Listen up: exercise is bad for you. Maybe.
In light of research released last week that suggests air pollution is now a bigger global killer than smoking, I decided to investigate: is that annoying guy who runs to work every morning actually doing himself more harm than good? Do all those toxic emissions he's breathing in cancel out the benefits of his 45-minute jog?
To start, some basic science. Running disproportionately exposes you to air pollution, for the simple reason that your lungs are gasping for more air and causing you to inhale on average three times as much as if you were walking. If you go for a run in central London, or Scunthorpe, or Swansea, or anywhere else with a high level of air pollution, you're going to be inhaling much more of those toxins.
"When you exercise, you take in more air than you do when you're walking. The minute volume – the volume of air you breathe in in a minute – is hugely increased. If it's polluted air, you're getting a much higher dose of that. You are losing a lot of the benefits of the exercise; you're not gaining as much as you would if you did it somewhere clean," says Dr Paul Cullinan, Professor in Occupational and Environmental Respiratory Disease at the National Heart and Lung Institute. "I'm not saying the exposure would outweigh the benefits of the exercise, but if you're going to do the exercise, you're much better off doing it somewhere clean."
Dr Audrey de Nazelle, who worked on a 2016 study about whether air pollution can negate the health benefits of cycling and walking, has some good news for the active among you.
"There's no hard evidence, because we haven't done the analysis based on running, but my sense from knowing the data from walking and cycling is that you're likely to be better off running than not running, even at high levels of pollution," she says. "The levels of air pollution you find in London, exercise is going to be more beneficial than no exercise, absolutely."
How far are we from a future in which London's air pollution level makes exercise a self-defeating activity? "We did a simulation at various levels of air pollution to try to see at which point – for cycling and walking – it would start having more harm than good," says Dr Nazelle. "In Delhi, which has really high levels of [pollution] concentration – 153 micrograms per cubic metre – cycling started being a risk, i.e. doing more harm than good, at 60 minutes. [With] running, you'd typically [be doing] more exercise than cycling, so that number isn't the same – and again, this is a simulation, but it can give you a sense of what those numbers might be. In London – at least at the time when we looked at these numbers – the average was 60 micrograms per cubic metre, so far below what would be seen as an issue for long-term health impact."
Dr Benjamin Barratt – a senior lecturer at King's College London, who worked on a 2017 study looking at the effects of people in their sixties and over walking in polluted areas – concurs: "Robust evidence of this issue is very difficult to establish. However, the general opinion is that air quality has to be very bad to cancel out the benefits of exercise, much worse than we experience in London. The exception may be for those with existing respiratory or cardiovascular conditions."
Indeed, it's important to note that these numbers only apply to healthy adults, and polluted air affects people who suffer from asthma, some types of heart disease and older people in general more severely. A 2017 study carried out by Imperial College London – where respondents went for walks along Oxford Street and through Hyde Park, respectively – showed that for those over the age of 60, the pollution inhaled when walking along Oxford Street was almost enough to cancel out the benefits of the exercise. All participants benefited from a stroll in the park, with improvements being noted in lung capacity, increased blood flow and decreased blood pressure. This effect was "drastically reduced" when participants walked along Oxford Street.
Short of moving to the countryside or Zone 4, advice around the healthiest way to exercise while living in a city mostly falls under the realm of common sense: avoid exercising in areas with heavy traffic, avoid pollution "canyons" – narrow areas with high buildings where pollution levels build up quickly – and generally just try to find some green space or, alternatively, quiet back roads. Air pollution is also lower in the early morning or evening, so try to run at those times if possible.
"Pollution levels drop off very rapidly when you're away from a busy road, within 50 yards or so," says Dr Cullinan. The message would be: if you're going to do it, [run] somewhere relatively clean."