Thailand's air is clean enough for most of the year, but in the dry months fine-particle pollution (PM2.5) regularly passes the national safety limit. Bangkok's bad spells usually come between December and February; the north is worst from about February to April, peaking in March.
PM2.5 means particles smaller than 2.5 micrometers, small enough to reach deep into the lungs. In Thailand they come mainly from traffic, industry and the burning of fields and forest, in Thailand and in neighboring countries. The UK Foreign Office warns that levels in Bangkok and Chiang Mai sometimes become dangerous and can worsen asthma, sinus and bronchial conditions. For a healthy adult on a short trip, a few hazy days are usually an inconvenience. For children, older travelers and anyone with heart or lung disease, it is worth planning around.
PM2.5 limits at a glance (October 2026)
- Thai 24-hour standard
- 37.5 µg/m³ since 1 June 2023 (it was 50)
- Thai annual standard
- 15 µg/m³ (it was 25)
- WHO 2021 guideline
- 15 µg/m³ over 24 hours, 5 µg/m³ per year
- Official readings
- Air4Thai, Pollution Control Department (website and app)
- Worst months
- Bangkok roughly December to February; the north roughly February to April, peaking in March
Thailand's 37.5 limit and what WHO recommends
Thailand tightened its air-quality standard in a Natural Resources and Environment Ministry announcement published in the Royal Gazette in July 2022. From 1 June 2023 the 24-hour average of PM2.5 must not exceed 37.5 micrograms per cubic meter, down from 50. The annual limit dropped from 25 to 15.
The World Health Organization is stricter. Its 2021 guidelines set 15 µg/m³ as a 24-hour value and 5 µg/m³ as an annual average. So a day that Thai authorities call acceptable at, say, 35 µg/m³ is still more than twice the WHO guideline. Many older websites quote WHO limits of 25 and 10; those are the 2005 values and are out of date. Thai officials still use 37.5 as the yardstick in 2026, and it is the line you will hear in Thai news when "red" or "unsafe" days are announced.
When Bangkok's air turns bad
Bangkok has its own pollution season, separate from the north. In the cool, dry months the air often sits still over the city, and exhaust, construction dust and smoke from burning in the surrounding provinces and neighboring countries build up. In January 2026 the Bangkok Metropolitan Administration (BMA) forecast stagnant air for the middle of the month, levels surged across all districts, and reporting at the time partly blamed fires in Cambodia.
When it gets bad, the city acts. The BMA asked government agencies and private companies to let staff work from home on 12 and 13 January 2026, banned the burning of rubbish and grass, and tightened checks on black-smoke vehicles. A year earlier, on 24 January 2025, 352 BMA schools in 31 districts closed for the day, the largest closure since 2020. If you see news of work-from-home requests or school closures, take it as a sign to limit time outdoors too.
The air is usually much cleaner in the rainy season, roughly May to October, when showers wash the particles out. Our Bangkok guide covers the city itself; this page is only about the air.
The north: Chiang Mai, Chiang Rai and the smoky season
Northern Thailand has a much heavier and longer haze season, driven by agricultural and forest fires and trapped by the mountain valleys. The Thai Meteorological Department records an average of 26.8 days with haze in Chiang Mai in March alone, and haze on most days from January to April. In early April 2026 the Public Health Ministry said 24-hour PM2.5 was above 37.5 in 41 provinces and at hazardous levels in 13, with Chiang Mai, Chiang Rai, Lamphun and Nan the worst hit, some areas red for more than ten days, and public health emergency centers opened in 12 provinces.
Our older article on Chiang Mai's smoky season explains why the burning happens and why the valley traps it. If you are choosing between destinations, the Chiang Mai, Chiang Rai and Pai guides are best read with the season in mind: November to January is usually the clearer window up north.
The view you came for may not be there in March. Mountain viewpoints, sunrise hikes and trekking trips in the north lose most of their point when haze hides the hills. If your trip is built around them, move it to the cool season or swap the north for the coast.
How to check PM2.5 with Air4Thai
The official source is Air4Thai, run by the Pollution Control Department. It shows hourly readings from monitoring stations across the country, including PM2.5, PM10 and ozone, with a color code. There is also an Air4Thai mobile app. Stations cover far more than Bangkok and Chiang Mai: the network reaches the beach provinces too, so you can compare Phuket or Krabi with Chiang Rai before you book the next leg.
A few tips for reading it:
- Look at the PM2.5 number, not just the color. Compare it with 37.5 (the Thai 24-hour limit) and 15 (the WHO guideline).
- Check the station nearest to you. Readings can differ a lot between districts on the same day, especially in Bangkok.
- Watch the trend over a few days. Haze in Bangkok often clears when the wind picks up; in the northern valleys it can stay for weeks.
The UK Foreign Office also points travelers to the international World Air Quality Index site. Its index numbers are calculated differently from Air4Thai's, so compare like with like: the raw PM2.5 concentration is the figure that means the same thing everywhere.
