Air pollution is a major environmental risk to health: the link between air pollution and health conditions such as respiratory and cardiovascular diseases are well established. Outdoor air pollution in cities and rural areas is estimated to cause more than 4 million premature deaths a year worldwide, primarily due to heart disease, strokes, chronic obstructive pulmonary disease and acute lower respiratory infections.

Recent research is also making a connection between air pollution and mental health conditions. A 2017 study from researchers at the University of Washington, for example, found that people who live in areas with high levels of air pollution report higher levels of psychological distress.

A study published in the British Medical Journal in 2018 found “a positive association between levels of air pollution across London and being diagnosed with dementia, which is unexplained by known confounding factors.” The specific pollutants most strongly associated were nitrogen dioxide (NO₂) and particulate matter (PM_2.5). The study looked at both vascular dementia and Alzheimer’s and found that the association with air pollution held true for Alzheimer’s, but not for vascular dementia.

Dementia, including both vascular dementia and Alzheimer’s disease, is the leading cause of death in England and Whales. Addressing air pollution as an environmental risk factor for mental health conditions could lead to substantial public health gain, the authors suggest, even if the impact was only to delay the progression of Alzheimer’s.

Another study found that children with existing mental health conditions were more vulnerable to the physical adverse effects of air pollution.

A study published last year in the journal PLoS ONE used nationwide population data from Korea to look at the association between long-term exposure to air pollution and mental health status. The study found a positive association between air pollution, and stress, depressive disorders and suicide, after adjusting for confounding factors (such as socioeconomic status and medical history). The authors conclude that long-term exposure to air pollution is a risk factor for a range of mental health disorders.

Other research has linked short-term exposure to peaks in air pollution to an increased risk of death among people with serious mental illness. Research, published in the journal Environment International, analyzed the link between haze days and mortality. Haze days are those on which pollutants gather in the air and limit visibility, usually dry days with low winds. The study found that haze events had a much more intense adverse effect on people with mental and behavior disorders than on the general public.

These studies have found associations between air pollution and mental health problems, but do not indicate whether the pollution causes mental health problems or how it might influence mental health. Researchers have suggested several possible ways air pollution can affect mental health. When air pollutants are inhaled they can lead to systemic inflammation, which in turn can worsen depression and anxiety. Increased levels of physical health problems from air pollution, such as asthma and congestive heart failure, can also increase anxiety and other mental illness. In addition, elevated levels of air pollution may lead to people to spend less time outside and to have a more sedentary, isolated lifestyle. Lack of physical activity and social isolation can be related to poorer mental health and greater psychological distress.