People breathing dirty air more likely to have mental health problems

People exposed to high levels of air pollution have much greater odds of suffering from a psychiatric illness such as depression, schizophrenia or bipolar disorder, according to a new study.

While no mental illness can be pinned on environmental exposures alone, the study, published today in PLOS Biology, adds to growing evidence that air pollution is toxic for our brains and may interact with other genetic and biological factors to impact when some people get mental disorders and how severe the disorders become.

"Our study shows that living in polluted areas, especially early on in life, is predictive of mental disorders in both the United States and Denmark," said University of Chicago computational biologist Atif Khan, the co-author of the new study, in a statement.

To examine mental health disorders, Khan and colleagues looked at a U.S. health insurance database of about 150 million people that covered 11 years of claims and a second dataset of all 1.4 million people born in Denmark between 1979 through 2002.

The researchers used 87 air quality measurements from the U.S. Environmental Protection Agency to estimate pollution exposure by county for the U.S. people; and used Denmark's pollution registry to estimate air pollution exposure for Danish people. In Denmark, the pollution exposure estimate was from when they were born until 10 years old.

The air pollutants include small particulate matter, diesel emissions, nitrogen dioxide, polycyclic aromatic hydrocarbons, and other contaminants that come from car exhaust, industry and wildfires.

In the U.S., they found the worst air quality was linked to a 27 percent increase in the rate of bipolar disorder and a 6 percent increase in diagnoses of major depression.

In Denmark the results were more alarming: When they compared people in the group with the highest levels of estimated exposure to those with the lowest levels they found:

• The rate of schizophrenia was 148 percent higher
• The rate of bipolar disorder was 29 percent higher
• The rate of personality disorder increased 162 percent
• The rate of major depression increased about 50 percent

The researchers controlled for other variables including age, sex, population density and income, lead author and University of Chicago professor and researcher Andrey Rzhetsky told EHN.

Rzhetsky said previous lab studies (on animals) have given clues as to how air pollution could spur mental disorders. Prior research in rodents shows exposure to pollution can cause inflammation and cell death in brain tissues—such changes can potentially lead to psychiatric disorders.

These "neuroinflammatory pathways" have caused depression-like symptoms in animals, Rzhetsky said.

However, the study had limitations and authors cautioned throughout the study that this does not prove that pollution causes mental health problems. For example, it is difficult to diagnose mental health disorders so the rates used by the researchers may be imperfect. Also, despite some lab studies, there is a dearth of studies on how air pollution might affect the nervous system.

The study was controversial, even among the scientists who reviewed it for publication in the peer-reviewed PLOS Biology. The journal published a companion commentary from physician, professor and researcher John Ioannidis of Stanford University today that outlines how they evaluated the evidence from Rzhetsky's study.

They "have offered a brilliant exploratory analysis with interesting hypothesis-generating hints for bipolar disorder and possibly other psychiatric diagnoses. Now, these leads need to be rigorously prospectively evaluated in other data sets," Ioannidis, who was not involved in the study, wrote.

Rzhetsky said even though the mental health and air pollution link is not yet determined, reducing air pollution exposure is a good thing.

"I don't see any harm in trying to reduce exposure and striving to live in cleaner places. There are lots of other diseases caused by pollution."