If you live near a major roadway, you may want to pay close attention. A new study has found a link between the onset of dementia and the proximity of your residence to highway traffic.

It's no secret that common air pollutants, particularly those associated with auto traffic, are bad for our health. But a team of researchers from Public Health Canada wanted to find out how these pollutants affect our brains. For the study, researchers tracked almost 7 million adults between the ages of 20 and 85 who lived in Ontario between 2002 and 2012. Using postal codes, they were able to determine which people lived near major roadways, with "major" defined as the equivalent of an interstate highway based on daily traffic volume. The team then compared this data with each person's medical records, looking specifically for three neurodegenerative conditions — Parkinson's disease, multiple sclerosis and dementia. The team's findings were published in The Lancet.

Researchers didn't find a link between living near a highway and a person's likelihood of developing Parkinson's or multiple sclerosis, possibly because the overall likelihood of developing these conditions is so small. But they did find a connection between living near a major roadway and a person's chances of developing dementia.

"There is something different going on with dementia," said Ray Copes, chief of environmental and occupational health at Public Health Ontario and a co-author of the study.

The numbers make the risk clear

Copes and his team found that living within 50 meters (about 164 feet) of a major roadway increased a person's chances of developing dementia by 4 percent. That likelihood decreased as the residential distance from the roadway increased. Those who lived 100 meters (328 feet) away from a major road were 4 percent more likely to develop dementia while those who lived between 101 and 200 meters (as far as a tenth of a mile away,) had an increased risk of 2 percent. Beyond 200 meters, the increase in dementia risk was negligible.

The link between dementia risk and residential proximity to a major roadway held strong even after researchers adjusted for factors such as education, body mass index, smoking or socioeconomic status. Next up, researchers are hoping to determine if there's one particular pollutant that is responsible for the link, or if noise plays a role.