HuffPost UK reader Jill asked: “People are not allowed to sing in huge, high ceiling churches, yet panting joggers regularly pass me much closer than two metres. Am I more at risk walking through a jogger’s ‘vapour trail’ than I am walking through another walker’s vapour trail?”
There’s perhaps nothing worse for your Covid anxiety than having a person run past you while breathing heavily – especially if you can physically see their breath puffing out into the air. But the risk of catching Covid-19 from someone running past you is actually very low.
Professor Chris Budd, is an expert in applied mathematics at the University of Bath and part of a forum for knowledge exchange in the mathematical sciences (V-KEMS), which has considered this question as part of a general study of the transmission of Covid-19 in a crowd.
He tells HuffPost UK: “Extensive numerical simulations based on careful mathematical models of aerosol transport have shown that it is, in general, safer to be closer to someone if they are running past you, than further away if you are stationary – such as in a room when someone is singing.”
This is because the time of the exposure is less in the first case than in the second, even though the aerosols have had less time to diffuse away.
“There is of course a trade off between distance away and the speed of the runner, but in general I would say that I would be safer if a jogger passed me quickly than if I was in a room with a singer,” he adds.
As you can probably expect, there’s not been an awful lot of research into this specific area – and what has been done has been met with some scepticism.
Earlier in the year a preprint study, meaning it hadn’t been peer-reviewed, put the fear into some when it suggested joggers were leaving behind huge vapour trails and keeping two metres apart wouldn’t cut it.
Researchers from The Netherlands and Belgium conducted simulations of the movement of droplets emitted by an exhaling walking or running person close to another walking or running person, and found it was problematic to be positioned in the slipstream (directly behind) the walking or running person.
Their research also suggested people needed to keep larger social distances than two metres – so keeping five metres behind someone walking and 10 metres away from someone running, which at the time a lot of people flagged was pretty impossible to do, especially in cities.
Angela Rasmussen, a virologist at Columbia University, told Vox at the time: “I think we should be very careful with making assumptions about transmission based on that ‘study,’ since it didn’t account for any variables related to transmissibility.”
How does Covid-19 spread?
One of the key ways the virus behind Covid-19 spreads is through close contact interactions between people – so that’s hugging, kissing, standing or sitting close (within six feet) to each other.
The virus can also spread in droplets and smaller aerosols expelled from the nose and mouth when a person is chatting, laughing, singing or even breathing. These particles can then be inhaled into the nose, mouth, airways, and lungs of others, and cause infection.
The concern with joggers is that they are often breathing heavily as they run past. But – and this is a big but – you are only exposed to their exhalations very briefly, and being outside, their germs are more likely to have been dispersed into the air pretty quickly, too.
Professor Paul Hunter, an expert in infectious diseases from the University of East Anglia, says the risks of Covid-19 transmission are driven by whether you’re indoors (a higher risk than being outdoors), how crowded it is (the more crowded, the higher the risk), how close you are to other people (the closer you are, the higher the risk), and the time you spend with somebody (the longer you spend, the higher the risk).
“Yes, a jogger passing through probably will create more virus around in the atmosphere than somebody else who’s walking,” he says, “so that increases risk from that perspective. But they are passing you a lot more quickly.”
He adds: “And generally speaking, joggers tend to put more space between you as they pass you on the path than other people. So taking all that together, I wouldn’t worry about it.”
What can you do if you’re still worried?
If you see a jogger coming towards you and you’re worried, keep to the side of the path away from where they are running or cross the road for peace of mind.
Should you wear a mask? Prof Hunter wouldn’t advise people to wear masks outdoors – unless they’re in a crowded outdoor area, like a beer garden – “because the risk outdoors is so much lower”.
On the whole, experts believe we shouldn’t worry too much. TV doctor Dr Sarah Jarvis appeared on Channel 5 earlier this year and said it would take around 50 runners breathing heavily running past you for a person to get the infection.
Prof Hunter concludes: “That doesn’t mean to say the risks are zero, it just means that the risks are in fact very low in relationship to other environments. To my point of view they are acceptably low as for me not to worry about it.”
Experts are still learning about Covid-19. The information in this story is what was known or available at the time of publication, but guidance could change as scientists discover more about the virus. To keep up to date with health advice and cases in your area, visit gov.uk/coronavirus and nhs.uk.