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CHIANG MAI BURNING SEASON
This webpage is a short summary of the scientific research on the burning season in Chiang Mai and Northern Thailand. It is covering most aspects related to this seasonal event (health, public data, chemical & physical properties, meteorology & climate, sources of pollution, prevention, policies & actions, etc.)
Some specific short answers can be found in the Frequently Asked Questions page.
This document is based on 'Comprehensive Review of the Annual Haze Episode in Northern Thailand (Pirard & Charoenpanwutikul, 2023)'. For readers who wish to know more about the burning season, the review covers many aspects of the annual haze episode in details.
The main purpose of this page is to circumvent all the misinformation and misunderstanding regarding this annual meteorological phenomenon by giving direct access (through the reference list in the linked article above) to scientific sources of information.
Go directly to:
Sources & Causes of Air Pollution
Air Quality Index (AQI) & Visibility
Health Effects of Air Pollution
Environmental Effects of Air Pollution
Public Opinions on Air Pollution
Official Actions against Air Pollution
Protection against Air Pollution
Toxicity of Air Pollution (cigarette equivalence)
Ranking of Most Polluted Cities
INTRODUCTION
The burning season (haze episode, smokey season, PM2.5 event, etc.) is an annual weather phenomenon in tropical Asia occurring at the driest time of the year, causing unhealthy levels of air pollution.
The burning of forests & crops charges the atmosphere with microdust that reach such concentrations that it affects daily life, particularly causing breathing difficulties for sensitive individuals.
The burning season extend from South of Bangkok to Southern China, between mid-January and mid-April and peaks in March. The start of the burning season depends how and where it is defined and can vary by 2-3 weeks. The intensity of the burning season greatly varies from year to year and can sometimes prolong to the month of May.
The main cause of the burning season is a combination of two factors: biomass burning (forest, crops) and meteorological conditions. A large amount of smoke is produced through burning and accumulated in valleys and river basins of Northern Thailand. It is not an urban pollution as it has a regional impact affecting remote valleys far from main cities.
Figure 1: Record of air pollution (PM2.5) in Chiang Mai between 2016 and 2023 as AQI color code. Green and Yellow are days with relatively low air pollution. Orange, Red and Purple are days with high air pollution with unhealthy to hazardous levels. The figure clearly shows February, March and April as the months with abundant red-purple colours. It also shows that some years (ex: 2022) are mild while other (ex: 2019, 2023) are bad.