The United Nations predicts that, by 2050, 68 percent of the world will live in cities where thick, polluted air is the norm. But pollution is already wreaking havoc now: Research published in JAMA Psychiatry on Wednesday shows that these invisible toxins could be partially responsible for higher rates of people experiencing hallucinations and delusions — especially among teens who spend their childhoods caught in their noxious tide.

"Adolescents who have psychotic experiences are at higher risk of going on to develop a psychotic disorder in adulthood.

As she writes in her paper, Helen Fisher, Ph.D., a reader in developmental psychopathology at Kings College London, teens in the UK who have been exposed to the highest levels of air pollution are significantly more likely to have experienced psychotic episodes, including severe hallucinations or paranoia. Those exposed to highest levels of nitrogen dioxide (NO2) — a dangerous gas already linked to respiratory disease — had 71 percent greater odds of having experienced a psychotic episode.

“Adolescents who have psychotic experiences are at higher risk of going on to develop a psychotic disorder in adulthood as well as a range of other mental health problems, including attempting suicide, and also are more likely to struggle to finish their education or get a job, be a teenage parent, have physical health problems, and generally have a poorer quality of life,” Fisher tells Inverse.

Los Angeles air pollution. Wikimedia Commons

A psychotic episode doesn’t mean that they actually had a mental disorder, Fisher explains. The symptoms that seem to be linked to air pollution exposure are less extreme versions of future mental health issues that may arise.

Her analysis is based on the life histories of 2,063 twins born in the UK between 1994 and 1995. These twins were enrolled in the Environmental Risk Longitudinal Twin Study, in which they were asked at age 18 whether they had experienced any psychotic experiences, like hearing voices or feeling like they were “being watched.”

Fisher looked for a relationship between that data and statistics on the teens’ exposure to air pollutants, estimated by using local air quality data from 2012. She estimated the levels of air pollution likely present at each person’s home and two locations where they spent significant time, like school or work.

Over 30 percent of the twins interviewed said that they had experienced an episode between the ages of 12 and 18. Taking a conservative approach, Fisher and her team defined 2.9 percent of those experiences as “definite” psychotic symptoms. Even after they adjusted for other potentially psychosis-related factors like substance use, they found that air pollution still explained 60 percent of their results.

“Our findings suggest that exposure to higher levels of air pollution was associated with adolescent experiences over and above factors in urban environments such as deprivation, crime and poor social cohesion — neighbours not trusting or supporting each other, or in other words, a poor sense of community,” Fisher says.

Pollution is also correlated with noisy traffic, which could be underlying Fisher's results Wikimedia

This study just shows a correlation, not a causal relationship, which means that there’s a lot left to figure out. Fisher admits that her results could in part be due to other aspects of urban life, like traffic noise or airport noise.

Though her study can’t explain why patterns between psychotic experiences and air pollution exist, Fisher has a few hypotheses. First is that pollution has previously been linked to inflammation in the brain, which could lead to psychotic disorders. There is some work supporting the idea that these gases do have neurotoxic effects, but the links to psychosis and pollution-driven toxicity are still unclear, and definitely not elucidated by Fisher’s study.

It is also possible that cities — and the pollution they come with — could cause stress more generally, which is also linked to poor mental health. Higher levels of air pollution, she says, are correlated with a lot more vehicle traffic, which is noisy.

“The air pollution we studied is largely due to heavy traffic in urban areas,” she says. “It is actually the correlated level of noise pollution that may be driving the association because it disrupts sleep and is stressful and these factors have also been linked to the onset of psychotic experiences.”

Researchers have conjectured that city life is hard on the mind since 1939, and since then, studies have suggested that urban environments may as much as double the risk of developing psychotic disorders like schizophrenia. With studies like this one, we’re slowly figuring out what it is about cities that makes them so hard on our mental health.

Fisher’s work may ultimately show that it’s actually noise pollution that drives the observed effects on psychotic episodes. But at the very least, it’s becoming clear that the concerns people have long held about cities are real. There’s just something about them that seems to be taking a toll on mental health — and given that nearly 82 percent of North Americans live in urban areas, it’s probably worth finding out what it is.

Partial Abstract:

Design, Setting, and Participants: The Environmental-Risk Longitudinal Twin Study is a population-based cohort study of 2232 children born during the period from January 1, 1994, through December 4, 1995, in England and Wales and followed up from birth through 18 years of age. The cohort represents the geographic and socioeconomic composition of UK households. Of the original cohort, 2066 (92.6%) participated in assessments at 18 years of age, of whom 2063 (99.9%) provided data on psychotic experiences. Generation of the pollution data was completed on October 4, 2017, and data were analyzed from May 4 to November 21, 2018.

Exposures: High-resolution annualized estimates of exposure to 4 air pollutants—nitrogen dioxide (NO2), nitrogen oxides (NOx), and particulate matter with aerodynamic diameters of less than 2.5 (PM2.5) and less than 10 μm (PM10)—were modeled for 2012 and linked to the home addresses of the sample plus 2 commonly visited locations when the participants were 18 years old.

Main Outcomes and Measures: At 18 years of age, participants were privately interviewed regarding adolescent psychotic experiences. Urbanicity was estimated using 2011 census data.

Results: Among the 2063 participants who provided data on psychotic experiences, sex was evenly distributed (52.5% female). Six hundred twenty-three participants (30.2%) had at least 1 psychotic experience from 12 to 18 years of age. Psychotic experiences were significantly more common among adolescents with the highest (top quartile) level of annual exposure to NO2 (odds ratio [OR], 1.71; 95% CI, 1.28-2.28), NOx (OR, 1.72; 95% CI, 1.30-2.29), and PM2.5 (OR, 1.45; 95% CI, 1.11-1.90). Together NO2 and NOx statistically explained 60% of the association between urbanicity and adolescent psychotic experiences. No evidence of confounding by family socioeconomic status, family psychiatric history, maternal psychosis, childhood psychotic symptoms, adolescent smoking and substance dependence, or neighborhood socioeconomic status, crime, and social conditions occurred.