Dementia affects tens of millions of people worldwide. Common risk factors include age, family history, and genetics. But new research points to an additional factor that might affect the chances of developing dementia: living near a major, busy road.

Dementia describes a wide range of brain illnesses that progressively lead to the loss of cognitive functioning. It affects reasoning, memory, behavior, and the ability to perform daily tasks.

The World Health Organization (WHO) report that approximately 47.5 million adults are currently affected by dementia worldwide.

The most common risk factors are age, family history, and hereditary background. While these are outside of one's control, there are additional risk factors that could be controlled. These include avoiding head trauma and other conditions that damage the heart and blood vessels, such as hypertension, heart disease, diabetes, and high cholesterol.

Emerging research highlights a new element that might influence the chances of developing dementia - living close to major, busy roads, such as highways or motorways.

Examining the link between major road proximity and dementia
Researchers from Public Health Ontario, Canada - in collaboration with several Canadian universities and Health Canada - have set out to examine the link between residential proximity to major roads and the incidence of dementia in Ontario.

Their results were published in The Lancet.

More specifically, the team, led by Dr. Hong Chen, looked at three major neurodegenerative diseases: dementia, Parkinson's disease, and multiple sclerosis (MS).

The scientists were motivated by existing research that has previously linked living near a major road to negative effects on the residents' cognition. Some studies have suggested that exposure to traffic and its side effects, such as noise and pollution, might contribute to neurodegenerative pathology.

In this new study, the Canadian researchers followed a total of 6.6 million Ontarians aged between 20 and 85 for over a decade, between 2001 and 2012.

The team used postcodes to determine the proximity of the residents to major roadways. The researchers also used the participants' medical records to see if they developed dementia, Parkinson's, or MS over the years.

Almost everyone (95 percent of the participants) in the study lived within 1 kilometer of a major road. Over the 10-year period, the researchers identified 243,611 cases of dementia, 31,577 cases of Parkinson's disease, and 9,247 cases of MS.

One in 10 dementia cases attributable to traffic exposure
Researchers found no association between living next to a major roadway and developing Parkinson's disease or MS. However, dementia was found to be more common among people who lived closer to busy roads.

The study revealed that up to 1 in 10 cases of dementia among residents living within 50 meters of a major road could be attributed to traffic exposure. Additionally, the closer people lived to the busy roads, the higher their risk of developing dementia was.

Between 7 and 11 percent of the dementia cases identified were attributable to major road proximity.

The risk decreased the farther away people lived from the main road. The results suggest that the risk of dementia was 7 percent higher for those living within 50 meters of a major roadway. This dropped to 4 percent for those living within 50-100 meters, 2 percent for those at 101-200 meters, and there was no increase in risk for those living more than 200 meters away.

Dr. Chen and team also found a link between long-term exposure to two common pollutants - nitrogen dioxide and fine particulate matter - and the incidence of dementia.

Although the link between dementia and road proximity weakened when researchers adjusted for these two pollutants, this association did not fully account for the entire near-road effect. This suggests that other pollutants, or even factors such as noise, could play a role.

Findings 'open up crucial global health concern'
Strengths of this study include its large scale, as well as the access that researchers had to detailed medical and residential information over a period of 10 years. The study also adjusted for factors including socioeconomic status, education, body mass index, and smoking.

Limitations of the study include its observational nature, which means that it could not establish causality. Furthermore, the pollution exposure was estimated based on the postcode, so the study could not consider the pollution that each individual may have been exposed to.

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