In 2020, it was estimated that the average Thai resident was exposed to an annual PM2.5 (particulate matter with a diameter size of 2.5 microns or smaller) concentration of 21.4 μg/m³ (micrograms per cubic meter), nearly two times the World Health Organization (WHO) target of 10 μg/m³.

For a few months each year, Chiang Mai, northern Thailand’s tourist-friendly cultural hotspot, ranks among the world’s most polluted cities. Chiang Mai’s air pollution is largely attributable to agricultural burning rather than fossil-fuel-related combustion experienced in other global cities.

Agricultural burning, also called open burning or “burning season,” is the practice of setting fire to cultivated fields to clear the land in preparation for the next crop cycle. In northern Thailand, agricultural burning practices are mostly used on sugarcane, rice, and maize crop fields between December and April.¹

The concurrence of burning season and Thailand’s driest, coldest months can result in extremely poor regional air quality, as this weather creates favorable conditions for particle pollution to accumulate and linger. Often, smoke from Thailand’s agricultural burns persists until Songkran, the Thai new year. Around this time, the impending rainy season helps clear the skies and tamp down polluted air.

Air quality during Thailand’s burning season commonly reaches and remains at levels considered “unhealthy” by the WHO and the air quality index (AQI) defined by the US Environmental Protection Agency (EPA). In Chiang Mai, the deterioration in air quality is often plainly visible. Brown smoke from crop-burning can frequently obstruct sunlight and views from Doi Suthep, a mountain that’s the site of a centuries-old Buddhist template.

This analysis examines the impact of agricultural burning on Chiang Mai air quality over a five-year period, evaluating the effectiveness of the Ministry of Agriculture’s ban on crop burning and proposing what more could be done to reduce dangerous PM2.5 levels.

The importance of PM2.5

Of all the pollutants emitted from agricultural burning, PM2.5, particulate matter with a diameter of 2.5 microns or less, is widely regarded as the most dangerous.

Average PM2.5 pollution levels during the agricultural burning period are often three to five times higher than the WHO’s 10 microgram per cubic meter (μg/m³) standard for annual PM2.5 exposure, with concentration levels during some hours climbing as high as 400 μg/m³.²

Due to its small size, PM2.5 can penetrate deep into the human respiratory system and into the bloodstream, causing far-reaching health impacts. PM2.5 originating from agricultural burning includes:

• smoke
• black carbon (soot)
• chemicals
• dust
• dirt

Epidemiological studies have linked exposure to PM2.5 to detrimental health effects, including:³

• asthma
• chronic obstructive pulmonary disease (COPD)
• respiratory infections
• stroke
• heart attack
• death

PM2.5 pollution standards

The following analysis uses the US EPA air quality index (AQI) to correlate PM2.5 concentration values to a widely accepted standard for health risk, supplemented by the WHO annual mean exposure threshold of 10 μg/m³.

Thailand has its own standard for acceptable air quality, recommending that PM2.5 levels not exceed 50 μg/m³ on a 24-hour basis. 50 μg/m³ closely correlates to the US EPA “unhealthy” limit of 55.4 μg/m³.⁴

To compare air quality levels using this standard, PM2.5 levels color-coded green, yellow, and orange (0-150) are deemed acceptable, while levels color-coded red, purple, and maroon (151-301+) are not (see Figure 1 for a visualization of the US AQI standard).

Figure 1: US AQI levels from 0-301+, equivalent PM2.5 standards by μg/m³, and health recommendations for each level. Source: IQAir" src="https://cms.iqair.com/sites/default/files/inline-images/1747_AQLChart_US_v3_WEB_210521b.jpg">

Methodology

This analysis examines PM2.5 levels reported by ground-level monitoring stations in Chiang Mai, Thailand across three months (January, February, and March) during 2021. This data is compared to the same time period in 2020, 2019, 2018, and 2017.

A three-month time period was selected for the agricultural burning season in Thailand because the actual start and end dates vary each year based on weather conditions and other variables. Comparing the same time period for each year helps reduce the influence of changes in weather that might affect measured PM2.5 levels.

Data was collected from 37 public PM2.5 sensors aggregated on the IQAir platform. Two are operated by the Thailand Pollution Control Department, part of the Thai government. All others represent validated non-governmental air quality monitors.

Findings

Per the three-month average during the open burning season, Chiang Mai experienced less PM2.5 pollution in 2021 than in 2020 and 2019, but higher PM2.5 concentrations compared to 2018 and 2017. During all 5 years, PM2.5 concentrations were recorded at dangerously unhealthy levels.

