Even low levels of air pollution may put children at higher risk for developing a mental illness, according to a recent study out of Sweden — the first of its kind to examine the effects of poor air quality on such a young cohort.

Standards set by the European Union and the World Health Organization, for example, cap yearly mean nitrogen dioxide levels — a chemical produced by motor vehicle exhaust and part of the pollutant cocktail that creates smog — at around 40 µg/m3 (micrograms per cubic meter). In Sweden, researchers noted a 9 percent increase in child mental illness for every 10 microgram-per-cubic-meter increase of nitrogen dioxide released into the air. Their findings, published in the journal BMJ Open, show that incremental hikes in air pollution can still negatively affect mental health.

"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 authors write.

Between July 2007 and December 2010, researchers from Umeå University followed the entire population under age 18 across four Swedish counties (which happened to include the three most populous cities in the country, including Stockholm). In total, they studied about 500,000 children in areas of varying population density, air pollution, and socioeconomic status. The research team compared concentrations of nitrogen dioxide, particulate matter, and fine particulate matter for each location using data from the Swedish National Register. They also compared the amount of sedatives, sleeping pills, and antipsychotics dispensed to the study group, using data from the Swedish National Board of Health and Welfare. More than 18,000 children received such medication at least once during a 3.5-year follow-up period.

The Swedish study is the latest among a growing body of research linking poor air quality to a host of mental and physical illnesses among all age groups. Anxiety, depression, and diseases of the central nervous system, including stroke, Alzheimer's, and Parkinson's, have all been linked to pollution. In China, exposure to chronic, low-level traffic exhaust, often laced with chemicals like lead and carbon monoxide, has also been found to adversely affect neurobehavioral functions in children and adults.

Among low- and middle-income countries, about 98 percent of cities with more than 100,000 inhabitants still fall short of meeting the WHO's global air quality guidelines. This spring, the global organization went so far as to declare ambient air pollution "the greatest environmental risk to health," following reports that global air pollution levels increased by 8 percent between 2008 and 2013  —  causing three million premature deaths.

"It is crucial for city and national governments to make urban air quality a health and development priority," Dr. Carlos Dora, the WHO's Interventions for Healthy Environments unit coordinator, said in a May statement. "When air quality improves, health costs from air pollution-related diseases shrink, worker productivity expands, and life expectancy grows. Reducing air pollution also brings an added climate bonus, which can become a part of countries' commitments to the climate treaty."

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