Severe levels of air pollution and visible smog has had many Bangkok residents reaching for protective masks to avoid its harmful effects. They are right to worry, according to the World Health Organisation, air pollution is estimated to be responsible for the premature death of at least 7 million people around in the world each year.
However, although the main concern for many is avoiding the potentially damaging effects of air pollution on one’s physical health, there is growing evidence that exposure to air pollution can also have negative effect on our mental health, cognitive performance, and brain functioning.
Exposure to elevated levels of air pollution is associated with increased symptoms of anxiety, depression, and psychological distress.
Understanding exactly how air pollution affects psychological well-being is not a straightforward task and researchers who attempt to tackle this question have typically taken the approach of examining census data from many thousands of people who have given information about aspects of their psychological well-being (e.g., anxiety) and their residential history, (amongst many other details).
Because it isn’t practical to measure the exact air pollution people are exposed to in their daily lives, researchers instead rely on using residential data to generate an estimate of people’s exposure to air pollution (either through examining the proximity of their house to a main road or using data from nearby air pollution sensors).
Researchers can then build statistical models to examine the association between this estimated exposure and some outcome (e.g., anxiety), whilst controlling for as many of the other confounding factors that could also influence this outcome (e.g., income, social status, age, occupation, etc.).
Using such an approach, researchers have examined data from over 71,000 female nurses in America and found evidence that estimated exposure to air pollution was associated with an increased risk of high anxiety levels. Similarly, estimated exposure to fine particulate matter (PM2.5) has been associated with increased levels of self-reported psychological distress in another large sample of American participants.
In 2018, researchers in South Korea reported evidence from 124,000 participants indicating that long-term exposure to air pollution was associated with lower perceptions of quality of life, higher levels of depressive symptoms, greater risk of diagnosis for clinical depression and increased suicidal ideation.
Researchers in Hong Kong recently attempted to identify all of the haze days with high air pollution that had occurred in the region between 2007 – 2014. They then used this to examine whether the days of excessive pollution where associated with any changes in mortality rates. In other words, is air pollution triggering people’s deaths. Not only did haze days slightly increase general mortality rates, suggesting it is physically harmful, the effect of the air pollution appeared to be especially pronounced for increasing the likelihood of deaths due to mental and behavioural disorders, suggesting that those individuals with mental health issues may be especially vulnerable to the negative effects of air pollution.
There are clearly a wide range of concerning associations between air pollution and mental health. However, one problem with these approaches is that it isn’t possible to conclude that air pollution actually has a direct, causal effect on the psychological well-being. In other words, we can’t be certain that it is the air pollution that is uniquely responsible for the increased symptoms of anxiety, depression, and psychological distress; despite researchers best attempts to control for all other explanations, it is possible that some other factor or combination of factors is driving the effects instead of air pollution.
Air Pollution Is Associated with Impaired Cognitive Abilities: Of Mice and Men (and Women).
Indeed, one of the problems of examining whether air pollution has a causal effect on psychological factors is that it isn’t ethical to run experiments where we can manipulate and expose people to harmful levels of air pollution; however, we can do such studies using animals.
In one recent study, Laura Fonken and colleagues exposed a sample of mice to the same sort of air pollution that an average (human) commuter might experience in a polluted city. For 10 months (about half their lives), these mice were subjected to high levels of PM2.5 for 6 hours per day, 5 days a week. At the same time they also had control group of mice who were exposed to normal, ambient air. After 10 months they tested both groups of mice on a series of commonly used tasks designed to assess their spatial learning and memory. The group of mice who were exposed to air pollution showed significant impairments in performance and when they examined the brains of the mice, they found that the long term air pollution exposure inhibited the growth of cells in brain regions that are critical for learning and memory.
Related cognitive impairments have also been observed in humans exposed to air pollution and air pollution is claimed to be a risk factor leading to considerable reduction in intelligence.
In a sample of more than 20,000 individuals in China, their (estimated) exposure to air pollution was negatively related to cognitive performance on language and arithmetic tests. When translated in to educational terms, they estimated that the effect of air pollution on reduced cognitive ability / intelligence was, on average, equivalent to losing an entire year of education.
Although this data was again correlational, it seems plausible that there is a causal effect of air pollution as the researchers tracked the performance of the same individuals as air pollution naturally varied, meaning that many other confounding factors that could explain the effect are already accounted for.
