Chiang Mai Air pollution
Cigarette Equivalent
(for general information about the burning season, click here)

Figure 1. Air pollution (particulate matter) maps of Northern Thailand with an adapted scale expressed as cigarette equivalent (1 cigarette = 66 μg/m3). Left: Annual PM2.5 map for the year 2021 (Wongnakae et al., 2023). Right: Peak PM2.5 on the 8th of April 2010, a particular bad haze season (Phayungwiwatthanakoon et al., 2014)

What is the cigarette-equivalent?

The original idea comes from several researchers who attempted a decade ago to express air pollution as an equivalent in tobacco smoking, a more understandable value for the general public. In 2011, an article by Arden Pope (Pope et al., 2011) and a blog of Beijing-based medical doctor Richard St-Cyr concluded that a day in Beijing is equivalent to 1/6th of a cigarette. The concept took off however in 2015 when a non-profit environmental organization declared that a day in Beijing pollution is equivalent to 38 cigarette a day (Muller & Muller, 2015). This extremely high and anxiogenic value got the attention of the media and has been applied everywhere since then without much thought, with an equivalence to air pollution of 1 cigarette = 22 μg/m3.

Figure 2. Original map of Eastern China with air pollution daily average translated into a cigarette-equivalent map with a value of 22 μg/m3 (Muller & Muller, 2015).

There is absolutely no doubt that both tobacco smoke and air pollution have detrimental effects on health. However, the idea that children in Beijing regularly (*) breathe the equivalent of two pack of cigarettes every day with the very, very severe health effects associated with it should raise some eyebrows. In this section, I explain quickly how these calculations are made and why values should not be taken completely in practical terms.

(*) the air pollution in Beijing has considerably improved in the last decade so some comparison are no longer valid

How these original values are calculated?

Both estimations are very simple calculations and is a sign that it might not be representative at all of the situation, especially when applied worldwide.

The 2011 Pope et al. estimation is based on particulate matter (PM) inhalation. Since one cigarette produces 1000 to 15000 μg of PM (tar content of 1 to 15mg), a direct relationship with the PM concentration in Beijing air pollution can be made. For an average cigarette with 6 mg of tar and the average Beijing pollution of 90 μg/m3 with 20 m3 inhaled per day, the relationship comes back to 0.3 cigarette/day as a yearly average.

The 2015 Muller & Muller estimation is based on mortality. They compared the mortality due to tobacco consumption in the US population (1.37 death per million cigarettes) and compared it with their own unpublished estimate of the number of death in China due to air pollution (1.6 million deaths) for an average country-wide annual PM2.5 value of 52 μg/m3 (Rohde & Muller, 2015). The equivalent dose of a cigarette from this calculation is 22 μg/m3.

As said, both approaches are too simplistic and misleading depending on the narrative associated with it. Pope et al. (2011) eventually brought some nuances and comments on why his particularly low values (0.3 cig/day) are probably underestimated. They acknowledge that there are substantial differences between voluntary, episodic but intense inhalation of smoke by an individual and a 24h exposure to small but significant amount by the whole population (See Dr. St-Cyr blog). Muller (2015), by default, is overestimating the value due to many factors but the most obvious is that the smokers population is seemingly not subtracted from the estimation. Considering that smoking practices are identical in the US and China (they are probably worse, especially considering second-hand smoke), with their assumption that half a million people die of smoking in the US, then it should be between 1.4 and 2.1 million deaths in China depending on how it is calculated (WHO put this value at 'above 1 million). Since it's unclear how they end up with 1.6 million deaths due to air pollution and what is the role of tobacco in it, it creates considerable uncertainty on the final value.

Can these values be used in Thailand and Chiang Mai ?

As a broad approach, for Bangkok maybe, since the air pollution is not that different from Chinese megacities. Based on these values, Chiang Mai burning season would be equivalent to 0.4 cig/day (Pope et al., 2011) and 4.5 cig/day (Muller, 2015) with daily variations from 2 to 15 cig/day. Chiang Mai however has a highly episodic air pollution with a drastically different composition (biomass burning is not equivalent to traffic and industrial sources). Therefore, it requires a recalculation and the specific conditions in Chiang Mai allows to approach the problem differently. with the original data available in Pirard & Charoenpanwutikul, (2023)

The approaches here are compositional (comparing toxic chemical components in cigarette smoke and haze in Chiang Mai, Bangkok and Beijing) and another approach based on excess mortality. Since Chiang Mai has a very episodical haze, the mortality of air pollution can be estimated by comparing the death rate during and outside the burning season and a comparison with other areas of Thailand when necessary. Since publications for Northern Thailand long-term effects have not established a statistically significant relationship with biomass burning pollution, only provincial average anomalies compared to national average have been considered (modulated on tobacco consumption).

