Exercising when air pollution levels are high can do more harm than good
Exercising when air pollution levels are high can end up doing more harm than good, writes Rachel Jacqueline
Do you check air pollution levels before exercising outdoors? You should. Although regular exercise is good for us, exercising in poor air may wipe any short-term fitness gains.
Worse still, you could be putting your body at risk of serious illness and - more worryingly - long term, irreversible damage.
Exposure to air pollution is a sad fact of life in any city, particularly in Hong Kong. The "vertical city" is a victim of its design, festering in roadside emissions and regional smog trapped among the city's many high-rises, particularly on days with little wind.
At rest, we have some basic protection measure against pollution - our nose. "The olfactory tissue and nose hairs act as a kind of first defence," says Dr Michael Tse of the Active Health Clinic at the Institute of Human Performance in Hong Kong.
Pick up the pace and we radically increase the rate and extent of exposure. "When you exercise, the rate of breathing is increased and you take deeper breaths," says Tse. "At higher intensities, you also breathe through your mouth to get sufficient oxygen, leading to a direct intake of pollutants."
The lungs and cardiovascular system take the first hit, with every lungful of polluted air damaging the fine tissue, causing inflammation. Signs of exposure include chest tightness, wheeziness, coughing and a burning sensation in the throat and lungs.
"As inflammation is repeated, a person is at greater risk of developing asthma, bronchitis, chronic obstructive pulmonary diseases, damage to the mucous producing glands in the airways, and has an increased risk of pneumonia," says Professor Anthony Hedley, creator of the Hedley Environmental Index and Emeritus Professor at the University of Hong Kong's School of Public Health.
Those with a predisposition for respiratory illness, such as asthma sufferers, are at an increased risk.
Seasoned Hong Kong-based ultra-runner Simon Southgate suffered the dangerous combination of air pollution and exercise a few weeks ago while running in the New Territories.
After a six-hour Saturday morning training run in moderate pollution, Southgate, 45, started to feel sick and had a dry cough.
He was diagnosed with exercise-induced bronchitis, given an inhaler, and prescribed heavy antibiotics to expel fine particulate matter trapped in his lungs.
"I was told I was at least the fourth person the doctor had seen over the weekend from exposure. I've had some problems with air pollution when training in the past, but nothing like this."
Southgate was unable to work and train for the best part of two weeks.
And it's not just our lungs at risk. "Air pollution has the potential to damage virtually every tissue in the body, including the blood vessels which supply critical organs," says Hedley.
A double-blind, randomised, crossover study by the University of Edinburgh published in the American Heart Association Circular in 2005 demonstrated that inhalation of diesel exhaust impairs blood vessel functioning.
They exposed 30 healthy men aged between 20 to 38 to diluted diesel exhaust or regular air for one hour during intermittent exercise at a moderate level and found damage to the heart and surrounding vessels, providing a link to the development of plaque or lesions in the blood vessels and heart attacks.
Hong Kong's air pollution is bad, scarily so. Levels are three times that of New York, twice that of London and rank the city behind Singapore and Tokyo for air quality.
"It's the biggest health crisis in Hong Kong," says Kwong Sum-yin, chief executive of independent NGO Clean Air Network.
"If you look at data over the past five or so years, you can see that our air pollution is twice or three times higher that the World Health Organisation [WHO] recommended levels," she says. "Previously air pollution has been an environmental issue, but now it's a public health issue."
Air pollutants are a toxic cocktail of noxious gases and each has a different effect on our bodies.
Ozone is like sunburn for your lungs, penetrating deep into the area responsible for vital gas exchange with the blood, resulting in breathing difficulties. Nitrogen dioxide and sulphur dioxides suppress the immune system, placing you at risk of respiratory infection.
Fine particles that hang in the air, particulate matter known as PM2.5, that may include heavy metals and toxic organic compounds, travel deep into our airways and, like in Southgate's experience, can clog our lungs.
With the worrisome state of our air, is it safe to exercise outdoors? The Air Quality Health Index (AQHI) issued by the Environmental Protection Department (EPD) indicates the cumulative health risk attributable to the three-hour moving average concentrations of these high-risk air pollutants.
Between AQHI 1 to 8, there are no recommendations from the EPD to reduce levels of exercise, although warnings kick in at AQHI 7 for those sensitive to air pollution such as children and the elderly.
Follow these guidelines, says Dr Alfred Tam Yat-cheung, chairman of the HK Asthma Society, and exercising outdoors is "safe". The key is finding a good location: "Provided you're away from the roadside, it doesn't hurt," he says.
Not everyone agrees, and the AQHI has been widely criticised. Kwong cautions that the new standards still fall well short of those recommended by the WHO, while Hedley believes the air is not fit to breathe, let alone run around in. "The AQHI and the government's air quality objectives are in no way protective of our health," says Hedley.
"The WHO guidelines don't even represent 'good quality air'. You could think of them as standards for 'safer air' … but the criteria continue to change the more we discover.
"The number of days I'd say when risks from poor air quality are significantly reduced enough to exercise in are few and far between," he says.
Even though Hong Kong is 70 per cent countryside, it makes little difference, says Lai Hak-kan, an assistant professor in epidemiology and biostatistics at the University of Hong Kong's School of Public Health.
"Residential locations here are densely packed … the positive impact of the country parks is not felt," he says. "There is a lot of overseas literature that finds proximity of distance from highways and traffic is very important … the closer you are the more exposure."
Exercising in country parks may help, but on high pollution days it's not enough. "The ambient levels of air pollution are extremely high across the whole territory," says Hedley.
Going indoors may make little difference. "It depends on whether the building is equipped with hi-tech air filters, and the extent to which furniture and fittings absorb particles and gases. In general, indoor air quality usually reflects outdoor air quality," he says.
So is there any hope? Recent research has shown the powerful antioxidative effects of exercise may combat pollution exposure, better yet, the body may even adapt.
A study of mice published in the journal Medicine and Science in Sports and Exercise in 2012 found mice that exercised in pollution over a five-week period underwent bodily changes to combat the harmful effects of the pollution; mice that didn't exercise, but were still exposed to the same levels of air pollution, suffered an alarming spike in lung inflammations and free radicals.
Although the same link is yet to be made in humans, a 2012 review of epidemiological data published in the New England Journal of Medicine estimated short daily bicycle trips in polluted cities took between 0.8 and 40 days from a person's average lifespan, but additional exercise lengthened it by three to 14 months.
A prudent approach to exercise is needed, one which minimises exposure to pollution during workouts whenever possible.
Southgate says he has not been put off, despite his scare. "I live in Hong Kong and don't have the luxury of training overseas. But if I have a long race coming up, I still have to train for it. I'm definitely more conscious of the air quality index. I'd plan my training around - if it is high, I won't train in it, or I'll go earlier or later in the day."