Most people might associate life near a highway with choking and wheezing on exhaust fumes, but there is a growing heap of evidence linking air pollution to a less obvious health effect:  Dementia.

According to study published in medical journal The Lancet, living within 200 meters of a major road is associated with at least some increased risk of dementia. 

The risk grows proportionately according to proximity.  People living 101 to 200 meters from a highway have an extra two percent chance of dementia.  That grows to a four percent higher risk among people living 50 to 100 meters away.  And the researchers found that people living within 50 meters of such a road had a seven percent greater risk of developing dementia.

"There is a gradient of increased risk as you get closer to major roadways," said Ray Copes, chief of environmental and occupational health at Public Health Ontario, Canada, who co-led the study.  "By the time you're 200 meters away, the risk is essentially down to baseline."

This is the latest of several studies that have found that people exposed to high pollution rates will, over time, demonstrate more cognitive decline and pre-dementia symptoms than those who breathe cleaner air. 

Scientists are zeroing in on how air pollution adversely impacts the brain.  Engine exhaust consists of fine particulate matter that is small enough to travel throughout our bodies, and a recent study found that certain particles common to air pollution could enter the brains of people who breathe them in.  Once in the brain, pollution particles lead to inflammation that could contribute to cognitive decline over time.