Girls who breathe in air pollution around the time of their first period might have irregular cycles, new research suggests.

Scientists found a link, although not a direct causation, between exposure to air pollution and an irregular menstrual cycle for young women, according to a study in the journal Human Reproduction. When females were exposed to high levels of particulate matter in high school, they had “slightly increased odds of menstrual irregularity and longer time to regularity in high school and early adulthood.”

The study explains that previous research has found air pollution affecting hormonal activity, and hormones play a role in regulating periods.

Previous research has also suggested a link between air pollution and infertility and other issues in the female reproductive system, like the hormonal condition polycystic ovary syndrome.

The new findings build upon these ideas. They were based upon data on almost 35,000 women, including the average amount of particulate matter in their area during their high school years and how long they reported it took them from their first periods before their menstrual cycle became regular.

There was a stronger association found in women who were older during their first periods and in women who lived in the Northeast or West, according to the study.

“Temporal exposure to air pollution in the adolescent and early adulthood window may be especially important,” the researchers wrote. “The data from this study agrees with existing literature regarding air pollution and reproductive tract diseases.”

Reducing air pollution, then, might improve women’s health.

“While air pollution exposures have been linked to cardiovascular and pulmonary disease, this study suggests there may be other systems, such as the reproductive endocrine system, that are affected as well,” corresponding study author Dr. Shruthi Mahalingaiah said in a statement from the Boston University School of Medicine. “Implications on human disease may come through reducing emissions on a global and individual level.”