Should you run today? A plain-English guide to exercising during the haze

When the sky turns that flat, milky grey and you can smell it, the honest answer for most healthy people is: adjust the run, don't cancel it. Ease the pace, shorten it, or move it indoors. You don't have to choose between hard intervals in the haze and lying on the sofa until October.

Singapore makes this easier than most places because the government publishes two official numbers, free and live. The trick is knowing which one to check, and what the bands actually tell you to do.

Two numbers, two questions

Singapore's National Environment Agency (NEA) runs two tools, and they are not interchangeable.

The 1-hour PM2.5 reading is for right now. Thinking about heading out for a jog in the next hour? This is your number.

The 24-hour PSI is a forecast plus a health advisory, and it is for planning ahead. Deciding whether to commit to a beach run or a long cycle tomorrow? Use this one.

Mixing them up is the most common mistake. A reassuring 24-hour PSI can sit alongside a 1-hour PM2.5 that is spiking, because haze moves and shifts through the day. So the myth that "PSI and PM2.5 are the same number, just check one" can genuinely mislead you. Different timeframes, different jobs.

One bit of background that explains why PM2.5 matters so much: Singapore's PSI is computed from six pollutants (sulphur dioxide, PM10, PM2.5, nitrogen dioxide, carbon monoxide and ozone), and it reports the highest sub-index as the headline figure. During haze, PM2.5 (the fine particles small enough to reach deep into your lungs) is usually the one driving the number.

The right-now call: 1-hour PM2.5

NEA splits the 1-hour PM2.5 into four bands, each with a plain personal guide. Here is the whole thing, translated into a go / ease-off / indoors call.

1-hour PM2.5 (ug/m3)	Band	Healthy adults	Vulnerable persons
0-55	Normal	Carry on as usual	Carry on as usual
56-150	Elevated	Reduce strenuous outdoor activity for the next hour	Avoid strenuous outdoor activity
151-250	High	Avoid strenuous outdoor activity	Avoid all outdoor activity
over 250	Very High	Minimise all outdoor activity	Avoid all outdoor activity

So your easy answer at a glance: in the Normal band, run normally. In the Elevated band, a healthy adult should take the intensity down (a brisk walk or easy jog instead of intervals), and anyone vulnerable should skip the hard stuff entirely. In the High band, healthy adults stop doing strenuous outdoor exercise, full stop, and vulnerable people stay indoors. Very High means everyone minimises being outside at all.

NEA's action words are deliberately precise, and worth getting right: reduce means do less, minimise means do as little as possible, and avoid means don't do it. "Prolonged" means continuous exposure over several hours; "strenuous" means a lot of effort. Both of those words point straight at your workout.

Tomorrow's plan: the 24-hour PSI

For planning the next day, the 24-hour PSI forecast carries its own advisory.

24-hour PSI	Band	Healthy adults	Vulnerable groups
0-50	Good	Normal activities	Normal activities
51-100	Moderate	Normal activities	Normal activities
101-200	Unhealthy	Reduce prolonged or strenuous exertion	Elderly, pregnant, children minimise it; chronic lung/heart disease avoid it
201-300	Very Unhealthy	Avoid prolonged or strenuous exertion	Minimise to avoid outdoor activity
over 300	Hazardous	Minimise outdoor activity	Avoid outdoor activity

The headline for the questions people actually ask: at PSI 80, you're in Moderate, outdoor exercise is fine. At PSI 120 (Unhealthy), a healthy adult should ease off prolonged or strenuous exertion, not necessarily cancel a gentle session. At PSI 180, still Unhealthy, the same applies but you're well into "take it easy or take it indoors" territory. At PSI 250 (Very Unhealthy), healthy adults should avoid hard or long outdoor exertion altogether.

Notice the myth this kills: that haze "only matters at Hazardous." The advisory starts changing behaviour from the Unhealthy band, PSI 101, and vulnerable groups are told to ease off a band earlier than everyone else.

Why intensity is the real dial

Here's the physiology that makes "ease off" more than vague nannying. When you go from rest to hard exercise, you breathe far more air, and you switch from nose-breathing to gulping through your mouth, which skips some of your nose's natural filtering. One dosimetry analysis estimated that shifting from rest to high-intensity exercise raised the rate of pollution you inhale roughly 9.5-fold.

