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Air Pollution Linked to Cognitive Impairment: A Review of the Research Evidence

University students at Anyang Normal University, Christmas Day, 2013, Henan Province, China. Cognitive impacts air pollution.

University students at Anyang Normal University, Christmas Day, 2013, Henan Province, China. Cognitive impacts air pollution.

A growing body of research demonstrates that air pollution affects the brain and causes cognitive deficits. When inhaled, fine particulate matter has been shown to be bad for your heart, lungs, and brain. While we have known that air pollution compromises cardiovascular and respiratory health since the 70s, the connection with impaired cognition is a more recent discovery. Inhaled fine particulate matter crosses the blood-brain barrier and adversely affects neurons. The research demonstrates that the neurotoxicants contained in air pollution cause neuroinflammation which can lead to neurodegenerative changes including diminished cognitive performance.

As reviewed in a 2012 paper titled “Smog in our brains,” published by the American Psychological Association the evidence is mounting linking air pollution to diminished cognitive abilities.

Elderly population

Research suggests that there may be a link between dementia and air pollution. A large, prospective study of women published in 2012 makes the case for this connection. The study is called, “Exposure to Particulate Air Pollution and Cognitive Decline in Older Women” and it was published in JAMA Internal Medicine. Lead researcher Jennifer Weuve, MPH, ScD, an assistant professor of internal medicine at Rush Medical College, and her colleagues found that long-term exposure to high levels of air pollution can cause and accelerate cognitive decline in older women (age 70 to 81). The researchers found declines in the test scores of older women in general cognition, verbal memory, category fluency, working memory, and attention. These declines were elucidated by pollution levels typically experienced by many individuals in the U.S. The researchers concluded:

“Higher levels of exposure to ambient PM are associated with worse cognitive decline. Importantly, these associations were present at levels of PM exposure typical in many areas of the United States. Therefore, if our findings are confirmed in other research, air pollution reduction is a potential means for reducing the future population burden of age-related cognitive decline, and eventually, dementia.”

These results corroborate the findings of an earlier study by Melinda Power, a doctoral candidate in epidemiology and environmental health at the Harvard School of Public Health. A possible connection between Alzheimer’s and air pollution was revealed in research by Lilian Calderón-Garcidueñas, MD, Ph.D., a neuropathologist at the University of Montana and the National Institute of Pediatrics in Mexico City. Her research found that dogs that had been exposed to higher levels of air pollution had increased inflammation and pathology compared to the brains of dogs exposed to less pollution. Her research shows an increase in amyloid plaques and neurofibrillary tangles, clumps of proteins, which are primary markers for Alzheimer’s disease in humans.

In an MRI study, Calderón-Garcidueñas found evidence of neuroinflammation associated with air pollution, which disrupts the blood-brain barrier and is a key factor in many central nervous system disorders, including Alzheimer’s disease and Parkinson’s disease.

Young population

There is a body of evidence that suggests children may be most at risk of cognitive impairments due to air pollution. The growing field of neuropediatric air pollution research offers some troubling findings.

Air pollution may even be harmful to the brains of developing children in utero. Frederica Perera, Ph.D, at the Columbia University Mailman School of Public Health, and colleagues found that children who had been exposed to higher levels of a certain urban air pollutant (polycyclic aromatic hydrocarbons a by-product of burning fossil fuels) while in utero, were more likely to experience attention problems and symptoms of anxiety and depression.

According to Shakira Franco Suglia, ScD, an assistant professor at Boston University’s School of Public Health, and her colleagues, children who were exposed to high levels of air pollution under-performed control groups on tests of memory and verbal and nonverbal IQ.

Randy Nelson, Ph.D, a professor of neuroscience at the Ohio State University, doctoral student Laura Fonken and colleagues found that mice exposed to high levels of fine particulate air pollution took longer to learn a maze task and made more mistakes than mice that had not been exposed to air pollution. These mice also showed signs of depression. Nelson also discovered less dendritic complexity in the hippocampus of mice exposed to air pollution. The hippocampus is a region of the brain known to play a role in memory.

Other research shows high school absentee rates and lower test result scores in children exposed to high levels of air pollution. This result was borne out even when controlled for socioeconomic differences and other confounding variables.

Calderón-Garcidueñas’ MRI study found higher inflammation and damaged tissues in the prefrontal cortex of children exposed to high levels of air pollution. The kids exposed to less air pollution performed better on tests of memory, cognition and intelligence than the kids who had high exposure.

This research is corroborated by a 2014 paper called, “The Impact of Short-Term Exposure to Ambient Air Pollution on Cognitive Performance and Human Capital Formation by Victor Lavy, Avraham Ebenstein and Sefi Roth. These researchers found that exposure to fine particulate matter and carbon monoxide decreased standardized test scores among Israeli high school students.

A 2015 study published in Medical Daily found that air pollution slows cognitive development in children due to brain inflammation. This study explored the cognitive functioning of Spanish students who attend schools adjacent to busy roads. The study’s lead author Dr. Jordi Sunyer, from the University of Barcelona, said air pollution results in chronic low-grade brain inflammation, which delays brain maturation. Sunyer and colleagues found deficits in working memory, superior working memory, and attentiveness.

A 2016 study reviews emerging evidence showing the relationship between air pollution exposure and neural, behavioral and cognitive changes in children. This research suggests that the breakdown of natural barriers (nasal, gut, lung, epithelial, and blood-brain barrier) allow the passage of toxic particles into the body of young people. This can cause neuroinflammation which contributes to cell loss within the central nervous system. The researchers hypothesize that this is a crucial mechanism by which cognitive deficits may arise.

More study

Much of current research and policy efforts link air pollution to respiratory and cardiovascular disease, however, we now have good reason to add cognitive deficits to the list of adverse impacts.

“This should be taken seriously,” says Paul Mohai, Ph.D., a professor in the University of Michigan’s School of Natural Resources and the Environment who has studied the link between air pollution and academic performance in children. “I don’t think the issue has gotten the visibility it deserves.”

A collaborative multi-disciplinary approach is required to effectively understand and address the neuroinflammation risks associated with air pollution. An integrated neuroscientific approach to studying these phenomena should incorporate clinical, cognitive, neurophysiological, radiological and epidemiologic research.

Pollution causes increased morbidity and mortality, it also causes cognitive deficits, especially in the young and the elderly. In addition to intellectual problems in the form of cognitive deficits, air pollution has also been linked to a higher risk of depression, suicide, and autism. Such pollution may also play a role in behavioral and social problems. The evidence for a wide range of neurocognitive impacts warrants more research.

Richard Matthews is a consultant, eco-entrepreneur, green investor and author of numerous articles on sustainable positioning, eco-economics and enviro-politics. He is the owner of The Green Market Oracle, a leading sustainable business site and one of the Web’s most comprehensive resources on the business of the environment. Find The Green Market on Facebook and follow The Green Market’s twitter feed.

Image credit: V.T. Polywoda, courtesy Flickr

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