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Cassian Pirard PhD   



When is the Burning Season?

It stars in mid-January and lasts until mid-April, but it can easily move by a couple of weeks depending on meteorological conditions.

How is it measured?

Air pollution in Chiang Mai is due to smoke. Smoke is a mixture of tiny particulates and gases. Gases are negligible in Chiang Mai and air pollution is mostly controlled by particulate matter. Particulates from fires with a size between 0.02 and 0.00001mm. Detectors measure the concentration of these particulates per volume and translate it into an Air Quality Index (AQI), giving a quick estimation of how harmful it is.

What is the AQI?

AQI is the Air Quality Index. It is a scale that takes into account pollutants in the air and translate into a health impact scale.

It has several categories, from 0 to 50 (Good); 50-100 (Moderate), 100-150 (Unhealthy for sensitive people), 150-200 (Unhealthy), 200-300 (Very unhealthy), 300-500 or higher (Hazardous).

How bad can it get?

Background levels during burning season are AQI 150, it limits the visibility of mountains around Chiang Mai and cause some discomfort to some people. However, it is not unusual to have days at AQI 200 or even 300. These are pollutions levels that affect almost everyone. Some years, AQI levels can go far above 500 for a few days. The visibility falls to a few hundred meters, the air has an orange tinge and strongly smell wood fire and affects everyone.

Is it better in the countryside?

No. since the source is mostly distant forest fires, being away from a big city does not change anything.

Pollution is a bit better at altitudes above 1500m but that’s about it.

What are the effects of the Burning Season?

Health issues (respiratory and cardiovascular diseases), decreased visibility, lower solar energy output, lower ground temperature.

How harmful is air pollution in Chiang Mai?

For healthy individuals, it mostly causes allergy-like symptoms

For sensitive individuals with respiratory and cardio-vascular conditions, it can bring acute symptoms and hospitalization.

Long-term effects are unclear for Chiang Mai. No strong evidence of correlation has been found with lung cancer.

How can I protect myself from it?

For indoor areas, air purifiers using HEPA filters

For outdoor activities, masks, N95 or equivalent will reduce air pollution to healthy levels

What is the cause of the Burning Season?

Mostly forest fires (90%) with small contributions from agricultural burning, traffic and urban sources. Smoke produced by forest fires is trapped into valleys by specific atmospheric conditions.


Is Chiang Mai the most polluted city in the world?

No. On a yearly average, there are hundreds, thousands of cities with higher pollution levels.

Occasionally, the air pollution level in Chiang Mai can rise to extremely high levels due to nearby forest fires. Then Chiang Mai can be, for a few days, the most polluted city. But this happens a few days every few years at best.

Most media and apps claiming Chiang Mai to be the most polluted, only use a very short list of selected cities and Chiang Mai happen to be on the top of that short list.

Is Chiang Mai air pollution worse than Bangkok?

No. On a yearly average, Bangkok & Chiang Mai have similar pollution levels. However, Bangkok has potentially more harmful chemicals (like any big city with lots of traffic and industries) and it is easier to protect yourself in Chiang Mai from very bad haze days than the constant mildly toxic levels of Bangkok.


Are rice/corn/surgarcane farmers responsible for it?

In Chiang Mai, most of the rice residue burning occurs before the burning season

There is no evidence from surveys, remote sensing and smoke composition that corn is being burned extensively

Sugarcane is not a common crop in Northern Thailand. However, it is a major contributor to air pollution in Isaan.

However, rural people are partly responsible as a large number of forest fires are started to help the collection of forest products.

Is it getting worse every year?

No. Air pollution varies a lot from year to year and there is a slight improvement in the last two decades when averaging the air pollution over the whole burning season.

However, it is imperceptible as it has little effect on very bad haze days, which are the most remembered events. 2019 had memorable high pollution levels (AQI > 600) and March 2007 had an average AQI, for the whole month, of ~250.

The impression that it gets worse is only due to more information available (AQI, home detectors, awareness) and people (mostly expats) being more vocal about it.

Is there anything done about it?

Yes, the yearly fire ban in Chiang Mai is effective with relatively small number of fires in recent years. However, most of the air pollution comes from the west (Mae Hong Son, Myanmar). Although agreements exist, these are not effective to control air pollution sources in those regions.

When is
How measure
How bad
Most polluted
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