(Centers for Disease Control and Prevention)

People who live near major state or interstate highways have a greater risk of developing dementia than those who live farther away, according to a study of 6.6 million people published Jan. 4 by The Lancet.

Researchers suspect air and noise pollution as the culprits. The study was observational, so it cannot establish causality, but it was designed to control for socioeconomic status, education levels, body mass index and smoking, so those factors are unlikely to explain the road-dementia connection.

The researchers tracked all adults aged 20 to 85 years and living in Ontario for more than a decade from 2001 to 2012. They used postal codes to determine how close people lived to a major road and analyzed medical records to see whether these individuals went on to develop dementia.

The risk of developing dementia lessened as people lived farther away from a main road, the investigators found, with a 7% greater risk in developing dementia among those living within 55 yards, a 4% higher risk at 55 to 109 yards, a 2% increased risk at 110 to 219 yards and no greater risk for those living more than 219 yards away. Up to 11% of cases of dementia among those who live within 55 yards of a major road could be attributable to traffic exposure, they said.

“Increasing population growth and urbanization has placed many people close to heavy traffic, and with widespread exposure to traffic and growing rates of dementia, even a modest effect from near-road exposure, could pose a large public health burden,” said lead author Hong Chen, Ph.D., of Public Health Ontario. “More research to understand this link is needed, particularly into the effects of different aspects of traffic, such as air pollutants and noise.”

Paper author Ray Copes, M.D., chief of environmental and occupational health at Public Health Ontario, said that the findings could be helpful in building design and municipal land use decisions.

In a related commentary, Lilian Calderón-Garcidueñas, M.D., Ph.D., a professor of biomedical and pharmaceutical sciences at the University of Montana in Missoula, wrote: “We must implement preventive measures now, rather than take reactive actions decades from now.”

The researchers also looked into whether traffic exposure was related to the development of Parkinson’s disease or multiple sclerosis but found no link.