Car emissions cloud the mind

January 5th, 2011 by Todd Neale

This article is published courtesy of Todd Neale, Staff Writer for MedPage Today.

Pollution from traffic was associated with worse cognitive function in a cohort of older men, researchers found.

A doubling of exposure to black carbon — a marker for traffic pollution — was associated with a 30% greater chance of having a low score on a screening test for dementia (OR 1.3, 95% CI 1.1 to 1.6), according to Melinda Power, a doctoral student at the Harvard School of Public Health, and colleagues.

Car emissions in John Street, St Peter Port (click image to expand - ©RLLord)

A composite score of six other cognitive function tests was also lower among men with greater exposure to black carbon, the researchers reported online in Environmental Health Perspectives.

“This is the first study to find an association between traffic-related air pollution and cognition in older men, and only the second to consider the relationship in older adults,” they wrote.

“Given the ubiquitous nature of the exposure, if traffic-related air pollution is causally related to cognitive impairment in older adults, implementation of interventions to reduce exposure, including establishment of more stringent emissions standards, would be expected to have substantial benefits,” they concluded.

Both animal and human studies have shown that traffic-related particles can induce oxidative stress and may have adverse effects on the central nervous system, including impacts on cognitive function.

Commuter traffic along Les Banques - Guernsey's east coast road endures a continuous stream of traffic for much of the day (click image to expand - ©RLLord)

To explore the issue, Melinda Power and her colleagues turned to the VA Normative Aging Study, an ongoing longitudinal cohort study. The current analysis included 680 men with an average age of 71. All completed a battery of seven cognitive tests at least once between 1996 and 2007.

The researchers estimated exposure to black carbon using a model encompassing the greater Boston area.

A doubling of black carbon level was associated with having a Mini-Mental State Examination (MMSE) score of 25 or less, as well as lower global cognitive function, after adjusting for age, education, first language, computer experience, physical activity, alcohol consumption, diabetes, dark fish consumption, percent of residential census tract that is nonwhite, percent of residential census tract adults with a college degree, indicator for first cognitive assessment, and indicator for part-time resident.

The deficit in global cognition — 0.054 standard deviations lower — was similar to that observed for an age difference of 1.9 years.

Adjusting further for previous lead exposure in a sensitivity analysis did not change the association with MMSE score, but rendered the relationship with global cognition nonsignificant.

There was some suggestion that the effect of traffic pollution on cognitive function may be enhanced in smokers and in overweight and obese individuals.

“As these conditions are pro-inflammatory, this is biologically plausible, but requires further confirmation,” Melinda Power and her colleagues wrote.

They noted that traffic pollution could affect the central nervous system either directly or indirectly.

Ultrafine particles like those found in diesel exhaust are small enough to pass through the air-blood barriers of the lung, enter the circulation, and travel to various tissues in the body, including the brain. They are also able to get to the brain via the olfactory nerve. Once in the brain, the particles cause neuroinflammation and oxidative stress.

The particles may also have adverse effects on cardiovascular health, which could indirectly affect central nervous system function.

The researchers noted some limitations of the study, including possible misclassification of personal levels of exposure to black carbon, possible residual confounding, potential bias from incomplete follow-up, the inability to attribute the findings to a particular exposure, and potential confounding by noise levels.

Melinda Power’s paper can be downloaded from the Environmental Health Perspectives website.

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