As part of our series for Brain Awareness Month, we consider the relationship between air pollution and cognitive decline, and research into how exposure may contribute to the development of Alzheimer’s disease and other dementias.
Air contaminants and pollution are well-established environmental hazards, with known respiratory and cardiovascular pathologies. But does evidence exist to suggest a causal link to neurocognitive impairment?
Recent Research into Air Pollution and Cognitive Decline
In recent years, the idea that air pollution might contribute to neurodegenerative diseases has attracted increased research. However, the most recent studies have not been conclusive.
In April 2020, an article from the American Academy of Neurology shared results from two studies, one of which showed a correlation between air pollution exposure and a steeper rate of cognitive decline, while another smaller study did not:
“5,330 people with an average age of 75 enrolled in the Washington Heights-Inwood Community Aging Project [WHICAP]; 1,093 people with an average age of 70 enrolled in the Northern Manhattan Study [NOMAS] … The group from the WHICAP study was followed for an average of 7 years with 6 rounds of follow-up testing every 18 months to 2 years. The Northern Manhattan study group of 1,093 participants was followed for 5 years with one follow-up round of testing.
Researchers used residential addresses of each participant to determine their exposures to 3 air pollutants. Those pollutants were nitrogen dioxide (NO2) and two groups of particulate matter … Average levels of air pollutants were similar for both groups of participants.”
The cohorts revealed a divergent result:
“The researchers found that in the WHICAP group, people with greater exposure to higher levels of air pollution had lower scores on the tests at the beginning of the study and more rapid rates of decline. Exposure to nitrogen dioxide was linked to an accelerated rate of cognitive decline comparable to one year of aging. Results were similar for fine and respirable particulate matter.
In the NOMAS group, researchers did not find an association between cognitive function and air pollution. Dr. Erin Kulick says the difference in results may be because the second group was much smaller and only had one round of follow-up compared to six rounds for the first group.”
Affects of Lifetime Exposure on Results
The air pollution levels in the groups’ communities remained within the Environmental Protection Agency’s acceptable range. Nonetheless, the authors speculate the results reflect exposure to higher levels in childhood or adolescence. The elderly in New York City may be paying a price for less stringent federal air pollution standards in the past. It also raises the question of whether current federal levels are low enough even now.
The mixed results of the aforementioned study bears contrast with prior research.
Killian and Kitazawa Study
A 2018 article published in Biomedical Journal indicated that “adults who are exposed to polluted air experience accelerated cognitive impairment. In China, Mexico and the U.S., elderly residents over 65 years old who live in areas with high air pollution generally performed significantly worse on a … a common cognitive test to assess dementia, than those living in cleaner areas.”
Authors Kilian and Kitazawa go on to summarize recent evidence indicating that chronic exposure to polluted air is a major environmental risk factor.
Moulton and Yang Study
In a 2012 article for the Journal of Environmental and Public Health, “Air Pollution, Oxidative Stress, and Alzheimer’s Disease,” Paula Valencia Moulton and Wei Yang state:
“Depending on their characteristics, air toxicants can reach the brain through several pathways — the effects of air pollution on the brain then manifest as neuroinflammation, oxidative stress, and neurodegeneration.”
While many factors contribute to Alzheimer’s disease, air pollutants could possibly accelerate these changes observed in the brain. The authors also asked whether lifetime rather than recent exposure to air pollution presents the greatest risk. This area, though, will require further research.
Assessment and Care
BrainCheck encourages physicians to consider the health impacts of air quality and pollution. These may directly affect thinking, memory and cognitive skills in patients.
Practices whose patients with long-term exposure to lower-than-average air quality should consider this risk factor when assessing cognitive health. BrainCheck supports remote cognitive testing and helps physicians track assessment of memory, language skills and executive function over time is advised if and when environments present a higher risk of exposure to air pollution.