That is exactly why NEA's advice keys on "strenuous" and "prolonged." A relaxed walk and a tempo run on the same hazy day are not the same dose of haze. Dropping from intervals to an easy jog, or an easy jog to a walk, materially cuts what you breathe in. The dial you're turning is intensity, not just whether you go out.

Going from rest to high-intensity exercise raised the rate of pollution inhaled roughly 9.5-fold, which is why the advice targets strenuous effort first.Personal-dose during cardiovascular exercise, 2026

Who should ease off earlier

NEA defines "vulnerable persons" as the elderly, pregnant women, children, and people with chronic lung disease (including asthma) or heart disease. Across both tools, these groups are advised to step down one band earlier than healthy adults.

This is the part where the dry tone goes off and we play it straight. For most healthy people, short-term haze over a few days generally causes no major problems beyond irritation of the eyes, nose and throat, and it's usually still fine to carry on outdoors with sensible adjustment. But if you have a heart or lung condition, are pregnant, or you're caring for young kids or older parents, the bands are advice, not a diagnosis. Anyone who feels unwell during haze, especially in those groups, should seek medical attention, and decisions about your own activity are best made with your doctor.

One thing you generally don't have to decide yourself: the government uses the 24-hour PSI forecast to guide the big institutional calls, like school closures and the suspension of outdoor training or work. So whether your kid's CCA training or outdoor sports goes ahead is decided for you. Your job is the personal jog-or-not call.

Should you mask up and run anyway?

Short version: no. Doctors here advise against wearing an N95 during outdoor exercise. It significantly limits your air intake and ramps up breathlessness, perceived effort and fatigue, exactly the wrong combination when you're already breathing hard. A randomised trial confirmed N95s raise perceived breathing resistance and discomfort at peak exercise, and a broader review found masks during exercise increase the feeling of effort, breathlessness and fatigue for limited protective payoff.

So the move when air quality is poor isn't to mask up and push through outdoors. It's to take the session inside.

Indoor swaps that actually work

Indoor exercise is the safe swap during haze, and it's preferable to outdoor exercise once the bands climb. But "indoors" isn't automatically clean air. If your windows and doors are open, indoor air just tracks the haze outside.

So make the swap properly. Keep windows and doors closed when outdoor air is unhealthy, and favour an air-conditioned or filtered space; a room with a HEPA air purifier cuts haze particles further. Then swap the session:

Outdoor run, cycle or row to a treadmill, stationary bike or indoor rower in a gym or at home
A long run to a strength session, mobility work or a home bodyweight circuit
A group run to an indoor class

You keep the training; you skip most of the dose.

Don't go sedentary, adjust

The tempting overcorrection is to write off exercise entirely until the haze clears. The evidence says don't.

A Danish cohort of more than 50,000 adults found that staying physically active was linked to meaningfully fewer deaths than being inactive, and the protective effect of activity broadly held even for people living in the most polluted areas. The long-term win from staying active is large. Exercising in dirty air can blunt some of those benefits and, in vulnerable people, raises short-term cardiovascular risk, which is exactly why the smart response is ease off and move indoors at higher bands, not business as usual and not the sofa.

Worth a reality check on "clean," too: even a Good or Normal band day can sit above the World Health Organization's strict long-term PM2.5 targets (5 ug/m3 annual mean, 15 ug/m3 over 24 hours). That's not a reason to stop training on a green day. It's a reason to keep cutting avoidable exposure where it's easy.

Bottom line

For most healthy people, haze changes the dial, not the decision: ease the intensity and move indoors at higher bands rather than going sedentary. Vulnerable groups step down a band earlier and should check with their doctor.

Where to check, and when it stops being your call

The live numbers are free on haze.gov.sg, nea.gov.sg, and the myENV app, the same official readings the bands above refer to. Check the 1-hour PM2.5 before you head out, and the 24-hour PSI when you're planning ahead.

For context on how real the top of the scale is: Singapore's worst haze on record came in June 2013, when the 3-hour PSI peaked at 401 on 21 June, an all-time high. The Hazardous band itself had been breached two days earlier, on 19 June. It's real, and it's rare. Most haze days sit far below that, in the band where the right answer is simply to run a little easier, or take it indoors.

This is informational coverage, not medical advice. If you have a heart or lung condition, are pregnant, or are caring for children or elderly family, treat the bands as a starting point and check the live readings and your doctor.

Sources

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