Cover Story

That yellow haze of smog hovering over the skyline isn't just a stain on the view. It may also leave a mark on your mind.

Researchers have known since the 1970s that high levels of air pollution can harm both cardiovascular and respiratory health, increasing the risk of early death from heart and lung diseases. The effect of air pollution on cognition and mental well-being, however, has been less well understood. Now, evidence is mounting that dirty air is bad for your brain as well.

Over the past decade, researchers have found that high levels of air pollution may damage children's cognitive abilities, increase adults' risk of cognitive decline and possibly even contribute to depression.

"This should be taken seriously," says Paul Mohai, PhD, 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."

Cognitive connections

Most research on air pollution has focused on a type of pollutant known as fine particulate matter. These tiny particles — 1/30th the width of a human hair — are spewed by power plants, factories, cars and trucks. Due to its known cardiovascular effects, particulate matter is one of six principal pollutants for which the Environmental Protection Agency (EPA) has established air quality standards.

It now seems likely that the harmful effects of particulate matter go beyond vascular damage. Jennifer Weuve, MPH, ScD, an assistant professor of internal medicine at Rush Medical College, found that older women who had been exposed to high levels of the pollutant experienced greater cognitive decline compared with other women their age (Archives of Internal Medicine, 2012). Weuve's team gathered data from the Nurses' Health Study Cognitive Cohort, a population that included more than 19,000 women across the United States, age 70 to 81. Using the women's address history, Weuve and her colleagues estimated their exposure to particulate matter over the previous seven to 14 years. The researchers found that long-term exposure to high levels of the pollution significantly worsened the women's cognitive decline, as measured by tests of cognitive skill.

Weuve and her colleagues investigated exposure to both fine particulate matter (the smallest particles, less than 2.5 micrometers in diameter) and coarse particulate matter (larger particles ranging from 2.5 to 10 micrometers in size).

"The conventional wisdom is that coarse particles aren't as important as fine particles" when it comes to human health, Weuve says. Previous studies in animals and human cadavers had shown that the smaller particles can more easily penetrate the body's defenses. "They can cross from the lung to the blood and, in some cases, travel up the axon of the olfactory nerve into the brain," she says. But Weuve's study held a surprise. She found that exposure to both fine and coarse particulate was associated with cognitive decline.

Weuve's results square with those of a similar study by Melinda Power, a doctoral candidate in epidemiology and environmental health at the Harvard School of Public Health. Power and her colleagues studied the link between black carbon — a type of particulate matter associated with diesel exhaust, a source of fine particles — and cognition in 680 older men in Boston (Environmental Health Perspectives, 2011). "Black carbon is essentially soot," Power says.

Power's team used black carbon exposure as a proxy for measuring overall traffic-related pollution. They estimated each man's black carbon exposure by cross-referencing their addresses with an established model that provides daily estimates of black carbon concentrations throughout the Boston area. Much like Weuve's results in older women, Power and colleagues found that men exposed to high levels of black carbon had reduced cognitive performance, equivalent to aging by about two years, as compared with men who'd had less black carbon exposure.

But while black carbon is a convenient marker of air pollution, it's too soon to say that it's what's causing the cognitive changes, Power says. "The problem is there are a lot of other things associated with traffic — noise, gases — so we can't say from this study that it's the particulate part of the air pollution that matters."

Still, the cumulative results of these studies suggest that air pollution deserves closer scrutiny as a risk factor for cognitive impairment and perhaps dementia.

"Many dementias are often preceded by a long period of cognitive decline. But we don't know very much about how to prevent or delay dementia," Weuve says. If it turns out that air pollution does contribute to cognitive decline and the onset of dementia, the finding could offer a tantalizing new way to think about preventing disease. "Air pollution is something that we can intervene on as a society at large, through technology, regulation and policy," she says.

Young minds

Research is also finding air-pollution-related harms to children's cognition. Shakira Franco Suglia, ScD, an assistant professor at Boston University's School of Public Health, and colleagues followed more than 200 Boston children from birth to an average age of 10. They found that kids exposed to greater levels of black carbon scored worse on tests of memory and verbal and nonverbal IQ (American Journal of Epidemiology, 2008).

More recently, Frederica Perera, DrPH, at the Columbia University Mailman School of Public Health, and colleagues followed children in New York City from before birth to age 6 or 7. They discovered that children who had been exposed to higher levels of urban air pollutants known as polycyclic aromatic hydrocarbons while in utero were more likely to experience attention problems and symptoms of anxiety and depression (Environmental Health Perspectives, 2012). These widespread chemicals are a byproduct of burning fossil fuels.

Meanwhile Mohai, at the University of Michigan, found that Michigan public schools located in areas with the highest industrial pollution levels had the lowest attendance rates and the greatest percentage of students who failed to meet state testing standards, even after controlling for socioeconomic differences and other confounding factors (Health Affairs, 2011). What's worse, the researchers analyzed the distribution of the state's public schools and found that nearly two-thirds were located in the more-polluted areas of their districts. Only about half of states have environmental quality policies for schools, Mohai says, "and those that do may not go far enough. More attention needs to be given to this issue."

