Bangkok's Smog Season and Your Kids: What Actually Helps

We came close to moving the family out of Thailand for part of every year.

Not permanently. The plan I kept turning over was two or three months in Australia, timed to miss the worst of the smog season and coming back once the air cleared. I never did it. But I thought about it seriously enough to price it out, and that tells you something about how bad it gets.

My daughter has mild cerebral palsy, which leaves her lungs weaker than they should be. Watching both of my children breathe that air, knowing one of them had less margin than the other, was a genuinely awful feeling and not something I had thought about before we moved.

It reached me too. In the bad weeks my chest felt tight and my eyes stung by the end of the day, and I am a healthy adult who was mostly indoors.

And then there were the stretches when school was simply cancelled. Everyone stayed inside with air purifiers running around the clock, and children who should have been outside on a playground watched the sky instead.

So if you have just arrived and you are wondering whether the smog talk is overblown, it is not. But it is also manageable, and almost everything that genuinely helps is cheap and unglamorous. Here is what is worth knowing.

When it actually happens

The bad air runs roughly from December through April, and the worst of it is usually January and February. The rest of the year, particularly the rainy season from about June to October, Bangkok's air is unremarkable and often good.

The reason it concentrates in those months is meteorological rather than industrial. In the cool season a layer of cold air settles near the ground under a layer of warmer air, which caps the city and stops pollution dispersing upward. Wind drops off as well. The same emissions that vanish into the atmosphere in July simply sit on top of you in January.

What fills that trapped layer is a mixture: vehicle exhaust from a city of ten million, industry around the edges, and crop stubble burning across the central plains and further north. The burning is the part that turns a bad week into a crisis week, and it is also the part nobody in Bangkok controls.

This is why the seasonality is so sharp. It is not that Bangkok pollutes more in January. It is that in January there is nowhere for it to go.

It is, slowly, getting better. Thailand's Pollution Control Department compared the November to early March window across two years and found Bangkok's average PM2.5 fell from 33 to 28 micrograms per cubic metre, a 15 percent improvement, while the number of days exceeding the Thai standard dropped from 65 to 29.

That is real progress and worth acknowledging. It is also progress from a bad baseline. Twenty-nine days over the national standard in a single season is still most of a month during which the air is officially not fit to breathe, and the national standard is itself more permissive than the World Health Organization's.

The number you are looking at may not mean what you think

If you take one thing from this article, make it this, because it catches almost everyone and it is invisible until somebody points it out.

Thailand's AQI and the US AQI are different scales, and the same air produces different numbers on each.

Thailand revised its own index on 1 June 2023 to bring it closer to WHO guidance, lowering the PM2.5 threshold from 50 to 37.5 micrograms per cubic metre and the red alert level from 91 to 75.1. That was a genuine tightening. But the Thai index is still built from multiple pollutants, and as the US Embassy in Thailand states plainly, it does not directly correspond to the US EPA's AQI, which is PM2.5 only.

The practical effect is that you can stand in one spot, open two apps, and see a comfortable-looking number on one and an alarming one on the other. Neither is lying. They are answering different questions.

The fix is simple: pick one scale, learn what its numbers mean for your family, and stop switching between them. Most international apps, including the ones most expat families already have, report the US AQI. The US Embassy runs its own PM2.5 monitor in Bangkok at the Rama IV and Ratchadamri intersection, which is a useful second opinion on a given day.

On the US scale the bands are 0 to 50 good, 51 to 100 moderate, 101 to 150 unhealthy for sensitive groups, 151 to 200 unhealthy for everyone, 201 to 300 very unhealthy, and above 300 hazardous.

For a family the honest rule of thumb is that children count as a sensitive group. That means the band that matters to you is 101 to 150, not 151 and above. If you wait for the number that says unhealthy for everyone before changing plans, you have already spent several days above the line that was actually about your kids.

What it does to children

Children are not small adults here, and the reasons are physiological rather than sentimental.

They breathe faster than adults relative to their body size, so they take in proportionally more air and more of what is in it. Their lungs are still developing well into adolescence. They spend more time being physically active, which means breathing harder. And they are closer to the ground, where vehicle exhaust concentrates.

The scale of exposure is not marginal. Reporting drawing on UNICEF data puts 13.6 million Thai children as regularly exposed to hazardous PM2.5 concentrations, and one analysis found Bangkok children exceeded WHO safety thresholds on 265 of 365 days in 2023, which is more than seven days in ten.

The established effects are respiratory: more coughing, more asthma symptoms and diagnoses, more nosebleeds, more infections. Beyond that there is a growing body of research linking childhood exposure to effects on cognitive development and attention, though that work is younger and the picture is still filling in.

The reasonable response is not alarm, it is action, because the protective measures are cheap relative to what they buy. A child with an existing respiratory vulnerability deserves a lower threshold for staying indoors than a child without one, and that is a conversation worth having with your paediatrician before the season rather than during it.

Masks, and the fit problem nobody mentions

Start with what does not work. A surgical mask, a cloth mask, or the black fabric masks sold at every BTS station do essentially nothing against particles this small. PM2.5 is fine enough to pass straight through a loose weave and around gaps.

What works is a respirator-class mask: N95, KN95 or KF94. These are designed to filter at least 94 to 95 percent of fine particles, and they are widely available here and inexpensive.

Now the part almost no article mentions. On a child, the filtration rating matters far less than the fit. A certified N95 that gapes at the cheeks or slides down a small nose performs worse than a cheaper mask that seals, because air takes the path of least resistance and that path is the gap, not the filter.

Fit is a matter of face shape rather than age. Children's faces vary enormously at the same age, so buy by trying rather than by the number on the box, and expect to test two or three styles before one works. KF94 masks often suit smaller faces better than standard N95 shapes because of their folded three-dimensional construction.