Even within young children there appear to be noticeable effects of air pollution on academic performance.
One study in China and one another (separate and larger) study in Spain examining the cognitive abilities of children found that those who studied in schools with high ambient air pollutants were significantly worse on a range of cognitive tasks compared to children from schools with low levels of air pollutant; greater exposure to air pollution predicted stunted cognitive development.
A Risk Factor for Dementia?
The precise mechanisms through which air pollution affects brain functioning is not fully understood, yet it is largely accepted that fine particulate matter (PM2.5), the kind of pollution that is at severe levels across Bangkok, has a damaging effect because these particles are small enough to be capable of reaching deep into the lungs from where they enter the blood stream. From here, the chemical toxins within the particles travel to the brain and can lead to (neuronal) inflammation, triggering a raft of immune responses that disrupt and lead to the loss of neurons.
Neuroinflammation and related processes are well recognised as underpinning neurodegenerative diseases such as dementia and this has lead some researchers to examine whether air pollution may also be a risk factor for dementia. Although no direct evidence currently exists linking air pollution and dementia, the evidence does suggest that air pollution is a plausible risk factor for dementia.
For example, one study tracked the residential data and dementia diagnosis of more 6.6 million Canadians over a decade and found that living next to a main road (and the increased air pollution that comes with this) was associated with a 12% increased risk of going on to develop dementia. Again, there are multiple potentially confounding factors at play here that make it difficult to prove that air pollution causes dementia, but it does further emphasise the need to examine more carefully the potential links.
Does Air Pollution Make Us More Immoral?
Aside from the potentially serious consequences of air pollution on our psychological well-being and cognitive abilities, there is also the possibility that bad air quality is linked to an increase in bad behaviours.
In one series of studies published in 2018, Jackson Lu and colleagues argue that air pollution leads to an increase in anxiety that in turn causes us to engage in more unethical and immoral behaviour. To examine this, researchers first studied the association between air pollution levels and crime rates in over 9000 American cities during a ten year period. After controlling for factors such as the level of poverty and unemployment, the cities with higher air pollution were associated with significantly higher crime rates compared to the low air pollution cities. Whilst this association is interesting, it doesn’t provide evidence of a direct, causal link between air pollution and criminal behaviour.
To address this, the researchers conducted a series of experiments where they asked participants to view photographs illustrating a city with either visibly high or low air pollution levels and vividly imagine living in the area and how it would feel to walk around and breathe in this air.
After participants had finished contemplating living in the area, they were then asked to complete a supposedly unrelated task where it was possible to cheat. For example, one study used a word association test in which participants earned small financial rewards for each correct response. Participants were then told that (due to a computer glitch) it was possible to cheat on this task if they hovered the mouse in a certain location but they were instructed to complete the task honestly.
Thus, the test was really examining their willingness to engage in unethical behaviour. Crucially, they found that participants in the polluted condition were significantly more likely to cheat on the task than those in the clean air condition. In further studies they show that simply imagining being surrounded by high levels of air pollution induces increased anxiety and it is this anxiety that interferes with our moral compass and ultimately leads to more unethical behaviours.
Indirect Effects and The Emotional Response to Air Pollution
There are also inevitable indirect effects on well-being that will occur as a consequence of chronic air pollution. When residents are advised to avoid being outdoors for prolonged periods of time a possible consequence of this is that they may adopt a more sedentary lifestyle, with reduced social interaction that leads to an increased sense of loneliness. All factors that are likely to contribute to reduced psychological well-being. In the short-term this is unlikely to have a serious effect but may become more problematic in the long-term.
Finally, apart from anxiety what other emotional responses might be evoked in response to air pollution?
Two important characteristics of air pollution is that it: (1) represents a real threat to physical and mental health that (2) individuals are largely powerless to deal.
Indeed, there are limited methods for protecting oneself, aside from wearing a filtration mask or moving to a less polluted city; the responsibility and power are instead held by governments and state agencies to implement effective policies. Notably, this combination of feeling threatened and powerless are key factors that underpin the emotional response of anger, and this anger may be even further exacerbated by an increased sense of powerlessness when the relevant authority’s approach is perceived to be ineffective, misguided, or incompetent.
โดย Dr. Harry Manley
About the Author.
Harry Manley is a lecturer in the Faculty of Psychology at Chulalongkorn University. Twitter @harrisonmanley