We know that breathing in air pollution from major roads is bad for our lungs, but it may be impacting people in another way. A new study has found that people living in close proximity to busy roads have higher rates of dementia. Currently, it is unclear whether the air pollution is the direct cause for higher rates, but the findings have been described as “interesting and provocative”.

Published in The Lancet, the study assessed close to 2 million people living in Canada over an 11-year period. They found that people living within 50 meters (164 feet) of major roads, had higher rates of the neurological condition, with as many as 11 percent of cases potentially caused by the high levels of traffic. The rate of diagnoses then dropped off the further from the roads people were living.

From age and genetics to diet and smoking, there are multiple risk factors that are thought to contribute to the development of dementia. The researchers accounted for many of these, including obesity and education levels, meaning they are unlikely to explain the link, but that does not necessarily mean that traffic or air pollution is the direct cause of the increased rates observed, though it does raise interesting questions.

“Studies like this are valuable in revealing new factors that could be implicated in Alzheimer’s disease and other dementias, and opening new avenues for further research,” said Dr Davis Reynolds, chief scientific officer at Alzheimer’s Research UK. “This study has identified major roads and air pollutants from traffic as possible risk factors for dementia, a finding which will need further investigation before any firm conclusions can be drawn.”

The research adds to the growing concerns about the impact that air pollution is having on an increasingly urbanized population. With over half of the world now living in urban areas, the effect breathing in such dirty air is having on public health is becoming a serious concern.

Last year, it was reported that between three and five million people are dying prematurely due to air pollution, and with the urban population only expected to expand, this figure is sure to rise. Most of these deaths are down to respiratory problems and lung cancer, but the fact that dementia may now also be implicated adds another layer to the situation.

How governments will respond to this is growing issue is still not clear, though some cities have taken bold steps, with Athens, Madrid, Paris, and Mexico City vowing to ban all diesel cars by 2025, while urging others major urban centers, such as London, to follow suit.