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Chiang Mai: the Most Polluted City in the World
(for general information about the burning season, click here)
A frequent news headlines during the burning season in March and April. Is this label of Chiang Mai as the most polluted city in the world really justified?
SHORT ANSWER:
No. In around 99.5% (monthly stats) of the time, it is put into this dreadful top position and should not be there. Why? Because journalists use a very selective list of cities (126 to be exact) of IQAir as a reference. These cities are neither the largest in the world, the most important, the most well-known (for pollution or otherwise) or the most polluted. Only a dozen of significantly polluted cities are in this list, ignoring around 1500 other equally polluted large cities in the world.
It does not change the fact that Chiang Mai can be awfully polluted some days per year, but the anxiogenic superlatives used is just disinformation attempts to maximize media exposure.
Figure 1. Distribution of large cities (>700000 inhabitants) with available AQI data and their annual AQI average for 2023. Cities from the IQAir list are in red while the cities ignored are in blue. Chiang Mai is circled in green and in an unremarkable place in the dense cloud of 1 million-ish cities. Bangkok is circled in orange.
LONGER ANSWER
During the Northern Thailand burning season, particularly in dry years, the air pollution in Chiang Mai can reach very concerning levels, providing us with orange skies, a visibility below a kilometer, an acrid smell and breathing difficulties for the least fortunates. This extreme situation (not just bad air quality) can last a few days and repeat itself a few times per burning season, depending on weather patterns and the presence of local wildfires.
Who made the list of cities?
The list abundantly shared on the internet only contains 126 cities. These cities are mostly well-known large cities but not exclusively. Despite my attempt to make sense to this selection of cities, I am still not sure what is the basis for this exclusive group.
The author of the list is IQAir. IQAIr is a commercial company based in Switzerland that provide air cleaning products & solutions. They have a very visible website for those who wish to be informed about air pollution and provide hourly updates on the air pollution in many worldwide locations. They also provide this list of "the most polluted / cleanest cities" based on a very limited number of selected cities in their database (the number and election of cities does not change). Due to their online exposure, the list is staple information for journalists who consume it without critical thought and when Chiang Mai hit the first spot on that list, it is certain to make headlines. Aside from that list, IQAir website is a valuable source of information providing links to 80000 measuring stations worldwide.
Another alternative and valuable source of information is AQICN and its duplicate WAQI, which uses around 250000 stations worldwide of various environmental protection agencies in each countries and certified stations.
entative at all of the situation, especially when applied worldwide.
How "city" is defined in this document?
Defining the population of a city is difficult as it entirely depends on what the definition of city covers in each country. A direct comparison of population lead to misleading conclusions on the true size of some cities. In some cases, a city administration covers most or all of its metropolitan area (ex: Shanghai is almost 30 millions people) and in other cases, more restrictive administrative definition exclude most of the urban area (ex: Washington D.C. is 690000 people but the metropolitan area directly surrounding it is 6.3 million). In Chiang Mai, the administrative unit (amphoe Muang) is 130000 people while the urban area (including satellites villages of Hang Dong, San Kamphaeng, Doi Saket, Mae Rim, San Sai,..) has a combined population of 1.2 million. The metropolitan area of Chiang Mai is not defined by the government due to the usual exclusiveness of Bangkok status but a conservative approach of the Chiang Mai - Lamphun metropolitan area is likely to be around 1.5 million people.
In order to have a relatively objective population estimation and importance of world cities, the metropolitan area or extended urban area is used for the definition and classification of cities here. For the data compilation, processing and comparison used here, all metropolitan areas with population in excess of 700000 inhabitants have been considered, which gives 877 cities as well as another ~100 entries of nation's capitals or specific regional capitals.
Figure 2. Distribution of large cities (>700000 inhabitants) with available AQI data and their peak monthly AQI average value for 2023. Cities from the IQAir list are in red while the cities ignored are in blue. Chiang Mai is circled in green and in among the highly polluted but by far not the highest. Bangkok is circled in orange.
Air Pollution Monitoring Coverage
Not all cities have a sufficient record of air pollution to be available in an online database. Aqicn.org states that its monitoring covers 130 countries and around 2000 major cities. However, some air quality records are not sufficiently continuous or the number of stations available and their position is problematic to be included in a worldwide comparison.
Thailand, along with Vietnam and Iran, are the 3 countries with significant air pollution issues that have 100% coverage of all significant towns. Western countries, Japan, Taiwan and South Korea also have complete coverage but the air quality in these regions is generally passable to good. China (94%) and Indonesia (89%) also have pretty good records of their main cities. On the other hand, highly polluted countries such as India (75%), Pakistan (45%), Bangladesh (17%) possess a patchy coverage as well as the whole African continent (28%).
It means that out of the 877 large cities used in this document, only 76% have an air pollution monitoring that is compiled in online databases and potentially available for a real-time listing of the most polluted city on Earth. In the 24% not covered, it includes many extremely polluted cities in South Asia and Africa.
