The weekend’s hot weather saw crowds of people flow to beaches and parks to bask in the sun and have picnics. Britons can sit outside for as long as they like, and since June 1, they are allowed to meet with up to five other people outdoors as long as they keep two metres apart. But coronavirus fear strikes many people seeing crowds in the park or queuing up for ice cream and public toilets next to strangers.
What are the chances of catching coronavirus outside? Generally, being outdoors is safer than being in a confined space with lots of people breathing out virus particles. That makes sense. But how does a picnic in the park compare with a BBQ in a private garden? Scientists are turning to data from contact tracing to find out which activities are the riskiest.
In confined spaces, there is little air movement to disperse viruses away from people’s breathing zones. An experimental study published in the New England Journal of Medicine described how the Sars-Cov-2 virus lingered for up to three hours in the laboratory, but acknowledged that the particles would drift down and settle on surfaces much faster in a natural setting.
Coronavirus is mainly passed on between people through droplets produced by breathing, talking, coughing or sneezing, but when they are outdoors, wind constantly moves the large volumes of air and dilutes these virus-laden droplets. “Of course, if the wind is blowing the virus towards you, there may be an increased risk of infection. But there will also be a massive dilution factor which will generally act to reduce the exposure even if the wind is blowing it in the right direction,” says Julian Tang, honorary associate professor of respiratory sciences at the University of Leicester. Sunlight, too, has a detrimental effect on most viruses, with the short-wavelength ultraviolet radiation (UVC) destroying viral RNA on surfaces so it cannot replicate further.
While it would be difficult to simulate how fresh air and sunlight dilute and destroy the virus outdoors, Tang suggests a simple scenario that many people will be familiar with for comparison: if someone is smoking in a room with closed windows, you can smell their smoke even if you’re sitting a few metres away. After a while, and a few cigarettes later, the smoke will grow denser and chances are you will inhale it. You may be able to smell their cigarette smoke if you’re sitting right in front of them in a park. But as you move around them, it becomes noticeable how the wind disperses the smoke from different angles and distances.
People can also stick to social distancing rules, such as staying two metres apart, more easily outdoors than in a cramped indoor space. It is, however, not only the distance to other people and the environment that explains how someone might catch the coronavirus. It depends on the time spent together and the activity as well.
“Looking at where and when people actually acquire their infections, without isolating them immediately after exposure, is very difficult,” says Tang. An infected individual may be able to recall who they came in close contact with on a day but one cannot be sure whether they became infected during their busy 20-minute train journey, from sitting through a university lecture just before that, or from having dinner with their family after the commute home.
Nevertheless, data emerging from contact tracing around the world does allow scientists to map how the coronavirus spreads in real-life settings and point to so-called super-spreader events. Muge Cevik, a clinical academic fellow in infectious diseases and medical virology at the University of St Andrews, has analysed different datasets to find that the majority of infections occurred in adults gathering indoors – be it in households, places of worship, crowded offices, care homes or public transport – and maintaining close contact for more than 15 minutes.
In one Chinese study of 1,245 confirmed cases, only one was traced to a conversation with an infected person outdoors. Another case study showed how one infected person in Chicago spread the disease to multiple people at a funeral and birthday party held three days apart (both events involved shared meals and hugging). Three of those likely infected at the birthday party then attended a church service that lasted for more than two hours and passed the virus on to another attendee sitting next to them. In China, 8,437 shoppers and employees of a supermarket were tracked after one of the employees tested positive for coronavirus. The risk of infection turned out to be much higher for the employees than for the shoppers, with nine per cent falling ill as a result compared with just 0.02 per cent of the shoppers.
So, could this be good news for BBQs and small garden parties? It depends, of course, on the nature of social interaction. “People feel much more comfortable spending time with friends and family, and they feel much more comfortable being in close contact with those people,” says Cevik. She explains that people are often more worried about catching the virus from a stranger passing by when a gathering with friends and family could actually be much riskier. “If you're outdoors maintaining social distancing and there's no symptomatic case in the group then your risk will be lower than if you have someone who's symptomatic in the group and you’re hugging and kissing each other,” she says.
Some of the clusters identified through contact tracing involved groups eating together, thus, people spending a long time together in a park or garden, sharing plates and utensils, without maintaining personal hygiene, will still be able to transmit coronavirus. Ian Hall, a professor of molecular medicine at the University of Nottingham, points out that water, soap and hand sanitiser may not be as readily available outdoors as they are in kitchens and bathrooms. “There's a bit of a temptation when you're outside having a barbecue to be less fussy about hand hygiene. Outdoors is likely to be safer, but there will still be some virus on some of the surfaces that you touch,” he says.
Maintaining good hygiene will also be key for the British restaurants, bars and pubs that are figuring out how to safely reopen from July 1. Those with a terrace, beer garden or car park will find it easier to accommodate a socially-distanced crowd. Tables will have to be cleaned between uses and disposable menus may become the new norm. But guests enjoying a meal or a cold pint in the sun will still have to enter the premises to use the bathroom. “Every circumstance is going to need a particular set of solutions and to a large extent that will depend very much on the local environment,” says Hall. “It's very difficult to create generic guidance for restaurants and pubs, which don't take into account their local environment.”
This article was originally published by WIRED UK
