A new study suggests that exposure to air pollution may be linked to increased mental illnesses among children.

The research, which was published in the journal BMJ Open, found that even small increases in air pollution exposure were associated with an increase in psychiatric problems.

In the study, researchers from Umeå University in Sweden examined the pollution exposure of more than 500,000 young people in Sweden aged below 18 and made a comparison against records of medicines prescribed for mental illnesses, such as sedatives and anti-psychotics.

"The results can mean that a decreased concentration of air pollution, first and foremost traffic-related air pollution, may reduce psychiatric disorders in children and adolescents," Anna Oudin of the Unit for Occupational and Environmental Medicine at the Department of Public Health and Clinical Medicine and lead researcher of the study said in a press release.

The results indicate that exposure to air pollution has increased the risk of having dispensed medication for at least one psychiatric diagnosis for children and adolescents. There is an increased risk of 9 percent with every 10 microgram per cubic meter (mcg/m3) increased concentration of nitrogen dioxide (NO2), regardless of the respondents' socioeconomic and demographic background.

The World Health Organization (WHO) limits NO2 exposure to 40mcg/m3, but levels can be higher in some parts of the world, The Guardian reports.

The study may not indicate a casual link between air pollution and mental health problems in children, but it shows a correlation between the two.

According to the WHO statistics, about 4 million people die prematurely every year due to outdoor air pollution. In the U.S. alone, air pollution causes nearly 200,000 deaths every year.

Low-income and minority communities in the U.S. are among the areas with high concentrations of air pollutants coming from industrial contaminants.

The researchers note that more work needs to be done in understanding the relationship between mental health and air pollution.

"The severe impact of child and adolescent mental health problems on society, together with the plausible and preventable association of exposure to air pollution, deserves special attention," the researchers wrote.