PETALING JAYA: You've probably seen it lately - the sky turns a dull grey, there is a faint smell of smoke in the air, yet runners are still out on jogging tracks.
With races and training continuing through haze season, it is worth asking whether trying to stay fit could sometimes mean taking on more risk than we realise.
Haze from vegetation fires carries tiny particles known as PM2.5, measuring 2.5 micrometres or less - about 30 times thinner than a human hair. These particles can travel deep into the lungs and reach the alveoli, the tiny air sacs where oxygen passes into the bloodstream.
Running increases that exposure because we breathe faster and more deeply than we do at rest. The longer and harder the effort, the more polluted air we take in, which makes prolonged outdoor exercise particularly concerning during poor air quality.
Haze exposure can cause coughing, wheezing, irritated eyes and shortness of breath, while also worsening asthma and other respiratory conditions. Children, older adults, pregnant women and people with existing heart or lung disease are especially vulnerable, although healthy runners are not entirely protected.
One useful illustration comes from a London study involving 119 adults aged 60 and above, who completed two separate two-hour walks: one along traffic-heavy Oxford Street and another in the cleaner air of Hyde Park. The group included healthy participants as well as people with chronic lung or heart disease.
After the park walk, lung function generally improved and blood vessels became less stiff, with some benefits still detectable about a day later. Following the street walk, those improvements were usually smaller, although the effect was not identical across every measure.
People with lung disease reported more wheezing and phlegm after the polluted walk, while those with heart disease reported more coughing. Healthy participants did not necessarily feel worse, yet breathing tests showed less improvement in lung function and poorer relaxation of the arteries compared with the park walk.
The pollution in that study came from traffic rather than haze, and the participants were walking rather than running. Even so, the lesson is useful: how you feel is not always a reliable guide to what polluted air is doing inside your body.
Over the longer term, particle pollution is associated with heart disease, stroke, chronic lung disease and lung cancer. One hazy run is not going to cause a lifelong illness, and science cannot put a precise number on the long-term risk from a single race - but repeated training in polluted air adds to cumulative exposure.
Know when to move the workout indoors
None of this means runners should stop exercising. Start by checking the Air Pollutant Index (API) at the station nearest you before heading out.
The health ministry advises avoiding outdoor activities when the API rises above 100, which is considered unhealthy. People with asthma or current respiratory symptoms should be more cautious and may need to stay indoors until readings improve further.
On hazy days, an easy treadmill session, stationary cycling or strength training in a space with cleaner, filtered air can keep fitness ticking over without adding unnecessary exposure. Indoor air matters, too, so keep windows closed when outdoor conditions are poor, use an air purifier with effective particle filtration, and avoid smoking or burning candles or incense indoors.
Air quality can change throughout the day, so waiting for a better window may make sense. Malaysia's intermonsoon period and the approach of the northeast monsoon may bring changing winds and more rain, but clearer skies are never guaranteed.
If coughing, wheezing, chest tightness or unusual breathlessness develops during exercise, stop and move to cleaner air. Persistent or severe symptoms should be medically assessed.
Race organisers have a role, too
Runners should not have to make these decisions alone, particularly during organised events. In 2019, the health ministry advised that outdoor sports events should be postponed when the API exceeds 100.
A sensible haze policy should set out postponement criteria before race day, monitor local readings before and during the event, and give medical teams a meaningful role in decisions.
Runners should also receive timely updates, fair withdrawal or deferral options, and clear emergency plans covering participants, volunteers and spectators.
Moving a run indoors or postponing a race is not giving up; it is training intelligently. A healthy running culture recognises that endurance matters, but so does knowing when the air itself has become part of the risk.
This article was written by Dr Faizul Haris Hatta and Dr Noor Artika Hassan, lecturers in environmental health at the Department of Community Medicine at International Islamic University Malaysia.