Haze and Running: When to Train, When to Rest, and What the PSI Numbers Actually Mean

The haze has settled in. Singapore's 24-hour PSI first crossed into the Unhealthy range in early September, and by late September all five regions were reading Unhealthy at once — the central region hit a PSI of 158 — as smoke from fires in Kalimantan and southern Sumatra continues to drift across the island. Since then readings have seesawed between Moderate and Unhealthy depending on the wind, with NEA issuing daily haze updates and a near-term forecast between high Moderate and mid Unhealthy. If you are mid-way through a marathon build or getting ready for a year-end race, that grey sky raises an immediate question: do I still run today?

The honest answer is: it depends on the numbers, and on you. Here is how to read the air, make the call, and keep your training on track without doing your lungs any favours they did not ask for.

Image above is from the BBC report by Tim Mcdonald and shows Singapore's port in the central business district seen during the haze and on a clear day. Read it here- Singapore anger as haze from Indonesia hits highest level this year

the https://www.bbc.com/news/world-asia-34355825

 

Why haze is a bigger deal when you are running

The main problem in haze is PM2.5 — fine particles small enough to travel deep into your lungs and, in high enough doses, into your bloodstream. Short-term exposure irritates the eyes, nose and throat. For people with asthma or heart and lung conditions, it can be far more serious.

Running makes all of this worse in a very simple way: you breathe more. On an easy run you move several times more air through your lungs than at rest, and much of it through your mouth, bypassing the nose's natural filtering. The harder the session, the bigger the dose. A reading that is manageable on a walk to the MRT is a different proposition 40 minutes into a tempo run.

Add Singapore's usual heat and humidity — which already push your heart rate and perceived effort up, as we covered in Heat Happens — and a hazy day is simply not the day to chase a workout.

 

The two numbers that matter (and when to use each)

Singapore gives you two official readings, and runners regularly mix them up.

24-hour PSI: for planning ahead

The Pollutant Standards Index you see in headlines is calculated over 24 hours. NEA recommends using the 24-hour PSI forecast to plan tomorrow's activities. The bands:

• 0–50 Good / 51–100 Moderate: normal training for most people
• 101–200 Unhealthy: cut back on prolonged or strenuous outdoor exertion; the elderly, pregnant women, children and anyone with heart or lung conditions should minimise it
• 201–300 Very Unhealthy: avoid prolonged outdoor exertion; vulnerable groups should stay indoors
• Above 300 Hazardous: minimise time outdoors altogether

1-hour PM2.5: for the run you want to do right now

 

Because the PSI is a 24-hour average, it lags. The sky can look apocalyptic while the PSI still reads Moderate. For a decision you are making in the next hour — like whether to head out the door — NEA points you to the 1-hour PM2.5 concentration instead, banded as Normal (0–55 µg/m³), Elevated (56–150), High (151–250) and Very High (above 250). In the Elevated band, the advice is already to reduce strenuous outdoor activity for the coming hour; vulnerable persons should avoid it.

Both readings are free on the haze.gov.sg site and the myENV app, broken down by region. Check the 1-hour PM2.5 for your part of the island before you lace up, not the islandwide headline number.

A runner's decision guide

Cutting through the bands, here is a practical way to make the call each day.

PSI under 100 and 1-hr PM2.5 Normal: train as planned. If the air smells smoky or your throat is scratchy, downgrade the session anyway — your airways are telling you something the sensors have not caught up to yet.

PSI 101–200 or PM2.5 Elevated: this is the grey zone, literally. Shorten the run, drop the intensity, or take the session indoors. If you have asthma or any heart or lung condition, or you are pregnant, skip outdoor training. A missed interval session costs you far less than an airway flare-up mid-build.

PSI above 200 or PM2.5 High and above: no outdoor runs. Not with a mask, not at an easy pace, not because it is week nine of twelve. An N95 has its place if you must spend hours outdoors, but running hard behind one is neither comfortable nor the point — if conditions call for a respirator, they are calling for a rest day.

Whatever the number says: stop if you get chest tightness, unusual breathlessness, a persistent cough or stinging eyes. And if you have been feeling run-down for days — haze or not — reread the warning signs of overtraining before you blame the sky.

 

 

Protecting your training block

A haze episode can last days or weeks depending on the fires and the winds, and October is peak training season for December's marathon weekend. The runners who come out of a hazy fortnight fittest are the ones who adapt early rather than push through.

Move quality sessions to the treadmill. Intervals and tempo work translate well indoors, and an air-conditioned gym with the windows shut is a genuinely different environment from the park connector. You will still sweat plenty — treadmill rooms are warm and airflow is poor — so treat fuelling like an outdoor session: fluids plus sodium, not just water. Our electrolytes range covers tablets, chews and capsules that work just as well on a treadmill as at MacRitchie.

Use the bad-air days for what your plan probably lacks anyway: strength work, mobility, and honest recovery. If the haze forces a down week, take it as one — the adaptations happen when you absorb training, not when you stack it. Our recovery tools can make those days feel less like lost time.

And time your outdoor runs deliberately. Readings shift with the wind over hours, not minutes. A morning that reads High can ease to Normal by evening — check again before you write the whole day off.

When the air clears

Ease back in rather than cramming missed mileage into a clean-air window. One or two easy runs first, then resume the plan where the calendar says you are, not where you feel you should have been. Training plans bend; lungs are less forgiving.

Not sure how to reshuffle your block around a hazy week? Drop by the store and talk it through with us — we are runners dealing with the same sky. For gear, fuel and recovery kit that keeps indoor training honest, explore the range.