Masks that actually filter smoke particles
Ordinary masks do little against PM2.5. The US Environmental Protection Agency (EPA) says paint masks, dust masks and surgical masks do not prevent smoke inhalation, and that cloth face coverings should not be used for smoke. What does work is a close-fitting particulate respirator: the EPA names NIOSH-approved N95 or P100 respirators, with the standard printed on the mask.
Fit matters as much as the filter. The EPA advises choosing a size that seals over the nose and under the chin and checking the seal every time you put it on, because any leak around the edge lets unfiltered air in. If you are heading north in the dry season, buy a few before you travel and try them on at home, so you know a size that seals on your face.
Indoors, keep windows closed on bad days and choose a room with good air conditioning. Some long-stay visitors buy an air purifier for their apartment. When you pick a hotel, a sealed modern building helps more than a breezy wooden bungalow in a haze month; our guide to Bangkok's neighborhoods helps with the location side.
Children, older travelers and people with asthma
Children, older people and anyone with asthma, COPD or heart disease are more sensitive to fine particles. We cannot tell you whether your own condition makes a haze-season trip unwise; that is a question for your doctor, ideally a few weeks before you go. Points worth raising with them:
- Whether you need a written plan for what to do if your symptoms get worse, and extra reliever medication. Check the rules in our article on bringing prescription medicine into Thailand before you pack it.
- Whether a respirator is suitable for you. The EPA notes that people with heart or lung disease may find respirators harder to breathe through and should get medical guidance.
- For children: the EPA says children aged 2 and older can wear respirators, but certified respirators are not made in sizes for very young children. For babies and toddlers, staying in clean indoor air is the main protection.
Families can make life easier by choosing the destination by season. Our guide to Thailand with kids covers the wider health picture, and the best-time-to-visit tool shows the weather and haze months by region. If someone struggles to breathe, call 1669 for an ambulance; our page on hospitals in Thailand explains how private and public care differ, and your travel insurance should be contacted early.
Heat and haze arrive together up north. March and April are also the hottest months, so a mask, closed windows and less time outside all pull in the same direction. Our advice on coping with the heat applies on the same days.
Planning a trip around the haze
If you have flexible dates, the simplest protection is timing. The month-by-month weather guide shows how the haze months line up with heat and rain in each region. Roughly:
| Region | Usual PM2.5 problem months | Usually clearer |
|---|---|---|
| Bangkok and the central plains | December to February | May to October (rainy season) |
| Chiang Mai, Chiang Rai, Pai, Nan | February to April, worst in March (haze often from January) | May to January |
| Beaches and islands | Check Air4Thai for the nearest station | Depends on wind and regional smoke |
The exact start and end move each year with rain and wind, so treat these as a guide and check live readings in the week before you travel.
Frequently asked questions
Is it safe to visit Bangkok in January because of the air?
For most healthy travelers, yes, but some days in January are well above Thailand's 37.5 µg/m³ limit. Check Air4Thai each morning, plan indoor sights on the worst days, and carry a well-fitting N95-type respirator if you are sensitive.
What is the safe level of PM2.5 in Thailand?
Thailand's legal 24-hour standard is 37.5 µg/m³ (since June 2023) and the annual standard is 15. The World Health Organization's 2021 guideline is stricter: 15 µg/m³ over 24 hours and 5 µg/m³ as an annual average.
Do surgical masks protect against PM2.5?
Not well. The US EPA says surgical, dust and paint masks do not prevent smoke inhalation, and cloth masks should not be used for smoke. A close-fitting N95 or P100 respirator that seals around the nose and chin filters far better.
When is the worst time for air pollution in Chiang Mai?
March is usually the worst month, within a haze season that runs roughly from January or February to April. The Thai Meteorological Department records haze on almost every day in March in Chiang Mai on average, and in April 2026 the north had red levels for more than ten days.
Which app shows official air quality in Thailand?
Air4Thai, from the Pollution Control Department, publishes hourly readings from stations across the country on its website and in its app. The UK Foreign Office also mentions the World Air Quality Index site, which uses a different index scale.
Sources
- Pollution Control Department – Air4Thai air quality readings (checked October 2026)
- The Nation – Thailand tightens PM2.5 standards (July 2022) (checked October 2026)
- World Health Organization – Global air quality guidelines, questions and answers (checked October 2026)
- The Nation – PM2.5 exceeds safety standard in 41 provinces (6 April 2026) (checked October 2026)
- The Nation – BMA asks for work from home as PM2.5 rises (January 2026) (checked October 2026)
- Bangkok Post – Bangkok air pollution forces 352 schools to close (January 2025) (checked October 2026)
- Thai Meteorological Department – Climatological data 1991–2020 (checked October 2026)
- UK FCDO – Thailand travel advice: health (checked October 2026)
- US EPA – Strategies to reduce exposure outdoors (wildfire smoke) (checked October 2026)