Figure 3: Representation of the cigarette equivalent of each major category of pollutant and mortality rates for Chiang Mai. This figure does not represent the specific toxicity of each component for various reasons explained in the text.

The Particulate Matter calculation

The PM2.5 (and total particulate matter content) approach is identical to Pope et al. (2011). For Chiang Mai, it comes back to 10-15 cigarettes per year but these values can be as high as 0.5 to 1 cig/day at the worst time of the haze season (>350 μg/m3). Bangkok and Beijing have similar values around 10 cigarette per year with peak values at 1 cigarette per week.

Heavy metals calculation

Metals are known toxic substances in cigarette smoke, particularly cadmium, arsenic, chromium, nickel and lead. Because concentrations are very low (in tobacco and in normal air pollution), cigarette-equivalent values can vary widely and do not automatically reflect an issue (ex: it could be 100x higher, but still 100x below what would be considered a public health issue). In Chiang Mai, lead, nickel and chromium are equivalent to 1 cig/day; mercury data has very large errors but is low on annual average. Cadmium and arsenic are at concentration equivalent to 1 cig/week and copper, zinc and vanadium at 1 cig/month.

The industrial and traffic lead emission in Bangkok and Beijing are equivalent to smoking 5 to 30 cig/day. Nickel, chromium and mercury are equivalent to 1 cig/day and copper, zinc and vanadium at 1 cig/week. Other elements (Al, Si, Ca, Mg, K, Na, etc.) can have very high cigarette equivalent values but this is due to their low concentration in tobacco smoke and high concentration in ground dust. These are not compiled in the final calculation has their toxicity is very low.

Figure 3. Daily cigarette equivalent plot for various metals concentrations in the air pollution of Chiang Mai, Bangkok and Beijing. Annual averages and maximum values are provided.

Gases content calculation

Gases are abundantly emitted by cigarette smoke due to the high temperatures achieved during smoking. Some gases such as carbon monoxide (CO), nitrogen oxides (NOx), hydrogen cyanide (HCN), carbonyl sulfide (OCS) and hydrogen disulfide (H2S) are a lot more present in cigarette smoke than air pollution. The equivalent dose for these gases is often below 1 cigarette per year. Sulfur dioxide (SO2) on the other hand, has higher concentration in air compared to tobacco smoke (where most of the sulfur is emitted as H2S) and as a result the cigarette/day equivalent during Chiang Mai burning season 100 and 10x higher in Bangkok while not being excessively high in absolute terms (=> not a major health concern). It is even more extreme for ozone, present in small amount in urban air pollution but basically absent from tobacco smoke.

Volatile Organic Compounds calculation

Volatile Organic Compounds (VOC) include simple and complex carbon-based molecules, some of them with high toxicity values. In Chiang Mai, benzene, xylene and toluene can be equivalent to 15 to 80 cig/day but annual values are below 1 cig/day. In Bangkok, these three chemicals can reach concentrations equivalent to 100 to 500 cig/day. Beijing has lower values but average annual values slightly higher than Chiang Mai.

Aldehydes (formaldehyde, acetaldehyde) are important toxic components in cigarette smoke, resulting in an equivalence of 1 cig/week during the burning season in Chiang Mai while Bangkok and Beijing do not have such high values but have higher annual averages.

Polycyclic Aromatic Hydrocarbons (PAH) during the burning season represent 1 cig/day but the highly carcinogenic PAHs (ex: benzo[a]pyrene) are less present. In Bangkok and Beijing, these compounds are 10x higher.

1, 3 Butadiene is a concerning tobacco smoke component but its presence in air pollution is very low, amounting to less than a cigarette per year. The same goes with many other tobacco specific toxic products such as acrolein, aniline, nicotine, phenols, nitrosamines, etc. which are basically absent from air pollution when compared to cigarette smoke.

