Every year between November and March, families in Bangkok have the same conversation. The sky turns flat and white, a haze settles over the expressway, and the question becomes whether it is safe to let a student play outside.
Most advice available online is either alarming or vague. This guide is neither. It sets out what the data actually says about Bangkok air pollution, what the official Thai thresholds are, what protects a student and what does not, and what a school should be able to tell you. Where authorities disagree, we say so rather than picking the answer that reads most cleanly.
Key highlights
- Bangkok PM2.5 runs from November to March and typically peaks in January and February. Traffic is the baseline source; crop burning and a seasonal temperature inversion make it worse.
- Two different AQI scales are in use. Air4Thai shows the Thai index, IQAir defaults to the US index, and the same air produces two different numbers. Read the micrograms per cubic metre instead.
- Indoors is not automatically clean. Bangkok homes measured an indoor to outdoor ratio of 0.76 during haze episodes. Filtration, not just closed windows, is what changes the number.
- Cloth masks do not work for PM2.5. Fit is the whole game, and it is genuinely difficult for young students.
- Authorities disagree about masks for young students. We set out all three positions rather than picking one.
- Ask your school four specific questions. Filtration coverage, the reading it acts on, what changes at each level, and how you will be told.
When Bangkok’s Air Is Worst, and Why
Thailand’s Pollution Control Department identifies November to March as the critical PM2.5 period for the Bangkok Metropolitan Region, and monitors a national haze season from 1 November to 15 May. Analysis of hourly data from 12 PCD monitoring stations across Bangkok found the highest monthly average PM2.5 in January, at 56.9 micrograms per cubic metre at a roadside station, well above the national standard.
Three things combine to produce it. Bangkok has a persistent baseline problem from traffic, which contributes an estimated 44% of fine particles in the metropolitan region. Biomass burning, largely rice and sugarcane residue in the central plains, contributes around 24% across the year and rises to as much as 35% during the dry season, with particles able to reach Bangkok within 24 hours depending on the wind. And in the cool dry months, a temperature inversion combined with low wind speeds caps the air above the city, so emissions that would normally disperse accumulate instead.
A useful distinction
Bangkok is not classified by the Pollution Control Department as a transboundary haze area. That designation applies to Thailand’s northern provinces between January and April, and the south between July and September. Bangkok’s problem is mostly its own traffic, seasonally amplified by regional agricultural burning and trapped by the weather.
The Numbers That Matter
Thailand tightened its national PM2.5 standard on 1 June 2023. Knowing where the Thai standard sits relative to the World Health Organization guideline is useful context, because meeting the national standard is not the same as breathing air the WHO considers healthy.
| Measure | Thailand national standard | WHO 2021 guideline |
|---|---|---|
| 24-hour average | 37.5 µg/m³ (since 1 June 2023) | 15 µg/m³ |
| Annual average | 15 µg/m³ | 5 µg/m³ |
Thailand’s standard corresponds exactly to WHO Interim Target 3, a recognised staging post on the way to the full guideline rather than an endpoint. In practical terms, a day that fully complies with the Thai standard can still carry more than twice the particulate load the WHO guideline allows. That is not a criticism of the standard, which was a significant tightening; it is context for interpreting a green reading on an app.
Why Two Apps Give You Two Different Numbers
This causes more confusion among Bangkok parents than any other aspect of air quality, and it is worth five minutes to understand once.
An Air Quality Index is not a measurement. It is a scale that converts a measurement into a category, and different countries draw the category boundaries in different places. Air4Thai and AirBKK report the Thai AQI. IQAir defaults to the US AQI. Both are correct. They are answering slightly different questions.
The Thai scale is built around the national standard, so an AQI of 100 corresponds exactly to 37.5 micrograms per cubic metre. Above 100, the standard is being exceeded. The bands work like this.
| Thai AQI | Band | PM2.5 (24-hour) | Official guidance |
|---|---|---|---|
| 0 to 25 | Very good (light blue) | 0 to 15.0 | Normal activity |
| 26 to 50 | Good (green) | 15.1 to 25.0 | Normal activity |
| 51 to 100 | Moderate (yellow) | 25.1 to 37.5 | Reduce prolonged strenuous outdoor activity; sensitive groups wear PM2.5 protection outdoors |
| 101 to 200 | Beginning to affect health (orange) | 37.6 to 75.0 | Wear PM2.5 protection every time outdoors; limit strenuous outdoor activity; sensitive groups avoid it entirely |
| 201 and above | Affects health (red) | 75.1 and above | Cease outdoor activities; use PM2.5 protection if going out is unavoidable; those with chronic conditions stay in a safe area |
The practical takeaway
At 75 micrograms per cubic metre, the Thai scale shows red at AQI 201, its most severe band. The US scale shows roughly 165, one band lower. Neither app is wrong. Read the raw PM2.5 figure in micrograms per cubic metre, which every app reports and which means the same thing everywhere, rather than comparing index numbers between apps.
