Exercising in smog: when the air cancels out your workout

Delhi's winter air can flip the maths on outdoor exercise. What the research says about break-even points, AQI bands and how to keep training through smog season.

Every November, the same question lands in Delhi group chats a little after 6 am: is it worse to skip the run, or to breathe this air while doing it? The sky is the colour of weak tea, the app says "very poor", and your shoes are waiting by the door.

There is a real answer, and it is more interesting than "never" or "who cares". For most cities on earth, the benefits of exercise comfortably beat the harm from polluted air. Delhi in winter is one of the rare places where that can stop being true.

Why exercising in polluted air is different from just breathing it

When you work hard, you breathe faster and deeper, and most people switch to breathing through the mouth. That means more air, and more of whatever is floating in it, reaches your lungs every minute you are moving.

The pollutant that matters most here is PM2.5, the fine particles smaller than 2.5 micrometres. The World Health Organization notes that both PM2.5 and the larger PM10 can penetrate deep into the lungs, and that PM2.5 can even enter the bloodstream, with effects mainly on the heart and lungs ¹. Outdoor air pollution was estimated to cause 4.2 million premature deaths worldwide in 2019, and in that same year 99% of the world's population lived in places that did not meet WHO air quality guideline levels ².

So the worry is fair. The useful question is how much is too much.

The break-even point: how much smog cancels out a workout?

In 2016, a team of researchers led by Marko Tainio modelled exactly this trade-off for cycling and walking, using air quality data from 1,622 cities in the WHO database ³. They looked for two thresholds. The tipping point is where extra activity stops adding benefit. The break-even point is where the harm from pollution fully cancels the gain.

For half an hour of cycling a day, the background PM2.5 level would need to reach 95 µg/m³ to hit the tipping point, and 160 µg/m³ to hit break-even ³. For half an hour of walking, both points sat above 200 µg/m³ ³. Fewer than 1% of cities in the database had yearly PM2.5 levels above 95 µg/m³ ³. At the average urban level of 22 µg/m³, you would need to cycle for 7 hours or walk for 16 hours a day before reaching the tipping point ³.

Now the twist. The most polluted city in the database was Delhi, with a background level of 153 µg/m³. There, the tipping point came at just 30 minutes of cycling a day, and break-even at 45 minutes ³. That figure was a yearly average, so the worst weeks of the year are, by definition, worse than it.

This was a model built on mortality estimates, not a trial on real runners, and it considered PM2.5 alone. Still, the message is clear. In most places, keep moving. In a city with Delhi's winter air, the dose and the timing start to matter.

What happens to your heart and lungs on a polluted route

A London experiment shows the effect on a single walk. Researchers asked 119 volunteers over 60, some healthy, some with stable COPD and some with stable heart disease, to walk for two hours in a quiet part of Hyde Park and, on another occasion, along busy Oxford Street ⁴.

In the park, everyone's lung capacity improved within the first hour, and in many cases the gain lasted more than 24 hours ⁴. Arteries became less stiff too, with a maximum change of more than 24% in the healthy and COPD groups and more than 19% in those with heart disease ⁴. On Oxford Street, the arterial benefit shrank to a maximum of 4.6% for healthy volunteers, 16% for those with COPD and 8.6% for people with heart disease ⁴. Same people, same walk, different air.

The smog does not make exercise bad. It makes where and when you exercise part of the workout.

A much larger study from South Korea followed 1,469,972 adults aged 20 to 39 ⁵. Among people living with high PM2.5, those who raised their activity to 1,000 MET-minutes a week or more had a 33% higher risk of cardiovascular disease, although the result fell just short of statistical significance ⁵. In cleaner areas, the same increase was linked to a 27% lower risk, also not quite significant ⁵. The firmer finding pointed the other way: in low to moderate PM10 areas, people who dropped from that level of activity to none had a significant 38% higher risk ⁵. These are associations, not proof of cause, but the lesson for a smoggy city is to avoid cramming in big outdoor volume on bad days, without giving up on training altogether.

What India's AQI numbers mean for your workout

India's Air Quality Index is not the same scale as the µg/m³ figures above, so you cannot match them number for number. The health advisory from the Ministry of Health and Family Welfare and the National Centre for Disease Control translates the AQI bands into plain guidance for healthy adults ⁶:

• Moderate (101 to 200): do less prolonged or strenuous outdoor exertion.
• Poor (201 to 300): avoid outdoor physical exertion.
• Very poor (301 to 400): avoid outdoor physical activities, especially in the morning and late evening.
• Severe (401 to 500): avoid outdoor physical activities.

The advice is stricter for older adults, children under five, pregnant women and anyone with asthma, other lung disease or heart disease, who are told to cut back from the satisfactory band upwards and to stay indoors with low activity on very poor and severe days ⁶. On poor to severe days, the advisory specifically says to avoid morning and late evening walks, runs, jogs and exercise, and to keep doors and windows shut at those hours, ventilating if needed between 12 pm and 4 pm ⁶.

How to keep training through smog season

Here is a simple plan for the weeks when the air turns.

1. Check the AQI before you lace up. Look at the reading from the monitoring station nearest you rather than a city-wide figure.
2. Move hard sessions indoors on poor or worse days. Strength work, skipping, bodyweight circuits, a stationary bike or stairs at home all count. If you track steps, this is the season to stop chasing a number outdoors; our piece on how many steps you really need explains why the target is more flexible than you think.
3. If you do go out, choose the hour and the route. The advisory warns against early morning and late evening sessions on bad days, so the middle of the day is the better bet, and quiet lanes or parks beat roads with slow, heavy traffic, which the advisory tells people to avoid ⁶.
4. Keep outdoor efforts short and easy. The break-even model suggests that duration is what tips the balance, so a 20 minute easy jog is a different proposition from a 90 minute long run ³.
5. Use a mask for short exposures, not hard intervals. A well-fitted disposable N95 or N99 may help when exposure is brief, while paper masks, handkerchiefs, scarves and cloth are not effective ⁶.
6. Look after the air at home. Run the air conditioner on recirculate, avoid purifiers that generate ozone, and skip incense sticks and mosquito coils in closed rooms ⁶.
7. Do not stop moving. The Korean data linked giving up activity with higher heart risk in cleaner air ⁵, and the smog will lift. Your fitness should still be there when it does.

If you have asthma, a heart or lung condition, are pregnant or take regular medication, speak to your doctor about how to adjust your exercise during high-pollution periods.

Clean air is something Delhi cannot switch on for you this month. Smart timing, a shorter route and a good indoor session are things you can.

General information for healthy adults, not medical advice. Speak to a doctor before changing your training or diet if you have a health condition, are pregnant or take regular medication.