Figure 3. Daily cigarette equivalent plot for various gases and volatile organic compounds in the air pollution of Chiang Mai, Bangkok and Beijing. Annual averages and maximum values are provided. CO: Carbon Monoxide, NOx: Nitrous oxides, SO2: Sulfur dioxide, NH3: Ammonia, HCN: Hydrogen cyanide, OCS: Carbonyl sulfide, CH2O: Formaldehyde, CH3CHO: Acetaldehyde, Bz: Benzene, Xyl: Xylene, Tol: Toluene, PAH: Polycyclic Aromatic Hydrocarbons, BaP: Benzo[a]pyrene, 1,3But: 1,3 Butadiene, Isop: Isoprene.

Mortality-based calculation

Calculation for mortality are based on published material on the increase of all-cause mortality but also mortality associated with cardiovascular, respiratory and cerebrovascular diseases in Northern Thailand when air pollution increases by 10 μg/m3. The relatively high quality of these datasets is due to an air pollution increasing by 10x or more between the rainy season and the burning season, giving considerable contrast in health effects. Cities with constantly medium to high background values are statistically less robust. The calculation here assumes a population of 7 million people for Upper Northern Thailand and a yearly air pollution modeled as 5% of 50 μg/m3, 5% at100 μg/m3 and 5% at 150 μg/m3 daily averages. A background level is calculated based on mortality anomalies between regions of Thailand, percentage of smokers and percentage of diseases associated with smokers. The effect of household and occupational environmental pollution cannot be objectively subtracted with the data currently available.

Based on these calculations, the burning season is equivalent to 1-2 cigarette per day for all-causes mortality and 1 to 10 cigarette per day for specific diseases. The higher values for specific diseases might reflect the higher impact of air pollution on sensitive individuals with pre-conditions (and in the context of this calculation, a healthy person smoking one pack of cigarette will have less acute health conditions than an asthmatic individual smoking the same quantity).

Other mortality estimations from WHO, Greenpeace and various environmental organizations are based on broad methodologies that are susceptible to high bias and/or inclusive of many effects not related to standard air pollution. Most of these estimations do not use factual data (other than PM2.5 levels) from the studied location with no attempt to modulate the impact of air pollution on mortality (ex: data is often calibrated in cities where industrial, traffic and urban pollution is potentially considerably more toxic than biomass burning or desert dust). When this mortality data is processed into cigarette equivalent, some results appear unrealistic (ex: 2 months of haze in Chiang Mai calculated on WHO stroke mortality gives an equivalent of 100 cig/day, 5 packs a day or 300-400 packs of cigarette for the duration of the burning season).

Figure 4. Daily cigarette equivalent plot for particulate matter (PM2.5 and PM) and all-cause and specific mortalities. [1] values are based on published material on the specific health effects of Chiang Mai and/or Northern Thailand pollution. [2] values are based on the national average mortality causes published by the World Health Organization. COPD: Chronic Obstructive Pulmonary Disease; IHD: Ischemic Heart Disease.

Final thoughts

To conclude this article, these numbers have to be considered with some skepticism in mind. Compositional comparison is informative but hide the fact that tobacco smoke has numerous toxic components that are simply absent from standard air pollution. Synergistic effects would also have to be considered in Tobacco smoke and air pollution to properly assess these compositional effects. Mortality-based calculations remain relatively inaccurate as they are subject to a large number of confounders that would have to be properly assessed both from air pollution and tobacco-smoking perspectives and would require a considerable amount of work to be quantified.

However, the average Chiang Mai value of 66 μg/m3 per cigarette calculated on mortality compared to the 3x higher value of Muller & Muller (2015) of 22 μg/m3 can be put in parallel with compositional comparison between Chiang Mai, Bangkok and Beijing. Although PM levels in Chiang Mai can be very high, on a yearly average, these are similar to these two megacities. On the other hand, toxic chemicals annual average concentrations show equal or systematically higher values in Bangkok and Beijing. A simple average of calculated toxic components in Bangkok and Beijing show that the air is 12x more toxic than Chiang Mai on an annual average.

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Chiang Mai Air pollution Cigarette Equivalent (for general information about the burning season, click here)

Chiang Mai Air pollution
Cigarette Equivalent
(for general information about the burning season, click here)