- Fine particulate matter (PM2.5) causes heart and lung disease and cancers, according to the WHO.
- Thailand's 24-hour PM2.5 standard is 37.5 µg/m³; the WHO's 2021 guideline for 24-hour exposure is 15 µg/m³.
- The north, including Chiang Mai, usually has its worst air in the burning season, roughly February to April.
- Check readings on the Pollution Control Department's Air4Thai service before planning outdoor activity.
- Call 1669 for chest pain, severe breathlessness or an asthma attack that does not respond to your reliever inhaler.
What PM2.5 is and why it matters
PM2.5 refers to particles 2.5 micrometres across or smaller, small enough to travel deep into the lungs and enter the bloodstream. The World Health Organization states that exposure to fine particulate matter causes cardiovascular and respiratory disease and cancers. In its 2019 estimates, most premature deaths attributed to outdoor air pollution worldwide were from heart disease and stroke, with the rest from chronic lung disease, respiratory infections and lung cancer.
Short spells of high pollution can cause sore eyes, a cough, a scratchy throat and headaches in healthy people. The bigger concern is for people with asthma, chronic obstructive pulmonary disease (COPD) or heart disease, whose conditions can flare, and for children, older people and pregnant women.
Thailand's standard and the WHO guideline
Thailand tightened its PM2.5 standard from 1 June 2023. The National Environmental Board lowered the 24-hour average limit from 50 to 37.5 micrograms per cubic metre and the annual average from 25 to 15, on a proposal from the Pollution Control Department (PCD).
The WHO's global air quality guidelines, updated in 2021, are stricter: 15 µg/m³ for a 24-hour average and 5 µg/m³ for the annual average. So a day that meets the Thai standard can still be well above the WHO guideline level. When you read a local report that air is 'within the standard', it is worth checking the actual number.
Readings are often given as an air quality index (AQI) rather than as µg/m³, and different apps use different index scales. Compare like with like, and use the concentration figure where you can.
Chiang Mai and the northern burning season
Northern Thailand, including Chiang Mai, Chiang Rai, Mae Hong Son and Lampang, usually has its worst air from around February to April, when forest fires, agricultural burning and smoke drifting from neighbouring countries combine with still, dry weather in the mountain valleys. The haze typically clears once the rains arrive.
During severe episodes, provincial authorities have urged people to stay indoors, asked employers to let staff work from home, and advised at-risk groups to avoid outdoor activity. If you are planning a long stay, a retreat or a medical trip to the north in these months, look at the previous years' readings for the same weeks and consider whether to travel at a different time, particularly if you or a child has asthma or a heart or lung condition.
Practical steps on bad-air days
Check the PCD's Air4Thai service, which publishes hourly readings from monitoring stations across the country, before planning outdoor activity. On high days, move exercise indoors, keep windows and doors closed, and run an air purifier with a HEPA filter in the room where you sleep. Many hotels and serviced apartments in the north can provide one on request.
If you need to be outside, a well-fitting particulate respirator mask (such as an N95, KN95 or FFP2) filters fine particles far better than a cloth or surgical mask; fit matters as much as the rating. Masks can make breathing harder for some people with heart or lung disease, so ask your doctor if you are unsure.
If you have asthma or COPD, carry your reliever inhaler, keep enough of your regular medicines, and follow your personal action plan. Ask your doctor before travel whether your treatment should be adjusted during a high-pollution period.
When to get medical help
See a doctor if a cough, wheeze or breathlessness is not settling, if you need your reliever inhaler more than usual, or if a child is struggling with symptoms. Hospitals in the north see a rise in respiratory visits during the burning season, so allow time.
Call 1669 for chest pain or tightness, severe breathlessness, blue or grey lips, confusion, or an asthma attack that does not respond to your reliever inhaler. Chest pain in someone with heart disease during heavy pollution should always be treated as an emergency.
This guide summarises published public-health guidance. It is not a substitute for first-aid training or for the instructions of the 1669 call handler or a doctor; if in doubt, call 1669.
During the burning season
- Check Air4Thai readings before planning outdoor activity.
- Keep windows closed and run a HEPA air purifier in your bedroom.
- Use a well-fitting N95, KN95 or FFP2 mask outdoors on bad-air days.
- Carry your reliever inhaler and enough of your regular medicines.
- Ask your doctor before travel if you have asthma, COPD or heart disease.
- Call 1669 for chest pain, severe breathlessness or an asthma attack not helped by your inhaler.
Frequently asked questions
When is the burning season in Chiang Mai?
Air quality in northern Thailand is usually worst from around February to April, and the haze typically clears once the rains begin.
What is Thailand's PM2.5 standard?
Since 1 June 2023 the 24-hour standard has been 37.5 µg/m³ and the annual standard 15 µg/m³. The WHO's 2021 guideline levels are 15 µg/m³ (24-hour) and 5 µg/m³ (annual).
How can I check air quality in Thailand?
The Pollution Control Department publishes hourly readings from its monitoring stations on its Air4Thai service.
Do face masks help against PM2.5?
A well-fitting particulate respirator such as an N95, KN95 or FFP2 filters fine particles much better than cloth or surgical masks. People with heart or lung disease should ask a doctor whether a mask is suitable for them.
Should people with asthma avoid Chiang Mai in March?
It is worth discussing with your doctor. Pollution in the burning season can trigger asthma attacks, so many people with asthma or heart and lung conditions choose to travel at another time or plan for mostly indoor days.
Sources
- World Health Organization — Ambient (outdoor) air pollution fact sheet
- World Health Organization — WHO global air quality guidelines (2021)
- Pattaya Mail — Thailand adjusts air safety threshold (PM2.5 standard of 37.5 µg/m³), 25 May 2022
- Pollution Control Department — Air4Thai air quality readings
- Department of Disease Control, Ministry of Public Health (Call Center 1422)
- GOV.UK — Thailand travel advice: Getting help