Test the seal at home. Have the child put the mask on and breathe out sharply: if you can feel air escaping around the nose or the sides, it is not sealing. Mask fitters, the elastic frames that go over a mask and pull it against the face, are cheap and can sharply cut leakage around a poor seal.

Be realistic about what masks are for. They are for the walk to school, the wait on a platform, the taxi with the window down, and unavoidable time outdoors on a bad day. No child will wear one for eight hours, and they should not need to, because the answer for time at home is not a mask.

Air purifiers, and how to size one properly

If you buy one thing for smog season, buy a decent air purifier for wherever your children sleep. Bedrooms before living rooms, because that is where they spend eight to ten uninterrupted hours every night.

Look for true HEPA filtration and ignore the marketing around ionisers, UV and plasma. Some of those technologies do very little and a few generate ozone, which is its own respiratory irritant. A plain HEPA filter and a decent fan is the whole product.

The specification that matters is CADR, the Clean Air Delivery Rate, which tells you how much clean air the unit actually delivers rather than how well the filter performs in isolation. A high-grade filter in an underpowered machine cleans a small room slowly and a large room never.

The standard sizing rule is to take the room's floor area in square feet and multiply by two thirds. That gives the minimum CADR for roughly five air changes an hour, which is the usual recommendation for allergy and asthma control.

For smoke conditions, which is what Bangkok's season actually is, the guidance changes: size the unit so its smoke CADR roughly equals the room's floor area in square feet, giving around seven air changes an hour. The higher target exists because outdoor smoke keeps infiltrating, so you are not cleaning the room once, you are continuously outrunning what is coming in.

In practice, for a bedroom of around 400 square feet, you want a smoke CADR somewhere around 250 to 400 rather than the 100 or so a cheap unit delivers. Two smaller units in the same room work perfectly well if that suits your budget, since CADR is additive.

Run them continuously through the bad months rather than switching them on when the air looks bad, and keep the bedroom doors shut. A purifier fighting an open door and an open window is doing very little.

Expect filters to need replacing considerably more often than the manufacturer's schedule suggests, because those schedules are written for cities with cleaner baseline air. Budget for it and keep a spare, because shops sell out during a bad spell.

And be honest about the limit. None of this makes your home a clean room. A well-sized purifier in a closed bedroom will hold indoor levels far below outdoors, which is a real and worthwhile difference, but it is a reduction rather than an elimination.

The school question

Schools vary enormously in how seriously they take this, and it is a fair thing to ask about directly, both before you enrol and during the season.

The questions worth asking: do you monitor air quality on campus with your own equipment or rely on a city reading, which index do you use, at what threshold do activities move indoors or get cancelled, do classrooms have air purifiers and are they sized for the rooms, and is the policy written down where parents can read it.

A school that answers those crisply has thought about it. A school that says it keeps an eye on things has not.

This is not theoretical. In January 2025 the Bangkok Metropolitan Administration closed 194 of its 437 schools in a single day, the most since 2020, with PM2.5 reaching 122 micrograms per cubic metre against a WHO 24-hour guideline of 15. Across the wider crisis more than 350 schools closed. Most international schools stayed open with indoor policies and filtered classrooms, which is part of what you are paying for, but the disruption to sport, breaks and outdoor learning is real and it lasts weeks.

Should you leave for the season?

So, the question I actually asked myself: is it worth leaving for the season?

For most families, no, and I will be straightforward about why we did not. The disruption was what killed it. Moving a household twice a year is a serious undertaking on its own, but the part we never solved was school. Two or three months away means either pulling the children out or running online schooling from another country, and neither is a small thing to do to a child every single year.

Set that against the cost of a rental abroad, the fact that it splits the family if one parent is working, and the plain reality that the measures above genuinely do work, and the sums stop being close. Two well-placed air purifiers cost a fraction of a few months somewhere else.

There is a longer-run version of this question, though, and I should be honest about it because it is the one we ended up answering. We never left for the season. We eventually left Thailand. The air was not the only reason and it was not the biggest one, but it was on the list, and it was on the list because it stops feeling like two or three months when you are inside it year after year. It accumulates.

For a small number of families it is worth considering seriously: a child with significant asthma or another respiratory condition that flares badly and predictably every season, where you have watched it happen more than once and the indoor measures were not enough. That is a medical conversation with a paediatrician who knows your child, not a decision to make from an article.

There is also a middle option that costs far less and gets most of the benefit. Time your home leave, or any long trip, to land inside January and February rather than during the summer holidays when everyone else travels. Southern Thailand and the islands sit largely outside the burning belt and stay clear when Bangkok does not, so even a week or two out of the city during a bad spell is worth something. Splitting a few long weekends across the worst weeks is the realistic version of the plan I never carried out.

What actually works, in order

In rough order of how much good they do per baht spent.

Air purifiers in the bedrooms, sized properly by CADR rather than bought on price, running continuously through the season with the doors shut. This is the single biggest intervention and it is not close.

Check one number every morning, on one scale, and let it set the day. Treat 101 and above as the line that concerns your children, not 151.

On bad days move what you can indoors and accept it. Bangkok is unusually well equipped for this, with malls, indoor play centres, pools and climbing gyms, which is one of the few genuine consolations of the season.

Keep properly fitting N95 or KF94 masks for unavoidable outdoor time, sized by trying them on rather than by age, and check the seal.

And keep it in proportion, with one caveat. Two or three difficult months sit inside a year that is otherwise a genuinely good place to raise children, the trend is slowly improving, and the families who cope best are the ones who prepared in November rather than reacted in January. The caveat is the one above: seasons add up. It is worth asking yourself every year or two whether it still feels like a fair trade, rather than only ever asking in the middle of a bad February.