How to Check the Air Before the School Run
- Air4Thai, run by the Pollution Control Department, is the official national network and covered 107 stations across 77 provinces as of early 2026.
- AirBKK, run by the Bangkok Metropolitan Administration, draws on 83 monitoring stations across the city and is the source BMA schools are instructed to check.
- IQAir is widely used by expatriate families and includes low-cost sensor data, but defaults to the US index.
One technical point makes a real difference to a morning decision. The Thai AQI is calculated on a 24-hour rolling average, so the index you see at 7am partly reflects yesterday. It will under-react to a sharp overnight spike. Air4Thai and AirBKK also publish hourly PM2.5 readings. For a same-morning decision about whether a student walks or takes a mask, look at the hourly figure. For a sense of the week, look at the index.
Why Students Are Affected More Than Adults
The World Health Organization is direct about the reasons. Students breathe more rapidly than adults, so they absorb more pollutants relative to body size. They are shorter, so they spend more time nearer the ground where some pollutants reach their highest concentrations. And their lungs and brains are still developing while this is happening.
The evidence on lung development is unusually strong because it was measured over years rather than inferred. The Children’s Health Study followed 1,759 students in southern California annually from ages 10 to 18 and found that the proportion reaching adulthood with clinically reduced lung function was 4.9 times higher in the most-exposed communities than the least: 7.9% against 1.6%.
The more encouraging finding came from the same research group a decade later. As air quality in those communities improved, lung function growth improved with it, and the share of 15 year olds with clinically low lung function fell from 7.9% to 6.3% to 3.6% across three successive cohorts. Reducing exposure demonstrably works. That is the reason any of this is worth the effort.
One further study deserves attention from any school. Researchers followed 2,715 students aged 7 to 10 across 39 primary schools in Barcelona, measuring pollution both in the playground and inside the classroom. Over 12 months, growth in working memory was 7.4% in the high-pollution schools against 11.5% in the low-pollution schools. The exposure that mattered was measured indoors, which is precisely the exposure a school can control.
What we are not claiming
You will sometimes see it stated that PM2.5 measurably increases school absence. We looked for the evidence and could not find a study robust enough to put in front of parents. The lung function and cognitive findings above are well established. The absence claim is not, and we would rather say so.
What Actually Protects a Student
Clean indoor air comes first
This is the single most effective intervention, and the one most families underuse. A 2025 study measuring 17 Thai homes found an indoor to outdoor PM2.5 ratio of 0.76 during haze episodes in Bangkok and 0.68 outside them. In other words, simply being inside with the windows shut removes only about a quarter of the particulate load. Indoor air quality was dominated by outdoor air quality.
Filtration is what changes that. Match the purifier to the room using its clean air delivery rate, or CADR. US EPA guidance gives a minimum CADR of 65 cubic feet per minute for a 100 square foot room, scaling proportionally, which converts to roughly 12 cubic metres per hour for each square metre of floor area. A 30 square metre bedroom therefore wants around 360 cubic metres per hour. Run it continuously with doors and windows closed, and replace filters on schedule, because an overloaded filter does not work.
Two cautions from the EPA. CADR rates particles only and tells you nothing about gases or odours. And avoid air cleaners that intentionally produce ozone, including some ionisers and plasma devices, because ozone is a lung irritant. If budget is the constraint, the EPA also endorses a do-it-yourself alternative: a MERV 13 or higher furnace filter attached to a certified box fan, which it describes as an effective option when rated units are not available or affordable.
