An international study led by the University of Chicago has found a possible link between air pollution and higher rates of mental illness.

The study, and led by Andrey Rzhetsky at the University of Chicago, collected data on more than 152 million people in the U.S. and Denmark, measuring their health and pollution levels.

For data compiled in the United States, researchers relied upon the Environmental Protection Agency's system air quality measurements -- for Denmark, they used a national pollution register.

Researchers then compared population data sets from each country.

They found that higher levels of air pollution were associated with higher rates of bipolar disorder and depression in both countries.

"Our study found that…bad quality air is predictive of the rate of psychiatric disease in the geographic location," said Rzhetsky to CTV News.

The poorest air quality in the US was associated with a 29 per cent increased risk of bipolar disorder compared to the cleanest air. Bad air quality was also linked with a six per cent increase in diagnosis rates of depression.

When they looked at Denmark, high exposure to air pollutants in childhood were linked to a 148 per cent increase in schizophrenia -- a 29 per cent increase in bipolar cases and a 50 per cent increase in cases of depression.

Scientists think is that small particles of pollutants in the air travel through the body into the brain and may cause inflammation -- leading to the symptoms of these psychiatric disorders. But the theory is based on animal studies with little evidence in people.

Other scientist question the study -- and the connection between air quality and the brain.

"It is very early to say that this is conclusive data" said Stanford University Professor John Ioannidis to CTV News, who was asked by the PLOS Biology journal to present his commentary in an accompanying article to the study.

"There is much more work that needs to be done…I think it is very early to say that air pollution is causing mental disease," he said. "Air pollution is a bad thing, whether there is an added mental health problem is very early based on this data."

Dr. Ray Copes, chief of environmental and occupational health at Public Health Ontario, told CTV News the study raises more questions than it answers.

But he said had the "potential to open up a whole new line of research" that examines the relationship between pollution and psychiatric conditions, "this is something researchers may want to look at more carefully in the future."

Air pollution has already been linked to a number of health problems including heart disease asthma and its role is even being questioned in rising rates of obesity and diabetes.

Rzhetsky says that if further studies confirm the link there is a potential solution to the problem.

"The implications of the study is that ...by cleaning our environment we might be able to prevent or even cure or actually relieve psychiatric diseases," he said.