With gyms closed, many more people are taking to the streets to get their daily exercise, but what does this mean for social distancing? We asked respiratory expert Christine Jenkins about the science behind the 1.5 metre rule and how best to protect yourself when exercising outdoors.
How far can your breath actually travel?
The amount of water vapour you have in your breath is an important determinant of how far it travels. Different portions of your exhaled breath, depending whether they contain mostly large or very small droplets, will travel different distances. As you would imagine, there is an intense area immediately around your face and then it progressively falls off, dispersing and distributing more sparely though the surrounding air as it gets further away from you. This radiation effect is not unlike the ripples you might see on the top of the water if you throw a stone.
Air travels out from a normal exhaled breath roughly 1 to 1.5 metres before it becomes exceedingly diluted, and water droplets are barely detectable. The extent of spread, and the dilution effect very much depend on the rate at which you breathe and the force with which you propel it - if you cough or sneeze, that greatly increases the distance your breath and the droplets within it will travel.
Is that the reason why we have the 1.5 metre rule?
Precisely. When you’re walking at a leisurely pace that does not require an increased ventilation rate then you’re not likely to be spreading droplets much further than a metre - a metre is a safe distance. There is a margin of error there, and the variability depending how fast you’re breathing may in fact mean it can go a bit further, but 1.5 metres is a safe distance in terms of the droplets in your breath.
By water vapour or droplets, do you mean spit basically?
The droplets in exhaled air are not from saliva. Sometimes when people forcefully cough there is saliva in the exhaled breath, because it’s a combination of what comes out of your air passages as well as what comes out of your mouth, but normally exhaled air contains droplets predominantly from within the air passages in the lungs.
Droplets held within the exhaled air are actually part of what we would call your ‘insensible loss.’ They include fine droplets less than 10 microns in size and many droplets of 10 to 100 microns. Your “insensible” water vapour loss from the respiratory tract is the amount of water vapour there is in your exhaled breath. It’s warm inside your lungs - 37 degrees is your physiological temperature - and there is a certain humidity attached to that too, and your breath contains that moisture and hence there is evaporative loss as you breath in and out and as your airways cool as well.
So this water vapour in your breath that isn’t saliva and is coming from within your lungs, and is a reflection of the relative humidity of the air you’re breathing out. Water lost from the lungs in this way is increased if you have a fever, and if you are breathing rapidly.
When we run and we are doing physical activity we are exhaling a lot harder. How does that alter the physical distancing measurement?
Well there are different parameters around how far the breath and the droplets within it are travelling and indeed they may be travelling a great deal further than 1.5 metres in this case. It depends on the forcefulness with which somebody exhales. Many of us would know when we’re out walking that many runners who pass by are breathing very heavily. Others will know that sometimes runners pass you by and you hardly know they’re there except for their footfalls. In other words, they are either very accustomed to running or very fit, and they don’t necessarily breathe out particularly hard or forcefully.
So it’s going to be very dependent on the rate at which the person is running, whether they cough when they’re running, whether they’re forcefully exhaling and how close they are to you. Of course it does greatly increase the possibility that you will be exposed to a greater amount of their exhaled air if you are less than 1.5 metres apart. But it is also about the forcefulness of the exhalation. Some runners may pass you by a little more quickly as well, and they can be breathing in when they pass you by. It is important to appreciate this is not a simple 2- dimensional problem!
Exercise is important for physical and mental health at this time but there’s also a need to maintain social distancing but there’s a lot of people out and doing different exercise and you do think am I running into someone else’s droplets here?
Unfortunately you could well be, and perhaps we all have to be courteous to each other when we are doing some very vigorous aerobic exercise - running particularly. We need to remember that we owe it to the people we are passing by to keep a wide berth and that berth probably has to be wider than even 1.5 metres for runners who are breathing very heavily. And similarly for runners, walkers whatever you are doing, coughing and sneezing and proper hygiene around that - covering up, using the elbow to cough or sneeze into, are crucial steps we must all take when we are out. The fresh air around us is still potentially laden with organisms and our lungs are normally very good at handling this but in a situation where there is an increased risk of infection, those risks do increase. Now I would not want to deter anyone from outdoor exercise at this point in time when we have a very low rate of community transmission. However, I do think courtesy is crucial and for people who are running especially, just a giving a little bit more of a wide berth would be a very safe thing to do.
And maybe hold your breath as you pass?
Indeed, and I would try to breathe out if somebody passes you by too closely. I would think to myself ‘well this is a time to be breathing out a bit more rather than breathing in’. I know we can’t do the breathing out all the time, but nevertheless a little bit of common sense here is really valuable. If someone passes you by very closely and is breathing heavily, I wouldn’t be taking a big deep breath in immediately after if I could avoid it.