Masks, and an honest account of the disagreement
Three credible authorities give three different answers, and most guides quietly pick one. Here are all three.
| Authority | Position on masks for young students |
|---|---|
| Pediatric Environmental Health Specialty Units, supported by the American Academy of Pediatrics | Age-graded. From about age 7 an adult small N95 will likely fit. Ages 2 to 7 are unlikely to achieve a good N95 seal, so child-size surgical masks may be the practical option. No masks under age 2. An N95 with a good seal cuts inhaled particles by roughly 90%; a surgical mask by roughly 20 to 30%; cloth masks are not recommended. |
| United States Environmental Protection Agency | Firmer. Advises not relying on masks for children at all, on the grounds that even N95 masks are not made to fit children and will not protect them. |
| Thailand Pollution Control Department | Directs sensitive groups to wear PM2.5 protection outdoors from the yellow band upward, and everyone from the orange band upward. |
What all three agree on is the order of priority: clean indoor air first, reduced exertion second, masks last. And one warning is worth repeating exactly, because it is the most common mistake: a student should not spend more time outdoors than usual simply because they are wearing a mask. Note also that KN95 and KF94 masks, widely sold in Thailand, are not certified to the same standard as NIOSH-approved N95s and may not offer equivalent protection.
Managing exertion, not just exposure
The mechanism to manage is breathing rate: more pollution enters the lungs when someone is breathing harder. That makes exertion the lever, not simply time outdoors. Shorten sessions, rotate players more often, lower the intensity rather than cancelling movement outright. US guidance is clear that there is no exact safe duration; worse air means more breaks, less intensity and closer attention to symptoms.
Two practical notes. Unlike ozone, particle pollution can be high at any time of day, so the common advice to exercise early does not apply to PM2.5 in Bangkok. And limit time near idling vehicles and busy roads, which in practice means the school drop-off queue.
What does not work
| Claim | Reality |
|---|---|
| Cloth masks filter PM2.5 | They do not, and are not recommended for this purpose. |
| A mask means outdoor activity can continue as normal | Explicitly warned against by pediatric guidance. |
| Closing the windows is enough | Bangkok homes still measured 0.68 to 0.76 of outdoor levels indoors. |
| Ionisers and ozone-generating purifiers clean the air | Ozone is a lung irritant. No US federal agency has approved ozone generators for occupied spaces. |
| Any purifier will do | It must be CADR-matched to the room, run with windows closed, and have filters replaced. |
| Air purifiers resolve asthma | EPA describes improvements as possible but not consistently large. |
| Antioxidant foods, extra hydration and houseplants protect against PM2.5 | These appear widely in advice columns. We found no health authority or peer-reviewed source supporting them as protective measures. Treat them as general wellbeing advice, not protection. |
What to Expect from a School
Schools in Bangkok are not working without a framework. All Bangkok Metropolitan Administration schools operate a mandated protocol: staff check AirBKK at 07:00, 11:00 and 15:00 and fly a coloured air quality flag matching the national bands, with defined actions at each level and a formal escalation ladder for closures. As of January 2026, 1,823 of 2,196 BMA kindergarten classrooms had been converted to dust-free rooms, with at least one safe zone in every one of the 437 BMA schools.
International schools set their own policies, and the good ones publish them. When you are evaluating a school, ask these four questions.
- What filtration is installed, and which rooms does it cover? Whole-campus or a handful of rooms is a large difference, particularly for the youngest students.
- Which data source do you act on, and is it measured on campus? A reading from a station several kilometres away is not the same as one on site.
- What changes at each level? Ask for the actual thresholds and the actual actions, in writing.
- How and when are families told? A decision made after pick-up time is not much use.
One point from US school guidance is worth carrying into that conversation: moving students indoors only helps if the building filters particles. Indoors without filtration is not a protective measure, which the Bangkok indoor to outdoor ratio confirms. It is also worth remembering that health authorities recommend at least 60 minutes of daily physical activity. A good policy relocates activity rather than deleting it.
At BASIS International School Bangkok, the campus uses a Pre-cool Fresh Air system designed to protect students from PM2.5. Families visiting during the dry season are welcome to ask the admissions team how the system works and how outdoor activity is managed on high-pollution days.
A Short Household Plan
- Pick one app and one metric. Check the hourly PM2.5 in µg/m³, not the index, before school.
- Filter the room where your family spends the most hours, usually a bedroom. Size it by CADR.
- Keep windows closed during the season and cool the space so they stay closed.
- Reduce indoor sources during episodes: frying, sweeping, incense and candles.
- Have properly fitting masks ready before the season, not during it.
- Agree in advance what reading changes your family plans, so it is not decided in a rush at 7am.
- If a student has asthma, review the action plan before November and keep relief medication accessible at school.
