The researchers found that young adults are more likely to exhibit symptoms of mental illness if they were exposed to higher levels of traffic-related air pollutants, especially nitrogen oxides, during childhood.
The findings indicate that the greater an individual’s exposure to nitrogen oxides, the more likely they are to show signs of mental illness at the transition to adulthood at age 18. This is when most symptoms of mental illness begin to emerge.
Study first author Aaron Reuben said that the link between air pollution exposure and young adult mental illness symptoms is modest, but because harmful exposures are so widespread around the world, outdoor air pollutants could be a significant contributor to the global burden of psychiatric disease.
The study showed that air pollution, which is a neurotoxicant, is a weaker risk factor for mental illness than better-known risks such as family history, but is equal to other neurotoxicants that are known to harm mental health, such as childhood exposure to lead.
Study co-author Helen Fisher of King’s College London’s Institute of Psychiatry, Psychology & Neuroscience was involved in a previous study that linked childhood air pollution exposure to the risk of psychotic experiences in young adulthood. This suggests that air pollutants increase the risk for psychosis later in life.
For the current study, the researchers measured exposure to air pollutants – particularly nitrogen oxides (NOx) and fine particulate matter (PM2.5) – by modeling air quality around study member’s homes at ages 10 and 18 years.
Overall, 22 percent of the individuals had exposure to NOx that exceeded WHO guidelines, and 84 percent had exposure to PM2.5 that exceeded guidelines.
The participants mental health was assessed at age 18, based on symptoms associated with ten different psychiatric disorders: dependence on alcohol, cannabis, or tobacco; conduct disorder and attention-deficit/hyperactivity disorder; major depression, generalized anxiety disorder, post-traumatic stress disorder, and eating disorder; and thought disorder symptoms related to psychosis. The symptoms were used to calculate a single measure of mental health, called the psychopathology factor, or “p-factor.”
Individuals with a higher p-factor score had a higher number of psychiatric symptoms with greater severity.
The effects of air pollution on mental health were observed across all symptoms of mental distress or dysfunction, with the strongest links to thought disorder symptoms.
While the findings are most relevant to high-income countries with moderate air pollution, like the U.S. and the UK, there are also implications for countries with higher levels of air pollution, like China and India.
“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,” said Fisher.
The study is published in the journal JAMA Network Open.