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CHIANG MAI BURNING SEASON
This webpage is a short summary of the composition of air pollution in Chiang Mai and Northern Thailand. Further information and references can be found in 'Comprehensive Review of the Annual Haze Episode in Northern Thailand (Pirard & Charoenpanwutikul, 2023)'.
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HEALTH EFFECTS
The toxicity of air pollution in Northern Thailand is not fully determined
but likely less than a large Asian city
For healthy individuals, symptoms are allergy-like (blocked nose, sore throat, itchy eyes)
For individuals with pre-conditions, acute symptoms and hospitalization can occur for asthma, COPD, cerebrovascular and cardiovascular diseases
Lung cancer seems to have no relationship with the level of air pollution
A cigarette-equivalent calculation based on health effects is available here
Air pollution and its effects on health have been studied for a long time. A large number of studies deal specifically with particulate matter which is the type of pollution seen in Chiang Mai. However, most studies are focused typically on urban pollution (traffic, industrial, urban activities, etc.) which is considerably different from biomass burning, the main pollution source in Northern Thailand. Differences exist in heavy metal contents, organic carcinogenic chemicals and the size of particulate matter itself and what is true for all these studies on urban pollution are not or might not be applicable for the air pollution in Northern Thailand. At this stage, it remains unclear if very high concentration of particulate matter in Chiang Mai is more toxic than an average urban pollution in large Asian cities. All media have more or less decided that Chiang Mai air pollution is heavily toxic but some scientific data indicates that it is likely less harmful than traffic and industrial haze or smog
. Air pollution in Northern Thailand has two general health effects, irritability and toxicity. The high concentration of particulate matter is a strong irritant. The general public and media assumption is that irritant means toxic but that is not case. Tear gas is an example of a strong irritant with no toxic component. Irritation is the main cause of health symptoms observed during the burning season. it is mild for healthy individuals but can be dramatic for sensitive patients. Toxicity is the other component and most data indicate a mild toxicity compared to similar particulate pollution in urban environments.
Harmful effects are due to the infiltration of dust deep into the respiratory and circulatory system, leading to potential alveolar obstruction but overall, causing a defensive reaction from the human body to eliminate particulate contaminants through a variety of immune responses.
The other effect is the absorption of toxic substances and infiltration of the smallest particulates inside the respiratory system membranes, causing harm by stimulating oxidative stresses, a general inflammatory response and genotoxicity.
Except in specific research studies, all size fractions, from 20 microns to 0.1 and below, are considered by authorities almost equally toxic. A distinction is made between PM10 and PM2.5 but no consideration is taken for composition, surface area, shape, source, etc. It creates a situation where seaspray with high salt microdust content and tobacco smoke could have a similar AQI based on PM2.5 and an inferred identical toxicity when in reality, salt microdust is harmless. What is true for the comparison between sea salt dust and tobacco smoke is also true, to a much lesser extent, between urban pollution and biomass burning smoke. In an effort to simplify data for the public and regulating bodies, air pollution is misrepresented as a simple function of concentration load in breathable air but it is important to keep in mind that analytical data show very significant variations far too complicated to be exposed here.
Causative agents
Most of carbonaceous particulate matter is theoretically inert but act as a mild irritant in upper and deep respiratory airways. Fine and ultrafine particulates can penetrate deeply into alveolar sacs, carrying a significant amount of chemicals that can be released in blood capillaries. Adsorbed toxic chemicals on the surface of particulates are the most concerning and long term issue as they use particulate matter as a carrier deeply into the human body where it can be absorbed into the bloodstream.
Figure 20: Simplified graphical representation of particulate matter penetration into the respiratory system
Polycyclic aromatic hydrocarbons (PAHs) are among those concerning chemicals since some species are well-known carcinogenic by altering the DNA replication process. PAH sources in Northern Thailand are relatively mild and from that carcinogenic PAH concentration alone, air pollution in Chiang Mai is similar to European or Japanese cities and several times lower than Bangkok despite the intense haze. The overall risk associated with PAH in Chiang Mai haze is of 1/10000 chances of developing a cancer after 70 years of exposure.
Dioxins (PCDD) are carcinogenic and immunotoxic compounds with the ability to bio-accumulate. The lack of large industrial complex in Chiang Mai or elsewhere in Northern Thailand makes these dangerous chemicals relatively insignificant except for smoke associated with garbage burning.
Some heavy metals have carcinogenic properties. Biomass burning however, does not release large amounts of heavy metals compared to urban sources. Again, Bangkok and many large cities have systematically higher levels of heavy metals than the thickest Chiang Mai haze (See Comparison of pollution).
General health symptoms
For healthy individuals, heavy haze impact is often limited to allergic reactions with mild to very mild symptoms including allergic rhinitis, congestion, sore throat, itchy eyes, headaches and more rarely skin reactions. For very high levels of particulate matter, shortness of breath can also appear. These mild respiratory symptoms are quite widespread, affecting 1/3 of individuals when PM2.5 reaches 35 μg/m3 (= AQI 100).
