People living in areas with higher levels of traffic-related air pollution are at increased risk for mental illness symptoms compared to those breathing cleaner air, a study published Wednesday by JAMA Network Open found.

Those exposed to large amounts of nitrogen oxides – the fumes produced by gas- and oil-burning vehicles and other sources – in childhood and adolescence were more likely to show signs of mental illness as they transition to adulthood, the researchers said.

As many as nine out of 10 people globally are exposed to high levels of air pollutants from fossil fuel combustion in vehicles, power plants and many manufacturing, waste-disposal and industrial processes, according to the World Health Organization.

The new findings reveal that “air pollution is likely a non-specific risk factor for mental illness writ large,” study co-author Helen Fisher said in a press release.

Those findings build on prior studies that show increased hospital admissions for many psychiatric illnesses during “poor” air quality days in countries like China and India, said Fisher, a researcher at King’s College London’s Institute of Psychiatry, Psychology and Neuroscience in England.

Previous studies have identified a link between air pollution and the risk of specific mental disorders, including depression and anxiety.

However, this study looked at changes in mental health that span all disorders and signs of psychological distress associated with exposure to traffic-related air pollutants.

The researchers analyzed air pollution exposure and mental health data among a group of 2,000 twins born in England and Wales in 1994 or 1995 that was followed to young adulthood.

All of the study participants underwent regular physical and mental health evaluations and provided information about the larger communities in which they live.

The researchers measured exposure to air pollutants by modeling air quality around participants’ childhood homes, using data provided by the British National Atmospheric Emissions Inventory and other sources.

Participant mental health at age 18 also was assessed for 10 different psychiatric issues, including alcohol, marijuana or tobacco dependence; attention-deficit-hyperactivity disorder; major depression; generalized anxiety disorder; and post-traumatic stress disorder.

These assessments were used to calculate a single measure of mental health, called the psychopathology factor, or “p-factor” for short, according to the researchers.

The higher a participant’s p-factor score, the greater the number and severity of psychiatric symptoms identified, they said.

Twenty-two percent of the study participants were found to have exposure to nitrogen oxides that exceeded WHO guidelines, and 84 percent had exposure to fine particulate matter, called PM2.5, that exceeded guidelines.

Participants’ exposed to higher levels of nitrogen oxides, but not PM2.5, during childhood had, on average, a two-point increase in their p-factor at age 18.

Age 18 is the age at which most symptoms of mental illness have emerged or begin to emerge, the researchers said.

While air pollution levels were greater in neighborhoods with worse economic, physical and social conditions, the study found that risk for mental illness based on exposure remained the same across all areas.

“We don’t know what the mental health consequences are of very high air pollution exposures,” study co-author Aaron Reuben said.

However, “because harmful exposures are so widespread around the world, outdoor air pollutants could be a significant contributor to the global burden of psychiatric disease,” said Reuben, a graduate student in clinical psychology at Duke University in Durham, North Carolina.