The study, published in Environmental Research, suggests industry-generated pollutants pose a greater health risk than traffic-generated manganese.

Murray Finkelstein, of the University of Toronto, compared the incidence of diagnosis and treatment of Parkinson's disease with markers of exposure to vehicle exhaust and industrial emissions in the cities of Toronto and Hamilton. The study examined a cohort of 110,000 subjects over three years.

In Toronto, the researchers found no association between Parkinson's diagnosis and exposure to manganese through traffic-generated air pollution.

In Hamilton, the odds of a physician diagnosing Parkinson's increased with the amount of ambient manganese in the air.

The study was sparked by interest in the effects of methylcyclopentadienyl manganese tricarbonyl, an agent added to Canadian gasoline for many years to reduce engine knocking, according to the Finkelstein.