Interview: How people exposed to air pollution are more likely to show symptoms of depression

Poor air quality in North India, including in the National Capital Region which has been enveloped in a thick smog for weeks in November, has could be hurting not only the physical health of millions of Indians but also their mental well being. Scientists from China who have been studying the effects of air pollution on mental health say that it is a major risk factor for depressive symptoms in developing countries.

The study conducted by researchers from the International Food Policy Research Institute, Peking University, Yale University and Beijing Normal University has shown that high levels of air pollution significantly raise the chances of people having symptoms of depression.

The researchers collected data from the China Family Panel Studies, a nationally representative survey of Chinese communities, families and individuals conducted in 2010, 2012, and 2014. The China Family Panel Studies analyses families and individuals from 162 counties in 25 provinces of China, including their economic activities, education outcomes, family dynamics and relationships, health, and subjective well-being. The final sample size for this research included 24,000 individuals.

The research team measured air quality using air pollution index based on concentrations of air pollutants, including sulfur dioxide, nitrogen dioxide, and fine particulate matter smaller than 10 micrometers referred to as PM10.

Researchers estimated the level of interview subjects’ air pollution exposure from the locations where the interviews took place. They looked for an association between the air pollution index in that locality and the subjects’ self-reported level of mental health symptoms, of “hedonistic happiness” and of “evaluative happiness”. Hedonic happiness refers to the immediate experience of happiness while evaluative happiness is an overall assessment of a person’s life and is less likely change with the external environment.

They measured different aspects of people’s mental health and short-term and long-term wellbeing from their answers to questions in a psychological distress survey. One question asked people to rate their overall satisfaction with their lives, which is determined by factors such as education, income and social status, on a scale of one to five. Another asked them to report symptoms of depression. A third question asked how often, in the previous month, a person found it hard to cheer up, which is a measure of short-term hedonistic happiness.

By studying the data, the study found that people exposed to higher levels of air pollution reported lower levels of hedonistic happiness.

The findings have contributed to the debate about the Easterlin paradox, that is, the observation that over time happiness does not display a strong correlation with income growth, even though rich people are generally happier than the poor at any given time. For instance, as the authors of the study point out, China’s levels of both hedonistic happiness and life satisfaction have stagnated despite economic growth and even after the return of welfare measures, improved employment and reduced income inequality. This study is among the first to simultaneously evaluate the significance of air pollution in explaining the stagnant or even declining happiness.

Xiaobo Zhang, senior research fellow at International Food Policy Research Institute, distinguished professor of economics at Peking University and lead author of the study, spoke to Scroll.in. Here are the excerpts of the interview.

Why did your group decide to study the link between air pollution and mental health?
When I first returned to China in 2012 from the United States, during the smog days in the Beijing, I experienced frequent headaches, generally felt gloomy and unhappy. While there is more evidence on the negative impact on physical health, the studies on the impact on subjective well-being is much more limited.

Considering that happiness is one of the most important social welfare indicators, I suggested that my PhD student Xin Zhang examine the link between air pollution and mental health as her dissertation research. Later on, I sent Xin to Yale University to further work with Professor Xi Chen (Department of Health Policy and Management, Department of Economics, Yale University) to finish the study.

In our study, we assess the impact of air pollution on several key dimensions, including depressive symptoms, moment-to-moment happiness, and evaluative happiness. We find air pollution reduces hedonic happiness and increases the rate of depressive symptoms. Our results shed light on air pollution as an important contributor to the Easterlin paradox that economic growth may not bring more happiness.

Both physiological and psychological pathways may explain the link between air pollution and mental illness. In the short term, air pollution causes more mental problems through headache and head tightness, eye irritation, attention problems, and fatigue. In the long term, the fine particulates (carrying chemicals and toxics) penetrating the brain may directly affect brain chemistry, structure and function of brains. These substances that penetrate the brain may also trigger an inflammatory response in the central nervous system. Besides, certain pollutants, such as CO, may inhibit the body’s ability to release oxygen and therefore affect mental health.

Is the adverse impact of air pollution on mental health a temporary effect? Do you think it can lead to chronic depression?
We found that a person is more likely to display depressive symptoms at the time of severe air pollution. It is a future research question to find out whether the impact is lasting or not. In another study, we do find air pollution has a lasting adverse impact on cognitive performance in math and verbal tests. The impact lasts for at least two years. We found such impact lasting even four years.

In your study, you say that hedonic happiness is impacted adversely by air pollution but life satisfaction is not affected? Why is that?
Our survey has two sets of questions: (1) Are you generally satisfied with your life? (2) Are you happy now? We find air pollution only matters to hedonic happiness shown in the second question (moment to moment happiness) but not life satisfaction (the first question). Air pollution lowers hedonic happiness and raises the rate of depressive symptoms. In particular, people who are more concerned with environmental problems, work outdoors, earn lower incomes, reside in less polluted areas, or have young children are more sensitive to air pollution.

The question of life satisfaction is more related to the assessment of overall life quality. It is not surprising that life satisfaction has little to do with the air pollution level at the moment. Life satisfaction is more associated with income level, inequality with a reference group, health status, and marital status.

Given that you find that air pollution and mental health are linked, do you see a rise in the incidence of mental health problems such as depression in countries like China and India where pollution levels are soaring?
It is very likely that incidence of depression will go up as air pollution gets worse. In another study, we find a negative impact on cognitive test scores (math and verbal).

The effect is particularly stronger for men, those working outdoors, and those with lower levels of education. The results also suggest that young adults respond more strongly to air pollution than elderly. There are several possible explanations. First, older people are probably more accustomed to living in the dirty air. Second, young people spend more time outdoors than senior people. Third, young people are likely more informed of air quality thanks to wide access to smartphones and the Internet, therefore paying more attention to air pollution. Also, the poor will suffer more [get more depressed] than the rich.