The study, which lasted over 25 years, followed young adults living in the UK and the US. To reduce the likelihood of individual biological health impacting the study they went to the extra measure of only following twins, all of whom (2000) were born between 1994 and 1995.

All participants had regular physical and mental health evaluations that gave the researchers further information about the communities in which they lived. Participants were measured against their exposure to air pollutants – nitrogen oxides and fine particulate matter, which are regulated pollutants in urban areas.

The World Health Organization (WHO) estimates as of 2021 that 9 out of 10 people worldwide are exposed to high levels of outdoor pollutants.

The most common pollutants are emitted by fossil fuel combustion vehicles, powerplants and also by industrial level manufacturing, waste-disposal and industrial processes. Over the course of the study the researchers found that 22% of participants had a worrying level of exposure to nitrogen oxides, so much so that it actually exceeded the WHO guidelines. They also found that 84% had high exposure to particulate matter.

This was then compared to mental health assessments. Participants were assessed at age 18 on symptoms associated with 10 different psychiatric disorders, these included: alcohol dependence, ADHD and generalised anxiety disorder. To measure this they created a psychopathology factor score: a high psychopathology factor score would mean a greater number of psychiatric disorders and severity of symptoms recorded.

The researchers found that those that had been exposed to greater levels of pollution had a higher psychopathology factor score by the time they reached 18, independent of other risk factors.

Dr Helen Fisher, the study’s co author and a Principal Investigator from Kings College London said, “This study has demonstrated that children growing up in our biggest cities face a greater risk of mental illness due to higher levels of traffic. While we might like to think of our towns and cities as green and open spaces, it’s clear that there is a hidden danger that many will not have even considered.”

These results, based on cohorts in higher income countries with only moderate levels of outdoor pollutants, raise questions about low-income developing countries with high air pollutant exposures such as China, Nepal and India. It is a question Dr Fisher is particularly concerned about and has noted that they are investigating it further.

Especially after the Mental Health Foundation’s Mental Health Awareness week’s survey on British teens relationship between nature and mental health in dense, urban areas, the findings from this research give even more weight to the need for governments and local councils/authorities to invest in ways to increase access to green spaces and decrease exposure to pollutants for those young people living in inner cities.