A multidecade study of young adults in the United Kingdom discovered that those exposed to higher levels of traffic-related air pollutants, particularly nitrogen oxides, had a higher rate of mental illness symptoms during childhood and adolescence.

While previous research has established a link between air pollution and the risk of developing specific mental disorders, such as depression and anxiety. This study examined changes in mental health associated with exposure to traffic-related air pollutants.

The findings, which will be published in JAMA Network Open on April 28, indicate that the more nitrogen oxides an individual are exposed to during childhood and adolescence, the more likely they are to exhibit any signs of mental illness during the transition to adulthood, which occurs at age 18 when the majority of symptoms of mental illness have emerged or are beginning to emerge.

According to Aaron Reuben, a graduate student in clinical psychology at Duke University and the study’s first author, the link between air pollution exposure and young adult mental illness symptoms is tenuous. However, “because harmful exposures are so widespread throughout the world,” he said, “outdoor air pollutants may contribute significantly to the global burden of psychiatric disease.”

According to the World Health Organization (WHO), 9 out of 10 people worldwide are currently exposed to high levels of outdoor air pollutants, which are emitted during fossil fuel combustion in automobiles, trucks, and power plants, as well as numerous manufacturing, waste disposal, and industrial processes.

Air pollution, a neurotoxicant, was a weaker risk factor for mental illness than other more well-known risk factors, such as a family history of mental illness, but was comparable to other neurotoxicants known to harm mental health, particularly childhood lead exposure.

Helen Fisher of King’s College London’s Institute of Psychiatry, Psychology, and Neuroscience, co-author and principal investigator of this study, previously linked childhood air pollution exposure to the risk of psychotic experiences in young adulthood, raising concerns that air pollutants may exacerbate the risk of psychosis later in life.

When combined with previous research indicating an increase in hospital admissions for a variety of psychiatric illnesses on “poor” air quality days in countries such as China and India, the current study adds to previous findings by establishing that “air pollution is likely a non-specific risk factor for mental illness in general,” Fisher said, adding that exacerbations of mental illness risk may manifest differently in different populations.

The study examined a cohort of 2,000 twins born in England and Wales between 1994 and 1995 and followed through early adulthood. They have participated in routine physical and mental health evaluations and provided information about their larger communities.

The researchers quantified exposure to air pollutants – particularly nitrogen oxides (NOx), a regulated gaseous pollutant, and fine particulate matter (PM2.5), a regulated aerosol pollutant with suspended particles smaller than 2.5 microns in diameter – by modelling air quality in the vicinity of study participants’ homes between the ages of 10 and 18 years, using high-quality air dispersion models and data from the UK National Atmosphere. Twenty-two percent of study participants had NOx exposure that exceeded WHO guidelines, and 84 percent had PM2.5 exposure that exceeded WHO guidelines.

Additionally, the research team at Duke and King’s IoPPN assessed participants’ mental health at the age of 18. The symptoms often distinct psychiatric disorders — alcohol, cannabis, or tobacco dependence; conduct disorder and attention-deficit/hyperactivity disorder; major depression, generalized anxiety disorder, post-traumatic stress disorder, and eating disorder; and thought disorder symptoms associated with psychosis — were used to calculate a single measure of mental health called the psyc.

The higher an individual’s p-factor score, the more psychiatric symptoms are identified, and the more severe they are. Individuals can also differ in their mental health across psychopathology subdomains, which group together symptoms of distress or dysfunction that manifests outwardly (externalizing problems, such as to conduct disorder), are primarily experienced internally (internalizing problems, such as anxiety), or are manifested through delusions or hallucinations (thought disorder symptoms). Air pollution has been shown to have a detrimental effect on mental health across these subdomains of psychopathology, with the strongest associations being with thought disorder symptoms.

Specifically for this study, the researchers assessed neighbourhood characteristics to account for disadvantageous neighbourhood conditions associated with increased air pollution levels and an increased risk of mental illness, such as socioeconomic deprivation, physical dilapidation, social isolation, and dangerousness. While air pollution levels were higher in neighbourhoods with poor economic, physical, and social conditions, adjusting for neighbourhood characteristics, as well as for individual and family factors such as childhood emotional and behavioural problems, family socioeconomic status, and a history of mental illness, did not affect the findings.

“We have confirmed the identification of a novel risk factor for the majority of major forms of mental illness,” Reuben explained, “one that is modifiable and on which we can intervene at the community, city, and even national level.”

The study team is interested in learning more about the biological mechanisms underlying the association between early life air pollution exposure and an increased risk of developing mental illness during the transition to adulthood. Previous research indicates that exposure to air pollutants can cause inflammation in the brain, resulting in difficulty regulating thoughts and emotions.

While the findings are most relevant to high-income countries with low levels of outdoor air pollutants, such as the United States and the United Kingdom, they also have implications for low-income, developing countries with higher exposure to air pollution China and India. “We do not know the mental health consequences of extremely high levels of air pollution, but it is an important empirical question that we are investigating further,” Fisher explained.