"A gas chamber."
That’s how New Delhi’s chief minister, Arvind Kerjrival, described the smog that recently blighted the Indian capital.
Last week, the city saw levels of air pollution that were 30 times the World Health Organisation’s recommended limit, prompting calls to temporarily ban large vehicles, close schools, and – in what seems an eminently sensible move - cancel a half marathon. Nevertheless, at the time of writing, the Airtel Dehli Half Marathon is still scheduled to go ahead as planned.
Are we killing ourselves by exercising outdoors in polluted areas? Or do the benefits of exercise protect us from any negative health impact of breathing polluted air? Let’s examine the evidence.
Types of air pollution
Polluted air contains different types of harmful constituents. Below are the main ones:
- Particulate matter, as its name suggests, refers to tiny particles of solids and liquids suspended in the air. They’re made up of various materials: metals, carbon, hydrocarbons formed from burning, biological matter such as pollen, dust from minerals, sea salt, sulfate, nitrate and ammonium. A lot of the items on this list are produced (either directly, or through secondary chemical reactions in the air) from man-made processes: the industrial combustion of fossil fuels and friction from vehicle brake pads/tires being two such culprits. But, Mother Nature is also culpable – sea spray, wildfires and volcanoes all generate particulate matter.
We can see some particles with the naked eye, but the more pernicious particulate matter is too fine to see. There’s good reason for this: whereas larger particles are filtered by hairs in the nose and upper respiratory tract, smaller particles can enter deeper inside the lungs and even penetrate the bloodstream, causing all sorts of damage to your respiratory and cardiovascular system. Scientists classify these smaller, more harmful particles into three groups:
- PM10 – ‘coarse particles’ have a diameter between 2.5 and 10 µm (micrometers/microns). They can enter your airways (trachea, bronchi, bronchioles).
- PM2.5 – ‘fine particles’ have a diameter less than 2.5 µm (micrometers/microns). These particles can get into your air sacs (alveoli).
- PM0.1 – ‘ultrafine particles’ have a diameter less than 0.1 µm. After getting into the air sacs of your lungs, they may pass into the bloodstream.
- Nitrogen dioxide (NO2) is a pollutant gas produced by power plants, domestic heating, and motor vehicles: the much-maligned diesel engine being a major source in urban areas. NO2 can irritate your airways, exacerbate lung conditions such as asthma, and increase susceptibility to respiratory infections.
- Sulfur dioxide (SO2) is another pollutant gas, released from power plants that burn fossil fuels, petroleum refineries, and industrial cement manufacturers. As well being an irritant gas, SO2 also contributes to the formation of another air pollutant, ozone.
- Ozone (O3) is a gas found higher up in the Earth’s atmosphere, where it absorbs harmful UV radiation. Closer to the ground, it is formed from chemical reactions involving sunlight and man-made pollutants such as nitrogen dioxide and hydrocarbons. This ground level ozone can irritate lungs, and hamper the respiratory system’s immune defenses.
Health risks of air pollution
So, just how bad is air pollution? According to the World Health Organization (WHO), air pollution is linked to 7 million premature deaths (under the age of 70) worldwide each year. In the UK, PM2.5 particulate matter is thought to contribute to 29,000 premature deaths per year. When nitrogen dioxide exposure is factored in, this figure rises to a staggering 40,000.
Unlike inhaling a lethal gas (e.g., hydrogen cyanide), air pollutants do not cause deaths directly. Rather, they increase the risk of and exacerbate cardiovascular and respiratory illnesses, such as heart attack, stroke, atherosclerosis, bronchitis, asthma and COPD (chronic obstructive pulmonary disease). “How?” you may ask. It’s complicated, but current research suggests that air pollutants cause oxidative stress (a process where damaging free-radical molecules build up in the body), which leads to inflammation and narrowing of the linings of airways and blood vessels.
It all sounds pretty grim. What’s also troubling, however, is that exercise could make us even more vulnerable to the health risks of air pollution. Well, in theory at least.
Exercising in air pollution
There are four main reasons why exercise might worsen the health effects of air pollution.
- Whenever you exercise, the amount of air you inhale each minute (your minute ventilation rate) increases. By extension, you also inhale more air pollutants. Indeed, studies show that high-intensity exercise can raise the fraction of particulate matter deposited in your lungs by between 6 and 10-fold.
- Air flows through your airways more quickly during exercise, which may carry pollutants deeper into your lungs, increasing the risk of ultrafine (PM0.1) particles being absorbed into the bloodstream.
- At a certain exercise intensity (around 100W of exercise output), you start to breathe mainly through your mouth rather than through your nose, meaning inhaled air bypasses your nose’s inbuilt filtration system, allowing larger particulate matter into your lungs.
- Strenuous exercise impairs the ability of your hairs lining your airways (cilia) to clear pollutants.
So, exercising in polluted areas must be bad for you, right? Not necessarily.
Juggling risks and benefits
It’s certainly true that air pollution can negatively impact both health and athletic performance. Studies of elite athletes suggest that dilation of blood vessels (vasodilatation) and the ability to change heart rate in response to training demands (heart rate variability) are impeded by inhaling air that is high in particulate matter. It’s worse for people with asthma and existing respiratory and cardiovascular disorders – (if you suffer from any of these, it’s wise to consult your physician before exercising in polluted areas).
Nevertheless, exercise confers several benefits for cardiovascular and respiratory health – reduced inflammation, lower blood pressure, improved blood flow, to name just a few. And the good news is that these benefits are generally thought to outweigh the adverse risks of air pollution. Put simply; it’s better to go out for a jog in downtown New York than sit inside on the sofa.
Exercise may also directly protect against the pollutants in inhaled air. Physically fit people with a low resting heart rate (below 70 beats per minute) seem to be resistant to blood pressure rises induced by increasing inhaled particulate matter. If you need help getting fit, why not check out our Get Fit plan.
Of course, the exact risk-benefit ratio of exercising outdoors varies according to the amount of exercise you do and the local levels of air pollution in which you do it. There may indeed be a ‘break-even point’, after which exercise in polluted air becomes deleterious to overall health. According to one study, for someone who cycles 30 minutes a day, this break-even point comes at a background level of PM2.5 pollution of 160µg/m3. To put that figure in context, only the cities of Zabol in Iran (with a PM2.5 of 217µg/m3), and Gwalior (176 µg/m3) and Allahabad (170 µg/m3) in India, have average levels of pollution above this. (As for New Dehli, last Monday saw PM2.5 levels of 438 µg/m3).
So, whether you’re in New York (9 µg/m3), Los Angeles (11 µg/m3), London (15 µg/m3), Toronto (8 µg/m3) or Sydney (8 µg/m3), the typical advice would be: get out there and exercise! Just try your best to avoid busy roads with lots of traffic and check the day’s air quality index before you head out the door