Exposure to high levels of traffic-related air pollutants in childhood is associated with poorer mental health in later life, according to scientists.
A 25-year study of young UK adults has found higher rates of mental illness symptoms among those exposed to greater levels of air pollutants – such as nitrogen oxides and fine particulate matter PM2.5 – during childhood and teenage years.
Nitrogen dioxide (NO2) and particulate matter (PM2.5) – which are emitted from vehicles, wood burning, industry and farming – are thought to be the two most dangerous air pollutants.
Air pollution can cause lung damage and is associated with up to 36,000 deaths in England every year.
The researchers said the link between air pollution exposure and mental illness symptoms in young adults appears to be modest but added it was still as equally harmful as factors like childhood exposure to lead.
About nine out of 10 people worldwide are exposed to high levels of outdoor air pollutants, according to the World Health Organisation (WHO).
As part of the study, published in Jama Open Network, the scientists looked at 2,000 twins born in England and Wales in 1994-1995 and followed them to young adulthood.
Aside from participating in physical and mental health evaluations, the participants were also measured against their exposure to air pollutants, including nitrogen oxides and PM2.5.
The test subjects were assessed for a range of psychiatric disorders at age 18, using a measure known as psychopathology factor (p-factor).
The researchers looked at symptoms associated with 10 different conditions, including alcohol dependence, attention deficit hyperactivity disorder (ADHD) and generalised anxiety disorder.
They found that over the course of the study, 22% of participants had exposure to nitrogen oxides that exceeded WHO guidelines while 84% had high exposure to PM2.5.
Those exposed to greater levels of pollution during childhood had a higher p-factor score by the time they had reached 18, independent of other risk factors, the researchers added.
Dr Helen Fisher, the study’s co-author and principal investigator from King’s College London’s Institute of Psychiatry, Psychology & Neuroscience, 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.”
According to the researchers, their findings are based on high-income countries with only moderate levels of outdoor air pollutants but have implications for developing countries with higher air pollution exposures, like China, Nepal and India.
Dr Fisher said: “We don’t know what the mental health consequences are of very high air pollution exposures, but that is an important empirical question we are investigating further.”