Bangkok’s smog season: when the air turns bad, how to check it, and what helps

For a good part of the year Bangkok’s air is unremarkable. Then, in the cool dry months, the skyline goes hazy, the apps light up orange, and everyone starts asking the same questions: is it safe to run outside, do I need one of those masks, should I buy an air purifier? The honest answers are all about timing and checking — the smog is seasonal and it is measurable, so the trick is knowing roughly when it comes, how to read the numbers without scaring yourself, and which of the popular fixes actually do something. Here is the plain-English version.

When it comes, and why

Bangkok’s smog has a calendar. It builds in the cool, dry season — broadly December through March, with the worst weeks usually falling in late February and March and sometimes bleeding into April — and it fades once the rains arrive. The rainy season, roughly May or June to October, is the city’s clean stretch, because rain washes particles out of the air and the monsoon winds blow them away. What makes the dry season bad is a combination rather than a single villain: the city’s heavy traffic, agricultural and regional biomass burning in and beyond the surrounding provinces, and — the seasonal amplifier — temperature inversions, where a lid of warmer air sits over the cooler air below and traps everything near street level while weak winds and no rain let it accumulate. Studies that try to apportion the sources land on different splits from year to year, so the honest summary is that it is traffic plus burning plus weather, not any one of them alone. And because burning intensity and the timing of the rains shift each year, the season’s severity genuinely varies — treat the calendar above as a pattern, not a promise.

Reading the number without scaring yourself

When you open an air app you are usually shown an AQI — a tidy 0-to-500-ish index with a colour. The catch that trips up newcomers is that there is no single AQI. Thailand’s own scale (from the Pollution Control Department) and the US EPA scale use different breakpoints, so the exact same air can show a higher, scarier number on a US-AQI app than on a Thai one. The fix is simple: look past the index to the raw PM2.5 figure in micrograms per cubic metre, which means the same thing everywhere, and check which AQI an app is using before you panic or relax. For reference, Thailand’s daily PM2.5 standard sits around the high-30s in micrograms, while the World Health Organization’s 24-hour guideline is much stricter at 15 — which is why Bangkok can be “within Thai limits” on a dry-season day and still well above what the WHO would call clean.

What actually helps — and what doesn’t

Start with the mask, because this is where most money is wasted. Against fine PM2.5 you need a respirator that seals to your face — an N95, KN95 or FFP2. A cloth mask or a loose surgical mask does comparatively little, not because the material is useless but because air leaks around the edges, and with particles this small the seal matters more than the rating on the box. So a snug KN95 beats a gappy N95. Indoors, the highest-leverage fix is a true-HEPA air purifier sized to the room — run it in the room you sleep in, keep the windows and doors shut, and don’t expect a small unit to clean a large open-plan space. A rough sizing rule is to treat a purifier’s stated coverage as about half its headline area if you want it to clear haze quickly. Beyond gear, the boring advice works: on bad days, spend time in sealed, air-conditioned indoor space — malls, offices and modern condos usually sit below street levels, though an un-filtered room with the windows open is not protected — and shift hard outdoor exercise to cleaner days or the afternoon, since still, cool early mornings during an inversion are often the worst time to run. Sensitive groups — children, older adults, pregnant people, and anyone with heart or lung conditions — should act at lower thresholds than a healthy adult.

On the train or in the open air?

One question worth settling: does taking the MRT or BTS spare you the pollution? Partly. An enclosed, air-conditioned train is generally better than sitting in open traffic on a tuk-tuk or the back of a motorbike, where you breathe raw exhaust. But the tidy belief that “the underground MRT has clean air” is a myth — enclosed rail environments carry their own dust from the tracks and tunnels, and studies find underground platforms can actually run higher on some particle measures than the street. The honest read: a sealed train beats open-air traffic on a smoggy day, but no commute is a clean-air haven, so the mask logic still applies if the air outside is bad.

Bangkok versus Chiang Mai

Travellers often ask whether Bangkok is “as bad as Chiang Mai.” They are different shapes of bad. In the peak burning weeks of roughly March and April, Chiang Mai — ringed by hills that fill with agricultural smoke — regularly posts far worse spikes, with readings that climb into genuinely hazardous territory. Bangkok’s peak is generally lower, but its pollution is steadier and more year-round, weighted toward traffic and the sheer size of the city rather than a short, ferocious burning season. Neither is clean on a bad day, and the only reliable comparison is the actual number for the day you are there.

Quick answers

When’s the air worst?

The cool dry season, roughly December–March, usually worst late February into March. Rainy season (about May–October) is the clean stretch. It varies year to year.

Do I need a mask?

On bad days, yes — a well-sealed N95/KN95. Cloth and surgical masks do much less against fine particles. Fit matters more than the label.

As bad as Chiang Mai?

Different: Chiang Mai spikes harder in the March–April burning; Bangkok is lower at peak but steadier all year. Check the day’s number.

How do I check it?

Air4Thai (official), or IQAir AirVisual / aqicn.org. Read the raw PM2.5 in µg/m³ and note whether it’s the US or Thai AQI.

Is it all year?

No — it’s seasonal. The rains usually clear it for months.