A new study finds the effects of air pollution may be more dangerous than previously thought.

According to a new report, air pollution is directly correlated to cognitive demise. The report, published in the American Heart Association’s journal Stroke, investigated 943 healthy adults over the age of 60 living in the New England region of the U.S.. Relying on MRI scans, the team of researchers compared each participants brain structure to levels of air pollution in the city in which they lived, finding a strong correlation between the two.

The study found a link between the levels of fine particle pollution and a reduction in brain size. Fine particle pollution is often found in power plants, car exhaust, and common burning.

The study found that for every increase of 2 mcg/m³, there was a 0.3 percent reduction in brain volume. Study author Elissa H. Wilker of the cardiovascular epidemiology unit at Beth Israel in Boston said the reduction is the equivalent of one year’s worth of brain aging. In general, smaller brain volume is caused by a loss of brain neurons that come with aging, researchers said, but in this case it seems to be correlated with the fine particulate matter.

The study is especially disconcerting due to disease symptoms experienced by participants affected by air pollution. According to researchers, a 46 percent increase in silent strokes was discovered in participants living in especially high pollution areas with higher-than-average levels of fine particulate air pollution.

“Mini-strokes don’t cause as much damage as a larger stroke, but sometimes they can do nasty things depending on where they are [in the brain],” said Dr. Beate Ritz, chair of the department of epidemiology at the Center for Occupational and Environmental Health at UCLA’s Fielding School of Public Health. “They won’t leave you totally disabled but they can make your health-related quality of life much lower. ”

Silent strokes (or mini strokes) are especially dangerous due the fact that they often occur without the victim knowing. Rates of recovering from strokes often depend on how quickly a victim can receive medical care, making silent strokes especially deadly.

While researchers conclude that the cause-and-effect relationship between air pollution in brain changes is strong, they said further research is necessary.

The study comes as a number of scientists have put forth theories on how air pollution affects the brain. A number of theories center on inflammation and it’s effects on brain development. The latest studies draws heavily on conclusions between air pollution, inflammation, and brain health.

It remains unclear exactly what steps cities and states can take to reduce air pollution. A number of state lawmakers have sought to curtail air pollution regulations in recent years, possibly leading to a spike in stroke rates. That said, this is the first study to examine the relationship between ambient air pollution in brain structure, so further research is likely needed.

The new results are likely to help researchers better understand “what could be going on between air pollution and serious outcomes like stroke and cognitive impairment,” Wilker said in an interview with LiveScience

The scans were done as part of the Framingham Offspring Study.