Ask us how the campus handles the dry season
Air quality is a reasonable thing to ask a school about, and we would rather you asked. We invite you to visit the BASIS International School Bangkok campus on Rama 2, see the facilities, and talk to us about how the school day is managed through the PM2.5 season. Contact the admissions team on +66 2 415-0099 or at admissions@basis.ac.th.
Frequently Asked Questions
When is PM2.5 season in Bangkok?
The Pollution Control Department identifies November to March as the critical PM2.5 period for the Bangkok Metropolitan Region. Levels typically peak in January and February. Nationally, PCD monitors the haze season from 1 November to 15 May.
What PM2.5 level is considered safe in Thailand?
Thailand tightened its national standard on 1 June 2023 to 37.5 micrograms per cubic metre averaged over 24 hours, and 15 micrograms per cubic metre averaged over a year. Both figures match WHO Interim Target 3. The WHO guideline itself is stricter still, at 15 for 24 hours and 5 annually.
Why does IQAir show a different number from Air4Thai?
They use different index scales. Air4Thai and AirBKK report the Thai AQI, while IQAir defaults to the US AQI. The same air can read 201 and red on one and about 165 on the other. Read the raw PM2.5 figure in micrograms per cubic metre, which is the same on every app, rather than comparing index numbers.
What mask is best for a child in Bangkok pollution?
Guidance differs by authority. Pediatric specialty units supported by the American Academy of Pediatrics indicate that from about age seven an adult small N95 will likely fit, that students aged two to seven are better served by child-size surgical masks because a well-fitting N95 is unlikely, and that no mask should be used under age two. The US EPA takes a firmer line, advising that masks are not made to fit children and should not be relied on. Thailand PCD guidance directs everyone to wear PM2.5 protection outdoors once the index reaches the orange band. All three agree that clean indoor air comes first and masks last.
Do cloth masks protect against PM2.5?
No. Pediatric guidance is explicit that cloth masks usually do not reduce exposure to smoke and air pollution and are not recommended for this purpose. Surgical masks are better than nothing but substantially less effective than a well-fitted N95.
Is staying indoors enough to protect my child from PM2.5?
Not on its own. A 2025 study of Thai homes measured an indoor to outdoor PM2.5 ratio of 0.76 during haze episodes in Bangkok, meaning indoor air still carried about three quarters of the outdoor concentration. Closing windows helps, but filtration is what makes indoor air meaningfully cleaner.
How do I size an air purifier for a room?
Match the clean air delivery rate, or CADR, to the room. US EPA guidance suggests a minimum CADR of about 65 cubic feet per minute for a 100 square foot room, rising proportionally, which works out at roughly 12 cubic metres per hour for every square metre of floor area. Run it with doors and windows closed and replace filters on schedule.
What should I ask a school about air quality?
Ask four things: what filtration is installed and which rooms it covers, which data source and reading the school acts on, what happens to outdoor activity at each level, and how families are notified. A school that can answer these precisely has thought about it.
References
- Royal Gazette, Pollution Control Department Announcement on the Air Quality Index of Thailand B.E. 2566. The official Thai AQI bands, PM2.5 breakpoints and prescribed public guidance.
- World Health Organization, Global air quality guidelines 2021. Current WHO guideline values and interim targets for PM2.5.
- World Health Organization, Air pollution and child health: prescribing clean air. Why young people are more vulnerable to particulate pollution than adults.
- Gauderman et al., New England Journal of Medicine, 2004 and 2015. Lung function growth and its improvement as air quality improved.
- Sunyer et al., PLoS Medicine, 2015. Cognitive development across 39 schools, with classroom-level exposure measurement.
- Kliengchuay et al., Heliyon, 2025. Indoor to outdoor PM2.5 ratios measured in Thai homes during and outside haze episodes.
- PEHSU and American Academy of Pediatrics, Masks to protect children from wildfire smoke. Age-graded mask guidance and comparative filtration effectiveness.
- United States Environmental Protection Agency, Guide to air cleaners in the home. CADR sizing table, filtration guidance and ozone warnings.
- AirNow, Air quality and outdoor activity guidance for schools. Action levels for school outdoor activity and the filtration caveat on moving students indoors.
- Air4Thai, Pollution Control Department. Official national monitoring network and Thai AQI readings.
- AirBKK, Bangkok Metropolitan Administration. City monitoring network used by Bangkok schools.