Regular exposure to particulate matter also indirectly leads to aeroallergen sensitization, making healthy individuals more prone to develop a histamine intolerance from other allergens and new allergic reactions. The prolonged sunlight absorption by haze also reduce vitamin D production by 10 to 20% when PM2.5 is 35 to 100 μg/m3. On the other hand, the absorption of UV by the polluted atmosphere makes sunburns less likely during the hot season.
Finally, the constant exposure to haze is also associated with mild psychological stress and cognitive impairment for some individuals, causing recurrent thinking about haze, irritability, insomnia and poor concentration. These psychological symptoms are worsened by constant anxiogenic media exposure and the omnipresence of AQI monitoring devices reports (See Public Opinion).
Short term diseases
The health effect of particulate matter regarding short term exposure and acute symptoms is quite well established and known to increase morbidity and non-accidental mortality risk for individuals with health pre-conditions. Significant effects are observed for respiratory and cardio-vascular diseases and adult mortality in high-risk groups. Health issues also affect particularly children due to their underdeveloped respiratory system and higher breathing rate as well as the elderly.
High level of particulate matter can increase mortality by 1 to 2% with a specific increase of 4-5 % for cardiovascular mortality and respiratory morbidity but these highly polluted day are also correlated with other weather parameters. Overall, the increase in particulate matter air pollution is strongly correlated with hospitalization rate for asthma, COPD, cerebrovascular diseases, myocardial infections and coronary and ischemic heart diseases. Other diseases such as pneumonia, influenza, pulmonary embolism also have mild indirect correlations. On the basis of increased specific mortality during the burning season, a cigarette-equivalent scale can be established to have a more understandable value on these health effects. On average, the air pollution in Chiang Mai follow the equivalence 1 cigarette = 66 μg/m3 for a daily average (see tobacco equivalence)
Figure 21:Thailand map of mortality anomalies compared to the national average. Positive (orange, red) are 25 to twice higher than the average while negative (blue, dark blue) anomalies are half to less than a quarter of the national average. Significant positive anomalies exist in the North for COPD, asthma and lung cancer but not for pneumonia.
Long term diseases
The long term effects of regular exposure to biomass burning particulate matter are unclear some studies have established that exposure to PM increases the risk of neurological and cognitive diseases (dementia, cognitive impairment, cognitive development) and metabolic (diabetes) but these research results come exclusively from urban pollution and conclusions cannot be transposed readily to biomass burning even if the media is very willing to do so.
Low birth weight and pre-term birth as well as higher infant mortality are correlated with high average PM2.5 but also with economic status as it affects the poorest population of continental South-East Asia (Northern Laos, Western Myanmar).
Lung cancer is an interesting case where scientific evidence is significantly different from the message given to the public through the media and official channels. Broad studies show an anomaly for lung cancer mortality in Northern Thailand compared to the national average and it has been instinctively associated with air pollution. However, this anomaly of around 50 to 75% higher than the national lung cancer rate, has an absolute risk (as number of cases per population) not different from the average lung cancer rate in Japan and significantly lower that the US, even when other factors than air pollution are taken into account. Further detailed analyses also show that the high lung cancer risk is found in Chiang Mai and Chiang Rai and more particularly in Amphoe Muang, Hang Dong, Doi Lo, San Pa Tong and Doi Saket districts of Chiang Mai province. Such spatial specificity is difficult to explain with air pollution that is broadly homogeneous over the whole northern region.
The lack of correlation between lung cancer rate and air pollution is still applicable as of 2025 and all province or region wide studies on the subject have failed to established a causal relationship. However, while no correlation can be found between the disease and air pollution, the survival rate of hospitalized lung cancer adult patient and after remission is lower than outside the burning season and albeit a weak correlation, it fits into the general medical evidence that diseased patients are more at risk than healthy individuals.
On the basis of the dozen of serious research articles on that specific subject, the media narrative that lung cancer deaths in Chiang Mai are abnormal and linked to air pollution is just poor journalistic work where assumptions are not checked with available evidence. What is more concerning is the announcement by the CMU medicine faculty that also link the two parameters, which goes against scientific evidence, published by that same faculty. Emotional and publicity-seeking statements under the disguise of an appeal to authority are unfortunately not uncommon in Thailand academia and a result of poor deontology considering that scientific evidence and consensus goes against it.
Since the correlative link between air pollution and the prevalence of lung cancer is weak, other causes have been investigated such as eating and cooking habits, indoor radon. Northern Thailand has relatively high radon emission levels compared to neighbouring regions, but it is only mildly above the world average and not different from the European average. Similarly to air pollution, radon distribution map in Northern Thailand does not match the distribution of lung cancer particularly. So although the victims of lung cancer are overly smokers (96% male, 52% female) and have some genetic predisposition to radon sensitivity, the reasons for the national positive anomaly of lung cancer in northern Thailand are still unknown.
Figure 22: Comparison between lung cancer anomalies in districts of 6 provinces of uppermost Northern Thailand (left) and radon concentration map. An air pollution map would not show significant differences between districts.