Unfortunately, India enjoys the dubious distinction of being home to some of the dirtiest cities on planet earth. While the metric for determining what is dirty can vary, one widely used metric is a city’s annual average particulate matter or PM2.5 level. PM2.5 refers to fine, inhalable particles, whose diameters are 2.5 micrometers or less. To get a sense for how small 2.5 micrometers is, consider a single hair from a human head. The average human hair is about 70 micrometers in diameter, therefore making it 30 times larger than the largest fine particle.

A recent ranking shows that when it comes to the world’s top 25 polluted cities using this PM2.5 metric, India is home to 13 such cities including the capital city of New Delhi. The link between air pollution and all manner of human ailments is well known in India and elsewhere and there are many studies that have documented the nexus between air pollution and diseases such as strokes, ischemic heart disease, chronic obstructive pulmonary disease, lung cancer, and pneumonia. 

Even so, what is less discussed, given the stigma associated with mental health issues in India, is the point that there is also a nexus between air pollution and mental health. Recent research sheds valuable light on this nexus with data from the United States. 

Findings of new research
This research uses modern econometric tools to investigate the causal impact of fine particulate air pollution (PM2.5) on mental health in the United States, as measured by the frequency of antidepressant prescription fills. Drawing on a comprehensive national dataset of medical claims, the research provides compelling empirical evidence that short-term exposure to elevated levels of PM2.5 significantly increases the likelihood of individuals filling prescriptions for antidepressants. Specifically, the analysis reveals that a one standard deviation increase in PM2.5 exposure during the previous week leads to a 5.8 percent rise in antidepressant fills in the current week.

That said, it is salient in such empirical studies to establish causality between exposure to air pollution and the filling of antidepressant prescriptions. To address this point, the researchers employ a so-called instrumental variables (IV) strategy using thermal inversions as an instrument. Why? This is because thermal inversions trap air pollutants near the surface, thereby creating exogenous variations in pollution that are not directly tied to human behavior. In this way, the IV approach strengthens the validity of the causal claims made and isolates the impact of air pollution from potential confounders.

The results obtained are robust across multiple model specifications and they withstand a battery of placebo tests, thereby ensuring their credibility. The research finds that the impact of PM2.5 on mental health is especially pronounced among seniors and individuals living in areas with chronically high pollution levels. These subgroups of people demonstrate greater vulnerability, which suggests the presence of compounding risk factors such as age-related sensitivity or chronic exposure effects.

The researchers further explore temporal dynamics and spatial heterogeneity in the pollution-mental health nexus. Their findings indicate that the effect of pollution is immediate and ephemeral: Increases in PM2.5 levels for one week are associated with higher antidepressant use in the following week, but this effect dissipates in subsequent weeks. This ephemerality tells us that acute air pollution episodes can trigger immediate mental health consequences that are not necessarily long-term in nature.

Local interventions
The research under consideration here also examines whether local policy interventions, such as the implementation of pollution control technologies or public health warnings, will attenuate the observed mental health effects. We learn that areas with stronger regulatory environments or greater public awareness campaigns tend to exhibit smaller impacts. In other words, local policy interventions can partially shield people from the insalubrious psychological effects of pollution.

One reason why the findings of this research ought to be instructive for a diverse nation such as India is that the research uses a rich dataset that encompasses millions of observations across different demographics, geographic regions, and time periods. This broad coverage permits a highly granular analysis, and it also enhances the generalizability of the findings. Moreover, the researchers take great care to ensure that they have correctly isolated the pollution effect when making claims about the deleterious impacts of exposure to air pollution for mental health.

A final point
Given the rising concern about air quality in urban and industrialised areas, this research rightly contends that mental health outcomes should be factored into cost-benefit analyses of environmental regulations. This is particularly salient because most air pollution studies focus primarily on physical health outcomes like respiratory or cardiovascular diseases, but the research I am discussing clearly highlights the less visible, yet equally serious, psychological toll that people in polluted environments confront. 

(Views expressed are personal)