Motor vehicle exhaust fumes, factory emissions, dust particles—all these things dirty the air we breathe, which may be harming our brains, according to a growing number of studies. For years, scientists have documented a link between air pollution and lung disease, and possibly heart disease. Now research is showing that air pollution may increase the chances of stroke, cognitive decline, Alzheimer's disease and other dementias, and Parkinson's disease. It may even damage the developing brains of young children.
“The epidemiologic data are pretty compelling,” says Ted M. Dawson, MD, PhD, FAAN, professor of neurology at the Johns Hopkins University School of Medicine, whose research focuses on Parkinson's disease and other movement disorders. The exact reasons are not fully understood, Dr. Dawson says, but researchers are working to identify which components of polluted air are most troubling and how they might damage the brain, either directly or indirectly.
“Most people think the brain is a protected area, but we know that it gets exposed to the environment through the gastrointestinal tract [eating] and olfactory system [smelling],” Dr. Dawson says.
Most neurodegenerative diseases are caused by a complex mix of genetic, environmental, and lifestyle factors, not a single cause, but exposure to high levels of air pollution may add to a person's genetic risk for a certain disease. Research from New York City, for instance, found that living near a highway is associated with an increased risk of stroke, and similar evidence has emerged for dementia.
In addition, the case is starting to grow for a connection between air pollution and Parkinson's disease, says Ray Dorsey, MD, MBA, professor of neurology at the University of Rochester Medical Center, who notes that prevalence of the disease is increasing in industrialized countries even when statistical data are adjusted for an aging population.
Dr. Dorsey, who co-authored Ending Parkinson's Disease: A Prescription for Action, points to a 2018 report published in the Journal of Public Health that estimated that air pollution, including indoor air pollution, costs the average person nearly three years of life.
In general, most people understand that breathing polluted air is bad for the lungs, especially for anyone with asthma or chronic obstructive pulmonary disease (COPD), but not many understand the connection between poor air quality and brain disorders, says Vladimir Hachinski, MD, DSc, FAAN, a neuroscientist and researcher at Western University in Ontario, Canada. Perhaps, he adds, that's because the possibility of their developing a condition like Alzheimer's disease seems far in the future.
Air pollution encompasses the gases and particles emitted from factories, power plants, and cars, buses, and trucks. One of the most familiar components is ozone, the ground-level gas or smog that can blanket a city. Air pollution is also composed of small particles of varying types and sizes, which get breathed in or ingested. Lead, which has known adverse effects on the brain, may be in those minute particles. Outdoor air pollution accounts for 4.2 million deaths globally every year, mostly due to stroke, heart disease, COPD, lung cancer, and acute respiratory infections, according to the World Health Organization.
While policy makers and public health officials craft and debate regulations that promote cleaner air, researchers are uncovering clues to how chronic exposure to air pollution may be harmful. One general theory, says Dr. Hachinski, is that the body mounts an inflammatory defense in reaction to exposure to pollutants.
“The body jazzes up to protect itself, which is good in the short term, but because the exposure to air pollution doesn't stop, the inflammation continues,” he says. Over time, constant exposure could lead to organ damage, particularly to the linings of blood vessels, which could explain the association between air pollution and stroke.
Another theory is that air pollution has a direct effect on the specific pathology underlying a given neurologic disease, which may be the case with Parkinson's disease and Alzheimer's disease.
Researchers are using laboratory mice to study, for instance, whether exposure to components of air pollution triggers the development in the gut and brain of the telltale marker of Parkinson's disease, the misfolded alpha-synuclein protein.
So far, most of the human studies have been observational, which cannot prove cause and effect (as randomized, placebo-controlled trials can). For example, an analysis published in Stroke in April 2018 of data gathered for a study of more than 3,200 people in Manhattan found that living within 100 meters, or about two city blocks, of a highway was associated with a significantly increased risk of stroke. The ongoing study, begun in 1993, used medical records, follow-up phone calls to participants, and air monitoring data by zip code to examine risk factors for cardiovascular disease.
During a median follow-up after 15 years, the study found that people who lived less than 100 meters from a major roadway had a 42 percent higher rate of ischemic stroke compared with those living more than 400 meters from a highway. The Manhattan neighborhoods included in the study are near the George Washington Bridge, a high-traffic area that includes a bus terminal.
The Manhattan study continues to collect data that researchers hope to use to explore a possible connection between chronic exposure to air pollution and cognitive decline and dementia, says Mitchell S. Elkind, MD, FAAN, one of its co-authors and professor of neurology and epidemiology at Columbia University.
“The challenge of studying air pollution is teasing out the different kinds of pollutants to determine which ones are the most harmful,” says Dr. Elkind. One of the main culprits is very fine particulate matter with a diameter of 2.5 micrometers (PM2.5), which comes mostly from combustion.
In another study, involving more than 18,000 older Americans with cognitive impairments, researchers examined PET scans to see if there was an association between living in an area with high air pollution and the presence of amyloid-beta, a biomarker for Alzheimer's disease, in the brain. The study, published in JAMA Neurology in November 2020, found that people living in areas with high concentrations of PM2.5 were more likely to have amyloid-beta, even after the researchers adjusted for other factors such as smoking, family history, household income, and high blood pressure. Elevated exposure to ozone did not seem to increase a person's risk.
A study conducted in South Korea and published in JAMA Neurology in May 2021 looked at a nationally representative cohort of 78,830 people over the age of 40 living in a metropolitan city. Using data from the Korean National Health Insurance Service and air pollution data matched to people's addresses, the study found that high exposure to nitrogen dioxide was associated with a 40 percent increase in the risk of developing Parkinson's disease, although other air pollutants such as ozone, carbon monoxide, and sulfur dioxide did not seem to elevate the risk. The study, which relied on air pollution readings taken at the district level, did not factor in possible occupational exposure to air pollutants such as heavy metals, pesticides, and trichlorethylene, an industrial solvent, which also have been linked to Parkinson's disease.
Danish researchers compared nearly 1,700 patients with Parkinson's disease to controls matched by age and sex and found an association between traffic-related air pollution and risk of Parkinson's. Their results were published in Environmental Health Perspectives in March 2016.
The brains of young children may be particularly vulnerable to air pollution, some scientists say. Research from Mexico City, which has bad air pollution, suggests that high exposure to dirty air may be associated with cognitive deficits and brain abnormalities identified in imaging of young children.
It's difficult to reach clear conclusions about the negative effects of air pollution on the brain because it's almost impossible to measure individual exposure, says Dr. Dorsey. People are exposed not only in their neighborhoods but also at work and on the way to work. And people living in the same neighborhood may have differing exposures based on the precise locations of their homes or how much time they spend outdoors.
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