus seems to have spread from one flat to others above it through their . 鈥淲hen you flush the toilet, there鈥檚 massive aerosolisation,鈥� says Raina MacIntyre at the University of New South Wales in Australia.
Some aspects of UK government guidelines now acknowledge a risk of aerosol transmission 鈥� but not all. In addition, people in the UK can act inconsistently, says Eilir Hughes, a family doctor in north Wales. While most people understand that spending time outside is safer than doing so inside and know to open windows when indoors with others outside their social 鈥渂ubble鈥�, they may frequently go to supermarkets and stand within metres of people going maskless, for instance.
Hughes is co-founder of Fresh Air Wales, which campaigns for measures against airborne spread. The group wants the UK鈥檚 safety slogan to become 鈥淗ands, Face, Space, Replace鈥�, indicating the need for people to refresh their air supply by opening windows. They also urge workplaces to use carbon dioxide monitors to check ventilation rates. The Isle of Anglesey County Council in north Wales is due to give such monitors to schools, care homes and other public buildings like leisure centres.
Unlike larger droplets, aerosols can float around the face shields and plastic screens used in many supermarkets and restaurants. They can also get around the loose blue face masks widely used by the public.
An October 2020 showed that healthcare staff in intensive care units are less likely to catch the virus than those working elsewhere in hospitals, even though they carry out the riskiest procedures on covid-19 patients 鈥� perhaps because ICU staff use higher-grade masks. This month, the British Medical Association asked Public Health England to recommend hospitals give all healthcare staff higher-grade masks.
Aerosol campaigners agree that surface transmission also happens, and definitely don鈥檛 want people to stop handwashing, although they tend to downplay the benefits of frequent surface cleaning. 鈥淎 casual touch of a surface is not going to get that much virus off it,鈥� says Julian Tang at the University of Leicester, UK, who is one of those calling for aerosol precautions.
But there is still much resistance to the idea that aerosols are the key transmission pathway. Last September, the US Centers for Disease Control and Prevention updated its website to say that aerosols are the main way the virus spreads 鈥� but later removed the statement, saying it had been posted in error.
A UK government spokesperson says: 鈥淟etting fresh air into indoor spaces can greatly reduce the risk of infection from coronavirus. We continue to be guided by the advice of our scientific and medical experts and keep the evidence on face coverings under constant review.鈥�
It is hard to claim that either stance is 鈥渇ollowing the science鈥� 鈥� the science is evolving as we learn more about this relatively new virus. Unfortunately, the uncertainty leaves the average person who needs to visit the supermarket not knowing if they should devote their efforts more to sanitising their trolley handles or giving other shoppers a wider berth than previously. Perhaps the safest course is to do both.
