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When is the Burning Season?

It stars at the end of January and lasts until mid-April, but it can easily move by a couple of weeks depending on meteorological conditions and how it is defined. Burning occurs from October to June but its only in February-March-April that conditions become critical (see Characteristics of air pollution).

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How is it measured?

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

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What is the AQI?

AQI is the Air Quality Index. It is a scale that takes into account pollutants in the air and translate it 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) (See AQI).

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How bad can it get?

Background levels during burning season are AQI 150. It limits the visibility of mountains around Chiang Mai and causes some discomfort to some people. However, it is not unusual to have days at AQI 200 or even 300. These are pollution levels that affect most people one way or another. Some years, AQI levels can peak far above 500 for a few days. The visibility falls to a few hundred meters, the air has an orange tinge and strongly smells wood fire and affects everyone (see Characteristics of Air Pollution).

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Is it better in the countryside?

No. since the source is mostly forest and/or agricultural 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.

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What are the effects of the Burning Season?

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

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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 Northern Thailand. No strong evidence of correlation has been found with lung cancer (see Health Effects).

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How can I protect myself from it?

For indoor areas, air purifiers using HEPA filters or positive pressure systems

For outdoor activities, masks, N95 or equivalent will reduce air pollution to healthy levels (see Prevention).

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What is the cause of the Burning Season?

In Chiang Mai, 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. In some other provinces, agricultural burning can be significant (see Sources & Causes).

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Is Chiang Mai the most polluted city in the world?

No. On a yearly average, there are hundreds of cities with higher pollution levels (a good bunch in India and China, but also Dubai, Athens, Milano,...).

Occasionally, the air pollution level in Chiang Mai can rise to extremely high levels due to nearby forest fires. Then, Chiang Mai could be, for a few days, the most polluted city but this happens a few days every few years at best and likely only in the early morning. The claim of most polluted is made on a very incomplete list used by many media and apps, where the 'competition' for the top spot is heavily biased towards Chiang Mai, ignoring hundreds of very polluted cities.

Details on this claim are available here: Chiang Mai: most polluted city in the world?

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Is Chiang Mai air pollution worse than Bangkok?

Not really. On a yearly average, Bangkok & Chiang Mai have similar pollution levels. However, Bangkok has more harmful chemicals (like any big city with lots of traffic and industries; sugarcane smoke is also potentially more toxic) and it is easier to protect yourself in Chiang Mai from very bad haze days than the constant mildly toxic levels of Bangkok (see Comparison of  air pollution).

 

Are rice/corn/surgarcane farmers responsible for it?

In Chiang Mai, most of the rice residue burning occurs in December-January, before the burning season. Some rice burning also happens in April-May.

Surveys, remote sensing and smoke composition refute the idea that corn is particularly problematic. It is burned in April-May but is not a large contributor to air pollution.

Sugarcane is not a common crop in Northern Thailand. However, it is an important source of air pollution in Isaan.

Beside agricultural fires however, rural people have a great part of responsibility in starting forest fires (see sources & causes).

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Is it getting worse every year?

No. Air pollution varies a lot from year to year. Trends over two decades of data are not conclusive between a very slight improvement and a very slightly worsening situation. The effect is so little that if they get better or worse, at this stage, it would take 50 years to notice it.

The impression that it gets worse is only due to more information available (AQI, home detectors, awareness, fear-inducing media exposure), particularly during very bad haze days (see Characteristics of air pollution).

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Is there anything done about it?

Yes, the fire ban in Chiang Mai is one action but it does not seem to be very effective. Numerous policies exist to deal with air pollution but systemic issues and a lack of meaningful application to local conditions have made these government actions ineffective. Things are slowly changing but a lot of critics towards government actions that were made more than 20 years ago remains applicable to this day (see official actions).