Skip to main content

Sky hazy due to pollution

Stefanos Fotiou

Air pollution has become a serious environmental and health issue in Thailand. The pollution levels in the country follow predictable patterns, which highlight the presence of a peak pollution season. For Bangkok, the highest levels of pollution are seen between November and February each year.

The first step in any effort to mitigate air pollution is to understand the magnitude of the problem and the sources of pollution. The most common measure of air quality is the Air Quality Index (AQI). The AQI is based on measurement of particulate matter (PM2.5 and PM10), Ozone (O3), Nitrogen Dioxide (NO2), Sulfur Dioxide (SO2) and Carbon Monoxide (CO) emissions. Using a composite index of air quality as the AQI is extremely important in understanding the magnitude of the problem. To go a step further and design mitigation actions someone has to perform more detailed analysis and “decipher” the meaning of a single numerical measure in order to identify the sources of pollution.

A hazy Bangkok skyline – or an invisible city skyline – is the most evident indication of air pollution, as small particles, generally referred to as particulate matter of 10 or 2.5 microns in width, are suspended in the air. Of these two sets of particles the PM2.5 draw big attention because of their high contribution to air pollution and the detrimental health effects they can cause in our respiratory systems. Although these small particles are measured by size, there are significant differences in their chemical composition depending on their origin. By analyzing this chemical fingerprint, we can determine the cause of pollution as well as different toxicity factors and health impacts of the PM2.5 particles.


ESCAP has initiated a large study in Thailand that is looking at several aspects of air pollution in Bangkok, Chiang Mai and Nakhon Si Thammarat. The results of this study will be published soon, and this article provides some main finding of this work. When analyzing the data we look at two levels: the source and the contributing factors. The source indicates the generation of the problem and the different pollutants and is related to different economic activities (from power generation to industrial production, agriculture and transport). The contributing factors refer to local weather conditions (temperature, wind, humidity) that are responsible for the temporal and spatial distribution of the pollutants and therefore for the specific AQI in a location. All these data, generated from ground-based measurement stations, are combined with observations from satellites and are fed into machine learning models to identify the most significant factors and the most effective tools for reducing air pollution levels.


The preliminary analysis of data and our study show the following: While internal combustion vehicles are an important source of air pollutants, they are not the primary sources of air pollution in most cities in Thailand. While vehicles do play a larger role in cities such as Bangkok or Nakhon Si Thammarat their overall contribution to the AQI in these cities is much smaller than other factors. In this context, smoke released by agricultural fires and forest fires are the main source of pollution and are related to unsustainable practices of natural resources management.


Additional sources of air pollution include industrial emissions from factories, coal-burning powerplants and construction. Another finding from the study is that NOx levels, associated mostly with industrial emissions, are relatively higher in Bangkok in comparison with Chiang Mai.


Looking at the problem of open agricultural fires and the practices associated with it we can highlight the locations with the most frequent incidents of open biomass burning. In the case of Bangkok, the hotspots closest to the city are primarily from cropland. Outside a perimetre of 100 kilometers, the profile changes somewhat with hotspots in urban locations growing in prominence as indicated in the graphs below.

Figure 1: Number and type of fires in various distances from Bangkok.

The relation of PM2.5 and open agricultural fires is also supported by the following graph that shows the PM2.5 levels in relation to the number of fire hotspots over a calendar year (July 2019 to June 2020).

Figure 2: PM2.5 levels and number of fires up to 400 Km from Bangkok (2019-2020).

The air pollution problem of Thailand has multiple faces and there are multiple sources responsible for the PM2.5. The causes vary greatly depending on location and dealing with the problem requires to go out of the box, address the real causes and have a portfolio of short-, medium- and long-term actions.

Specific actions should include:

  • Short-term: Promote sustainable agricultural practices (like circular models for straw utilization and sustainable agricultural mechanization) and enhance prevention and suppression of forest fires.
  • Medium-term: Engage cooperatively with neighboring countries to work together in reducing air pollution emissions from agricultural burning.
  • Long-term: Decarbonize industrial activity, invest in clean energy and zero-emission public and private transport.
Print this article
Stefanos Fotiou
Former Director, Environment and Development Division
Matthew Perkins
Programme Officer
Environment and Development +66 2 288-1234 [email protected]