The Impact of Air Pollution on Mental Health

The World Health Organization estimates that air pollution causes roughly 3 million premature deaths every year (Vigo et al 2016). Air pollution is a generic term that includes a variety of pollutants including gases, metallic and organic compounds, and particulate matter (PM). Most negative physical and mental health consequences of air pollution are caused by the effects of PM on the body and brain.

PM in the atmosphere is caused by both human activity and natural processes. Leading causes of PM include combustion of fossil fuels (e.g. from automobile exhaust), industrial processes, and large-scale agricultural operations. Natural sources of PM include windblown dust and wildfires. Exposure to high concentrations of PM during periods of high air pollution that take place when certain atmospheric conditions prevail or in communities located close to wildfires can exacerbate lung and heart conditions and increase deaths and hospital admissions. Children, the elderly, and individuals with respiratory and cardiovascular disease are especially susceptible to the negative health impacts of air pollution. In recent years global warming has resulted in dramatic increases in the frequency, size, and severity of wildfires in Australia, the Western United states, Siberia, the Amazon basin, and other world regions.

This post is offered as a concise summary of accumulating research findings on the mental health consequences of exposure to atmospheric particular matter (PM).

Harmful Effects of Particulate Matter (PM) on Health: Possible Mechanisms of Action

The deleterious effects of PM from industrial air pollution, large-scale agriculture, automotive exhaust, and other sources on respiratory and cardiovascular disorders are well understood. However, studies on the potential mental health impact of air pollution—including smoke from wildfires—are at an early stage. Accumulating findings of animal and human studies support that PM causes inflammation in the central nervous system which, in turn, may increase the risk of developing depressed mood, anxiety disorders, bipolar disorder, and other mental health problems (Barron et al. 2017).

There is limited evidence that exposure to PM (or other pollutants) may adversely affect cognitive development (i.e. when exposure takes place in early childhood), result in chronic stress, interfere with normal cognitive performance, and possibly increase the risk of developing schizophrenia and dementia.

Animal and human studies have established that exposure to particulate matter (PM) in the air causes oxidative stress resulting in inflammation in the brain and body, has direct neurotoxic effects on different structures in the brain, and interferes with normal production of cortisol—the body’s major stress hormone—by disrupting the hypothalamic-pituitary-adrenal axis. The effects of air pollution on brain function may also be mediated by complex inter-relationships between inflammation, oxidative stress, and physical inactivity, obesity, and lack of sleep. Depending on the individual’s medical and psychiatric history, the chronicity of exposure, and type of exposure, some or all of these mechanisms may play a role when an individual is exposed to PM in the air, resulting in increased risk of developing depressed mood, anxiety and possibly other neuropsychiatric disorders. The deleterious consequences of PM on mental health may be more severe when exposure takes place in childhood and adolescence.

Exposure to particular matter (PM) may increase the risk of developing specific psychiatric disorders. A systematic review and meta-analysis of English-language studies published between 1974 and 2017 found a statistically significant association between long-term exposure to PM less than 2.5 microns in diameter (PM<2.5 microns) and the risk of depression; a possible association between long-term exposure to PM<2.5 microns in diameter and anxiety; and a possible association between short-term exposure to PM<10 microns in diameter and suicide risk (Braithwaite et al 2019).

The significance of these findings is limited by differences in study designs. These findings are similar in magnitude to deleterious effects of long-term exposure to PM<2.5 microns on general health. For example, it has been established that long-term exposure to PM<2.5 microns is associated with a 10 percent increase in all-cause mortality (Brook et al 2010). The authors found a statistically significant association between short-term PM<10 exposure and increased suicide risk, and evidence for an association between short-term PM<2.5 or PM<10 exposure and depression-related emergency room visits. No evidence was found for an association between PM exposure and risk of bipolar disorder or schizophrenia.

In addition to increasing the risk of depressed mood and other mental health problems, exposure to air pollution may have large-scale negative effects on subjective feelings of well-being. The authors of a recently published study that analyzed over 210 million social media ‘tweets’ reported that individuals exposed to PM<2.5 may have generally lower feelings of well-being (Zheng et al 2019).

Implications for Future Research and Policy

The relationship between exposure to PM and other components of air pollution and increased risk of developing depressed mood and other mental health problems is probably multi-factorial. Underlying mechanisms and the roles they play with respect to different psychiatric disorders is poorly understood and may vary across psychiatric disorders. Further toxicological animal studies and epidemiological human studies are needed to clarify causal relationships between exposure to air pollution in the context of socioeconomic status, other sociocultural factors and health factors, and increased risk of depressed mood and other psychiatric disorders.

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Since less-developed world regions often have more polluted air, the above findings have important policy implications for programs that address the public health concerns of underserved populations. Such health inequalities call for policies addressed at improving air quality (Gascon et al. 2015; Woodcock et al. 2009).

Bottom line

The impact of air pollution—including fine particulate matter (PM)—on physical and mental health is a direct consequence of global warming and can be expected to worsen as global warming increases in the coming decades. The catastrophic wildfires currently raging in Australia have led to increased public awareness of the enormous public health consequences of this problem globally.

Most negative physical and mental health consequences of air pollution are caused by the effects of PM on the body and brain. Accumulating findings of animal and human studies show that PM causes inflammation in the central nervous system, which, in turn, may increase the risk of developing depressed mood, and possibly also anxiety disorders, bipolar disorder, and other psychiatric disorders (Barron et al. 2017). Long-term exposure to air pollution may also have large-scale negative effects on general well-being. Large studies are needed to further investigate possible causal relationships between PM and other components of air pollution and risk of developing depressed mood and other psychiatric disorders. Finally, aggressive policies aimed at improving air quality are needed to address public-health inequalities that result in greater exposure to PM in less developed world regions.