­­Breathing dirty air causes stress hormones to spike, new research suggests, which could help explain why long-term exposure to pollution is associated with heart disease, stroke, diabetes, and a shorter life span.

Haidong Kan of Fudan University in Shanghai, China, and colleagues looked specifically at the health effects of particulate matter (PM), small particles <2.5mm in diameter, from industrial sources, that can be inhaled and become lodged in the lungs. While PM 2.5 levels have gone down in North America in recent years, they are on the rise worldwide.

“This research adds new evidence on how exposure to PM could affect our bodies, which may (ultimately) lead to higher cardiovascular risk,” Kan told Reuters Health in an e-mail interview. “Our result may indicate that particulate matter could affect the human body in more ways than we currently know. Thus, it is increasingly necessary for people to understand the importance of reducing their PM exposure.”

The new study, published online in Circulation, included 55 healthy college students in Shanghai, a city with pollution levels in the middle range compared to other Chinese cities, according to Kan.

He and his colleagues put working or non-working air purifiers in each student’s dorm, and left them in place for nine days. After a 12-day ‘washout’ period, the researchers swapped out the filters and left them for another nine days. At the end of each nine-day period, the researchers tested levels of a wide range of small molecules in students’ blood and urine.

Ninety-seven metabolites were associated with short-term PM exposure, the researchers found. Levels of cortisol, cortisone, epinephrine and norepinephrine rose with dirtier air.

The changes observed with higher PM levels suggest that breathing polluted air activates two key mechanisms involved in the stress response, the hypothalamus-pituitary-adrenal and sympathetic-adrenal-medullary axes, Kan said.