Air pollution in Bangkok: when PM2.5 season hits, what the numbers mean and how to protect your lungs

Bangkok's air is fine for much of the year and genuinely bad for a few months of it. If you plan for those months the way folks back home plan for wildfire smoke or hurricane season, you'll be fine.

The short version

• PM2.5 season is the cool, dry stretch, roughly December through March, when still winter air traps smoke from crop burning and traffic exhaust over the city [1].
• Thailand's standard is a 24-hour average of 37.5 µg/m³ of PM2.5. It was tightened from 50 µg/m³ on June 1, 2023, and the top "red" level now starts at 75.1 µg/m³ [2].
• The US EPA AQI is stricter. Since 2024, the EPA calls anything above 9.0 µg/m³ "Moderate" and above 35.4 µg/m³ "Unhealthy for Sensitive Groups" [3]. A Thai "yellow" day can be an American "Moderate" one.
• Bad weeks trigger action. In January 2025, nearly 200 city-run schools closed in a single day [1], and the government made Bangkok's trains and buses free for a week [4].
• What works: a HEPA purifier sized to your bedroom, a well-fitting N95 or KN95 for outdoor time, and an app alert on your phone.

When and why the smog rolls in

Bangkok doesn't have a winter you'd recognize from Chicago, but it does have a cooler, drier season. That's the problem. Cool, stagnant air sits over the city like a lid and holds in smoke from agricultural burning in the surrounding provinces, plus exhaust from millions of vehicles [1]. When the wind picks up or the rains come back, the haze clears.

How bad does it get? A few real snapshots:

The worst readings tend to show up in outer districts with heavy trucking and open land, like Nong Khaem, Bang Khun Thian and Lat Krabang, but central areas like Pathum Wan, Khlong Toei and Sathon slip into orange on bad days too [5][6]. No neighborhood is a hideout.

Outside the season, many days are perfectly breathable. Locals and long-timers will tell you the rainy months are when you open the windows.

Thai AQI vs US AQI: why your apps disagree

This trips up almost every American. You open one app and it says yellow; your friend's app says orange or even red. Both are "right." They're using different yardsticks.

Thailand's system, run by the Pollution Control Department, has five color levels: blue, green, yellow, orange and red [2]. Health warnings kick in once the 24-hour PM2.5 average passes 37.5 µg/m³ (orange), and red starts at 75.1 µg/m³ [2]. The Bangkok city government describes orange as the level where pollution is "beginning to affect health" [6].

The US EPA's AQI uses these PM2.5 breakpoints, updated in 2024 [3]:

So a reading of 50 µg/m³ is "orange, starting to affect health" in Thailand and "Unhealthy for Sensitive Groups" in the US, with an index well over 100. At 70 µg/m³ it's still orange in Thailand but solidly "Unhealthy" on the US scale. For comparison, the World Health Organization's guideline is no more than 15 µg/m³ as a 24-hour average on most days of the year [1].

Our advice: pick one scale and stick with it. Most Americans use the US AQI because it's what AirNow showed them during wildfire season back in the States, and because the health guidance is familiar. Just remember that Thai news and school announcements use the Thai colors.

The apps and data sources

Check the 24-hour average for planning your week and the hourly number for deciding whether to go for that 6 a.m. run. Readings can swing a lot between morning and afternoon.

Purifiers, masks and your apartment

Air purifiers

A HEPA purifier is the single best buy for PM2.5 season. Electronics chains and department stores in Bangkok stock the big international brands, and prices run from budget to sticker shock. A few rules of thumb:

1. Size it to the room. Check the clean air delivery rate (CADR) and the room size on the box, and buy one rated for a bigger space than your bedroom. Thai apartments are often listed in m², so a 30 m² bedroom is about 320 square feet.
2. Bedroom first. You spend a third of your life there. One good unit in the bedroom beats a weak one in the living room.
3. Keep the windows closed on bad days and run the AC on recirculate.
4. Stock spare filters before the season starts. They sell out when the haze hits the news.
5. Buy a cheap PM2.5 monitor if you want to see whether the purifier is earning its keep.

Older condos can be drafty, with gaps around sliding balcony doors. If you're apartment hunting in October or November, ask about window seals the way you'd ask about the AC.

Masks

Surgical masks do little against fine particles. For outdoor time on orange and red days, wear a well-fitting N95 or KN95, and replace it when it gets dirty or loses its seal. Kids' sizes are sold in pharmacies, but a good fit on a small face is hard, which is one more reason to keep children indoors on the worst days.

Who should be most careful

Small kids, older adults, pregnant women and anyone with asthma or heart or lung disease. If you or a family member has asthma, talk to your doctor before the season about an action plan. Bangkok has plenty of English-speaking private hospitals, but remember that Medicare generally doesn't cover care abroad, so make sure your international or Thai health insurance is in place first.

School closures, work from home and family planning

When pollution stays above the standard for several days, the city can close schools it runs. In January 2025, affected schools had to make up the time with weekend or online classes [5]. International and private schools make their own calls, and many have indoor-air policies that cancel outdoor recess or PE above a set AQI. Ask your school for its policy at enrollment.

The city also runs a voluntary work-from-home scheme and urges residents to work from home when they can; in January 2025, only about 100,000 people had signed up in a city of roughly 10 million [1]. Heavy trucks can be restricted in parts of the city on bad days [1].

A practical family game plan:

1. Book your getaway. Many expat families plan their US home leave, or a beach trip south, for January or February.
2. Find indoor activities. Malls, indoor play centers, swimming in covered pools and libraries carry kids through a bad week.
3. Run outside early or not at all, and move your workout to the gym when the AQI is up.
4. Talk to your employer about remote work days before the season, not during it.

If you'd like to compare purifier models or hear which schools handle haze days well, ask in our community group for Americans in Thailand.

Sources

1. Free Malaysia Today (AFP): Nearly 200 Bangkok schools close over smog (January 23, 2025)
2. Thailand.go.th: Thailand adjusts Air Quality Index criteria for PM2.5 (2023)
3. US EPA: Final updates to the Air Quality Index for particulate matter (February 2024)
4. Thailand Public Relations Department: Bangkok offers free public transport for a week to combat PM2.5 (January 2025)
5. The Nation: Bangkok closes 53 schools as PM2.5 smog shrouds capital (January 22, 2025)
6. The Star: Bangkok air quality slips into orange zone (January 1, 2026)

Keep reading

• Healthcare and health insurance in Bangkok
• Schools in Bangkok: American and international schools
• BTS, MRT and river boats in Bangkok
• Medicare when you live abroad