Demographics of listed cities
Using these criteria above for cities designation, the IQAir list shows some surprising omissions among very large cities . Metropolitan areas such as Lagos (Nigeria, 27 millions), Mexico City (Mexico, 22 millions), Kano (Nigeria, 16 millions), Tianjin (China, 14 millions), Rio de Janeiro (Brazil, 14 millions), Chennai (India, 12 millions), Hyderabad (India, 11 millions) are not present. It leaves some questioning on why a city like Chiang Mai with 1.2 million people is present but not cities 20 times bigger.
Chiang Mai is however far from being the smallest city in the list and some insignificant (in terms of population) towns such as Wroclaw (Poland, 0.125 million), Bern (Switzerland, 0.134 million), Pristina (Kosovo, 0.227 million) and Manama (Bahrain, 0.297 million) are listed. One could assume the last three are capitals, but if Lagos or Mexico City are issing, obviously , there is no reason for these to be specifically included.
Figure 3. Distribution of large Asian cities (>700000 inhabitants) and their annual AQI average. Cities from the IQAir list are represented as diamonds. Data show the highest pollution levels are found in India & Pakistan cities
Air Pollution in Listed Cities
Among the list of cities used here with air pollution data available (~700 cities), no relationship was found with the IQAir list. Some of the most polluted cities with an AQI above 100 on an annual average (=extremely polluted cities) can be found such as New Delhi and Lahore but there are another handful of urban areas with over 1 million people that are not represented. In annual AQI above 50 (very polluted cities), agglomerations like Dhaka, Kabul and Karachi are present but not another 40+ cities with almost constantly high level of pollution. Chiang Mai annual average is a lot below at 33.4 and unremarkable among Asian cities.
The full list of cities was also looked at on peak monthly averages and again, cities like New Delhi, Lahore, Dhaka and Kabul top the list at AQI above 150 but another dozen cities are ignored. For averages betweeen AQI 150 and 100, a bracket that include Chiang Mai for March or April during bad smoky seasons (once every 3-4 years on average), there are three other listed cities (Karachi, Accra and Tehran) and another 50 unmentioned cities.
For daily and hourly pollution peak, with the methodology used here in this study, it was too time consuming to manually remove false datapoint and extreme outliers due to other causes than city-wide pollution. There is no reason to believe that Chiang Mai would be particularly outstanding on that basis.
Figure 3. Distribution of large Asian cities (>700000 inhabitants) and their monthly peak AQI average. Cities from the IQAir list are represented as diamonds. Data show the highest pollution levels are found in India & Pakistan cities
Other variables
There is one variable investigated in this document where Chiang Mai dominates the IQAir list. That is the online following of updates on air pollution in Chiang Mai. Even with an extended definition for the population of Chiang Mai, there are 10 times more people (12 million) receiving updated about air pollution in the city. The second city is Wroclaw in Poland with 2 times more people (0.263 million followers) . Seoul, Bern and Batam (Indonesia) have a ratio followers/population of 70%; Bangkok and a handful of other cities are around 50%, etc. One could thinkg the list is an online artifcact of interest, but at the other end of this list, significantly polluted cities in Africa have following around 0.1% of the city population.
There is also no clear correlation between the number of followers, city populatio and the level of pollution. The highest public interest in air pollution while lacking (at a world level) a reason to be concerned is hold by cities in the US, ocassionally labeled as most polluted (in the US or worldwide) such as Los Angeles, Denver, etc. Canberra also has a high following despite having very low pollution levels, possibly due to the very short duration but intense effect of bushfires. Chiang Mai has a very disproportionate interest in air pollution when comparing with dozens of very large, very polluted cities that counts followers in the thousands.
Figure 5. IQAir list of cities with their AQI monthly peak values and the ratio between followers of the AQI of the city and the population of the city. Chiang Mai (circled in green) is a clear outlier while Bangkok (circled in orange) is the high value group.
Is Chiang Mai the most polluted city in the world?
Based on the IQAir selected list, Chiang Mai can be quite often at the top of the list as it has only around 20 cities in direct competition (time-indepedent) and less than 10 in time-dependent records (other cities that reach very high pollution levels in March & April). The reality is that India, for examle, has 3 listed cities (Delhi, Kolkata and Mumbai) all in direct time-dependent competition with Chiang Mai but 50 other equally polluted cities at that time that are not part of the comparison. 50 cities is only considering urban areas of 1 million people and above, not a lower 'municipality' threshold that would put Chiang Mai as a 150000 people urban area that would be in a list of hundreds of very polluted cities.
So is it really the most polluted city in the world? Most of the time, likely not. There is a high possibility that during very intense burning season, between 6 and 10 am (local time), the hourly pollution peak average reach a level where Chiang Mai tops the world's list of large cities (>500000). But by 10am, the pollution generally start to subside. It can still be very high, but so is the spring pollution in China, India, Pakistan, Iran, etc. On paper, using simple statistics, every time Chiang Mai is presented as the most polluted city in the media based on the IQAir listing, there is only 1 chance out of 200 that it is actually true when considering all cities with population above 100000 people that have the potential to reach similar level of pollution at that time.