(COLORADO) — Exposure to air pollutants such as wildfire smoke has been found to have long-lasting effects on youth, specifically children, according to a study by The University of Colorado in Boulder.

The study, published in August, comes after smoke from a California fire has thickened in places like Colorado and Las Vegas.

“We need to understand what these extreme events are doing to young people, their brains, and their behavior,” said first author Harry Smolker, a research associate with CU’s Institute of Cognitive Science, whose study is among the first to explore the behavioral impact of particulate exposure on adolescents.

The research analyzed data from 10,000 pre-teens — aged 9 to 11 — who participated in the study. Researchers looked at how many days in 2016 youth were exposed to fine particular matter (PM 2.5) above 35 micrograms per cubic meter (35ug/m3), the level which the Environmental Protection Agency (EPA) deems unsafe. It also compiled data from a questionnaire that asked parents at four points over three years about signs of depression, anxiety, and other symptoms in their children.

When looking at the data, researchers found that each day of exposure at unsafe levels significantly boosted the likelihood of a child having symptoms of depression, anxiety, and other “internalizing” symptoms up to one year later.

“We found that a greater number of days with fine particulate air pollution levels above EPA standards was associated with increased symptoms of mental illness, both during the year of exposure and up to one year later,” Smolker said.

Constant high levels of exposure had a greater influence on risk than annual averages or maximum levels did, thus suggesting each day of breathing poor-quality air counts. For each day of unsafe exposure, risk went up 0.1 points on a scale of 1 to 64.

Some studies show that PM 2.5 may be small enough to cross the blood-brain barrier, fueling acute and longer-term brain changes.

“Wildfire smoke events are becoming more and more common, and this study adds to a growing body of evidence that they can impact our health,” said co-author and Assistant Professor of Geography Colleen Reid.

Some youth may even be genetically predisposed to be even more vulnerable. On high pollution days, hospital admissions for depression, suicide attempts, and psychotic episodes have been shown to increase. Studies also suggest that those who are exposed to high levels when pregnant are more likely to have cognitive impairments later in life.