For years, science has known that pollution from smokestacks and exhaust pipes can promote diseases, including heart disease and asthma. Now, researchers in Sweden say polluted air also appears to be bad for mental health.

The researchers came to their findings by cross-referencing pollution exposure with a registry of dispensed psychiatric medication for children and adolescents in Sweden. The results are persuasive, in part, because the study sampled an unusually large population, around half a million people.

In Sweden and other Scandinavian countries, researchers have access to nationwide medication registries, which cover every citizen. The researchers combined this registry with a registry of addresses and with the national model of air pollution. They found that children who live in polluted areas were more likely to have received a dispensed medication for a group of psychiatric illnesses.

“We saw that for an increase of 10 micrograms per cubic meter in NO2 (nitrous dioxide), the risk of dispensing one of these medications increased by 9 percent, if you look at the overall estimate,” says lead author Anna Oudin, a researcher at Umea University in Sweden. “We looked at this in four different regions, but overall it was 9 percent. It's comparable to what we've seen for the increase in cardiovascular disease.”

While proving a cause is not possible, the results definitely show a link between levels of air pollution and the amounts of psychiatric medications that are being dispensed in these populations, Oudin says. The association remains clear even at very low pollution levels.

“We looked at both rural areas and urban areas and we saw the same association basically everywhere, independent of how we looked or what we adjusted for,” Oudin explains. “We've done our best to rule out any other factor, but we can never be entirely sure, of course.”

The study looked at common pollutants like NO2, PM10 and NO2 PM 2.5, which are smaller particulates easily inhaled deep into the lungs. NO2 PM2.5 is usually seen as a marker for vehicle exhaust; PM10 can come from many different sources.

The study used data on a broad group of psychiatric medications, from sedatives used to treat sleep disorders to antipsychotics. “It’s very broad and we can't separate these from each other based on the data we have,” Oudin says. “We can't say if it's a certain medication that is driving the association or if it's all the medications.”

More detailed data on the medications might reveal links between air pollution exposure and depression or anxiety, for example, or autism or psychosis. That will be the next step in their research, Oudin says.

Oudin believes her work could influence how people look at mental illness. Researchers studying psychiatric disorders typically look at things like genetics, social environment and trauma, but the new results indicate that the environment could play an important role. The awareness of the environmental factors could even help reduce the social stigma associated with mental illness, Oudin hopes.

The research could also possibly change the way governments regulate air pollution. “When regulators and policy makers think about this, they usually take into account the health risks of air pollution and weigh them against the costs of reducing air pollution, but mental health issues are not included in those cost calculations,” Oudin says.

“We know that air pollution can get into the brain and cause inflammation and that psychiatric disorders can be caused by inflammation. So I think there is a need for these kind of large epidemiological studies, and I think there will be more to come in the next few years.”

This article is based on an interview that aired on PRI's Living on Earth with Steve Curwood.