The fine particles found in air pollution can cause high stress hormone levels and other negative cellular-level changes, according to a study released Monday that is peeling back the layers on why air pollution exposure can lead to heart disease.

The first-of-its-kind study published in the American Heart Association’s journal Circulation measured the health levels and environments of 55 college students at a university in Shanghai, China. The students received alternate treatments in random order, with some having high-efficiency air purifiers and others having “sham” purifiers, in which researchers had removed the filter gauze.

All treatments ran for 36 hours, beginning at 7 p.m. on a Friday. During that time, all windows and doors were tightly closed, and the students had to stay in their dormitories as much as possible, other than attending class. Researchers conducted a health test, and collected blood and urine samples at the end of each study period.

After 24 hours with real air purifiers in use, exposure levels for fine particulate matter were in the safe range based on air quality guidelines from the World Health Organization.

Past studies have suggested that fine particulate matter – the kind in air pollution from cars, factories, power plants, fires and smoking – might negatively affect cardiovascular and metabolic health. But just how that happens biologically is unclear. In this new study, researchers used “metabolomics,” a way to study and measure changes in substances at a cellular level that produce a body’s energy.

Researchers said the study suggests that the human central nervous system reacts to changes in exposure to particulate matter.

“We found marked changes in serum metabolites, including hormones, glucose, amino acids, and lipids,” the study authors wrote. “Consistent with previous publications from us and other groups in China, we also observed increases in blood pressure and levels of biomarkers of inflammation and oxidative stress in relation to higher” fine particulate matter exposure.

Still, more study is needed, said the study’s senior author Haidong Kan, M.D., Ph.D., a professor of environmental health sciences at Fudan University in Shanghai. “Although we found significant health benefits with air purifiers, the actual health protection people could get from air purifiers in real living conditions is still not well-determined.”

The study was a small one, and it is unclear whether it can translate to results in other countries. Air pollution levels are much higher in urban China than in the United States or Europe.

“Future studies should examine whether the health benefits from short-term air purification can improve long-term health,” Kan said, “and whether these findings are also found in people who live in low pollution areas.”