The growing link between pollution and mental health

By Dr Layla McCay, Director, Centre for Urban Design and Mental Health

In many cities around the world, air pollution feeds into people’s decisions about how they spend their time. They may avoid walking, cycling or group exercise for fear of health problems when air quality is poor. Missing out on these healthy activities can be detrimental to people’s health, but it is well known that air quality also directly influences people’s physical health.

According to the World Health Organisation, air pollution increases people’s risk of heart disease, stroke, lung cancer, asthma and other lung diseases. Air pollution caused three million premature deaths in 2012 alone, and is largely responsible for 7.6 percent of all deaths worldwide. However, physical illness may not be the only way in which air pollution affects our health. What is less well understood is the effect of air pollution on mental health.

Over the past several years, evidence has started to point to links between air pollution and mental health. In a recent Journal of Urban Design and Mental Health, researcher Jacob King conducted a new review examining what is currently known about this proposed link. He found that while knowledge is still at an early stage, research findings show emerging evidence that convincingly links air pollution to a range of mental health outcomes from depression and anxiety to psychosis, dementia, childhood cognitive development, and suicide.

Research findings show emerging evidence that convincingly links air pollution to a range of mental health outcomes

King’s review of current research found that increases in NO2 exposure have been associated with significant increases in depression and use of psychiatric medication, and that emergency department attendances for depressive episodes seem to be more likely when air quality is poor. Another study found that nurses with increased exposure to PM2.5 particulate matter were more likely to have anxiety symptoms, and in a Swedish study, children living in an area with higher levels of PM10, PM2.5 and NO2 were more likely to be dispensed psychiatric medication. Furthermore, CO, O3, NOx, and fine and coarse particulate matter exposure were found to be associated with developing dementia. None of these studies prove a causative link. However, they do provide reasonable evidence that increasing atmospheric concentrations of a number of air pollutants are associated to varying degrees with a range of mental health disorders.

At this time, it is hard to quantify the size of that effect. This is because people’s exposure to air pollution as they move around the city is notoriously difficult to isolate and measure accurately. Some studies estimate air pollution exposure in terms of the distance from someone’s home to a busy road. But there may be other relevant factors associated with living near a busy road that affect mental health independently of air pollution, such as noise, or traffic safety concerns, or poverty, or transit connectivity.

Another challenge is that most of the research has been conducted in high-income countries. Particularly in low and middle-income settings, the research suggests that some of the negative effects of increased air pollution on mental health may be mitigated by the economic, cultural and social benefits of living in the city. Again, the different impacts of these various factors can be difficult to pick apart and measure accurately.

We also do not currently understand how air pollution might exert this effect on mental health. There are two main theories: either air pollution might create an inflammatory response in the brain (the neuro-inflammatory hypothesis), or else exposure to air pollution might help ‘turn on’ the expression of certain genes that are implicated in mental illnesses (the gene-environment interaction hypothesis). People also seem to be psychologically affected by the perception of air pollution. These mechanisms are currently speculation and more research is warranted.

Around the world, cities are cleaning up air quality, and investing in urban planning and design strategies to separate pedestrians from air pollution. There is already a clear public health case for this investment. The current evidence for links between air pollution and mental health may be still emerging, but it is promising. At this stage it is reasonable to add mental health promotion to the case for improving air quality. It would also be appropriate to add the reduction of air pollution exposure to the toolkit of known measures that can support good mental health in the built environment. This should most appropriately be considered alongside other factors for which the links to mental health are more established, such as access to nature, physical activity, safety, sleep quality, and positive social interactions.

To read the research in full, see: Journal of Urban Design and Mental Health