In 2021, the city experienced average monthly PM2.5 concentrations drastically above the WHO’s 10 μg/m³ target (see Figure 2):

• January: 42.5 μg/m³
• February: 47.7 μg/m³
• March: 83.3 μg/m³

Figure 2: Average annual and monthly PM2.5 concentrations, color-coded by US Air Quality Index categories. Source: IQAir" src="https://cms.iqair.com/sites/default/files/inline-images/Figure2-ChiangMai-monthly-averages.jpg">

From 2017-2021, March was the most polluted month, with average PM2.5 concentrations frequently double that of the other months in the three-month monitoring period.

Only 1.5 percent of hours met the US AQI “good” standard

Over the three-month monitoring period in 2021, only 1.5 percent of hours averaged PM2.5 concentrations below 12.0 μg/m³, the US AQI standard for “good.”

By comparison, this threshold was met by:

• 0.7 percent of hours in 2020
• 2.6 percent of hours in 2019
• 2.0 percent of hours in 2018
• 1.2 percent of hours in 2017

More hours in 2021 had levels deemed “unhealthy for sensitive groups” or worse than any other year examined (see Figure 3). Even though the frequency of “good” air quality was up from 2020, so too were average PM2.5 concentrations above 35.4 μg/m³.

Figure 3: Chiang Mai’s distribution of hourly PM2.5 concentrations from January 1 - March 31, shown as US Air Quality Index categories. Source: IQAir" src="https://cms.iqair.com/sites/default/files/inline-images/Figure3-ChiangMai-percent-horus.png">

Burning season is beginning earlier

Trends indicate an increasingly earlier start in agricultural burning that may be the result of:

• climate change, which has pushed agricultural workers to start the season earlier to avoid droughts
• government bans on crop burning during January and February

Average PM2.5 concentrations during December and January of 2021 and 2020 were double that of the same months in 2017, 2018, and 2019 (see Figure 4 for PM2.5 levels during each month as divided by daily PM2.5 concentrations).

Figure 4: Daily PM2.5 levels in Chiang Mai from 2017-2021, indicated by US AQI color. Source: IQAir" src="https://cms.iqair.com/sites/default/files/inline-images/Figure4-ChiangMai-calander-year.jpg">

Why do farmers participate in open burning?

Open burning practices are illegal in Thailand. Farmers, however, often have little recourse to find alternatives given their financial situation. Hiring workers to cut stalks and collect leaves manually can delay sowing of the next crop cycle and raise the total cost of cultivation by 30 to 40 percent.

Mechanized harvesting provides an alternative, but it is often prohibitively costly and typically used less than 10% of the time.⁵ Farmers fear that costly solutions could threaten their market share, reduce demand, and endanger their livelihood.

Many farmers are also indebted to large agribusinesses and suffer from falling prices and diminished yields. This can make the transition to more laborious methods or more expensive technology unfeasible. In order to navigate around the burn ban imposed in 2020, many farmers have resorted to burning their fields at night when smoke sources are less likely to be detected.

What is being done to curtail agricultural burning?

The government, while banning the practice on a federal level, does not provide any resources to ease the transition to cleaner alternatives. No money, machinery, or labor is offered to farmers to promote compliance.

Some federal organizations have taken measures to begin helping farmers make the transition from agricultural burning to more healthy, sustainable methods of preparing for the next season.

In 2021, the Department of Agricultural Extension campaigned in Northern Thailand to demonstrate environment-friendly ways to handle farm waste. It is yet to be seen whether these efforts will have an observable impact on air pollution levels in the region.

When agricultural smoke becomes particularly severe, the Thai armed forces are sometimes employed by the Thai government to put out the fires.

Cloud seeding, a type of weather modification that changes a cloud's structure to increase the chance of precipitation, is also used under the discretion of the Royal Rainmaking Project. The rain spawned from cloud seeding can help to tamp down smoke and clear the skies, although the long-term climate impact of this practice has yet to be determined.

Despite government efforts to reduce the concentration of airborne particulates, air pollution continues to pose a significant health threat to residents in Thailand and beyond, as smoke from crop burning can travel for thousands of miles into neighboring countries.

Other solutions for effectively combating crop burning may include:

• government subsidies
• government-provided mechanized harvesters
• diversification of crops
• breaking up large agribusinesses to allow farmers to earn more and gain more control over their farms

Summary

While average PM2.5 concentrations were down from 2020 (a record burning season) to 2021, they were roughly in line with 2019 and nearly 20 percent higher than 2018 and 2017.

The 2021 season also had the most hours with PM2.5 concentrations over 35.4 μg/m³ (“unhealthy for sensitive groups” or worse), placing children, elderly, and people with pre-existing heart and lung disease at a higher risk than previous years.

Farmers in northern Thailand often resort to crop burning as a means for clearing land of residue, pests, and weeds. While this practice allows farmers to turn over fields quickly with fewer resources, it is costly for public health and the environment.

A review of PM2.5 concentration data over the last 5 years fails to show a drastic change in the 2021 season from years past despite increasing public knowledge about the air quality impacts of crop burning. To achieve safe air quality, a more committed and multifaceted approach is required.