Although Michigan and Massachusetts may experience areas of poor air quality, their pollution problems pale in comparison to those of Mexico City, for example. In a series of studies, Lilian Calderón-Garcidueñas, MD, PhD, a neuropathologist at the University of Montana and the National Institute of Pediatrics in Mexico City, has investigated the neurological effects of the city's infamous smog.

In early investigations, Calderón-Garcidueñas dissected the brains of dogs that had been exposed to air pollution of Mexico City and compared them with the brains of dogs from a less-polluted city. She found the Mexico City dogs' brains showed increased inflammation and pathology including amyloid plaques and neurofibrillary tangles, clumps of proteins that serve as a primary marker for Alzheimer's disease in humans (Toxicologic Pathology, 2003).

In follow-up research, Calderón-Garcidueñas turned her attention to Mexico's children. In one study, she examined 55 kids from Mexico City and 18 from the less-polluted city of Polotitlán. Magnetic resonance imagining scans revealed that the children exposed to urban pollution were significantly more likely to have brain inflammation and damaged tissue in the prefrontal cortex. Neuroinflammation, Calderón-Garcidueñas explains, disrupts the blood-brain barrier and is a key factor in many central nervous system disorders, including Alzheimer's disease and Parkinson's disease. Perhaps more troubling, though, the differences between the two groups of children weren't just anatomical. Compared with kids from cleaner Polotitlán, the Mexico City children scored lower on tests of memory, cognition and intelligence (Brain and Cognition, 2008).

Brain changes

It's becoming clearer that air pollution affects the brain, but plenty of questions remain. Randy Nelson, PhD, a professor of neuroscience at the Ohio State University, is using mouse studies to find some answers. With his doctoral student Laura Fonken and colleagues, he exposed mice to high levels of fine particulate air pollution five times a week, eight hours a day, to mimic the exposure a human commuter might receive if he or she lived in the suburbs and worked in a smoggy city (Molecular Psychiatry, 2011). After 10 months, they found that the mice that had been exposed to polluted air took longer to learn a maze task and made more mistakes than mice that had not breathed in the pollution.

Nelson also found that the pollutant-exposed mice showed signs of the rodent equivalent of depression. Mice said to express depressive-like symptoms give up swimming more quickly in a forced swim test and stop sipping sugar water that they normally find attractive. Both behaviors can be reversed with antidepressants. Nelson found that mice exposed to the polluted air scored higher on tests of depressive-like responses.

To find out more about the underlying cause of those behavioral changes, Nelson compared the brains of mice that had been exposed to dirty air with brains of mice that hadn't. He found a number of striking differences. For starters, mice exposed to particulate matter had increased levels of cytokines in the brain. (Cytokines are cell-signaling molecules that regulate the body's inflammatory response.) That wasn't entirely surprising, since previous studies investigating the cardiovascular effects of air pollution on mice had found widespread bodily inflammation in mice exposed to the pollution.

More surprisingly, Nelson also discovered physical changes to the nerve cells in the mouse hippocampus, a region known to play a role in spatial memory. Exposed mice had fewer spines on the tips of the neurons in this brain region. "Those [spines] form the connections to other cells," Nelson says. "So you have less dendritic complexity, and that's usually correlated with a poorer memory."

The changes are alarming and surprising, he says. "I never thought we'd actually see changes in brain structure."

Nelson's mice experienced quite high levels of pollution, on par with those seen in places such as Mexico City and Beijing, which rank higher on the pollution scale than U.S. cities. It's not yet clear whether the same changes would occur in mice exposed to pollution levels more typical of American cities. Another limitation, he notes, is that the animals in his study were genetically identical. Nelson says he'd like to see similar studies of wild-type mice to help tease out whether genetic differences might make some people more or less vulnerable to the effects of pollution. "I would suspect there are people who are wildly susceptible to this and people who are less so, or not at all," he says.

Few studies have investigated connections between depression and air pollution, but Nelson's wasn't the first. A study by Portuguese researchers explored the relationship between psychological health and living in industrial areas. They found that people who lived in areas associated with greater levels of air pollution scored higher on tests of anxiety and depression (Journal of Environmental Psychology, 2011).

Back in Ohio, Nelson plans to study how much — or how little — pollution is necessary to cause changes in the brain and behavior. He's also beginning to look at the effects of air pollution on pregnant mice and their offspring. Though more research is needed to fully understand how dirty air impairs the brain, he says, the picture that's emerging suggests reason for concern.

In the United States, the Environmental Protection Agency reviews the scientific basis for particulate matter standards every five years or so, and completed its last review in 2009.

To date, the EPA hasn't factored psychological research into their standards assessments, but that could change, according to a statement the EPA provided to the Monitor. "Additional research is necessary to assess the impact of ambient air pollutants on central nervous system function, such as cognitive processes, especially during critical windows of brain development. To this end, as the number of … studies continue to increase and add to the weight of overall evidence, future National Ambient Air Quality Standards assessments will again assess and address the adequacy of existing standards."

In the meantime, says Weuve, there's not much people can do to protect themselves, short of wearing special masks, installing special filtration systems in their homes and offices or moving to cities with less airborne pollution. "Ultimately, we're at the mercy of policy," she says.

The good news, Nelson says, is that the mental and cognitive effects of air pollution are finally beginning to receive attention from the mental health research community. "We sort of forget about these environmental insults," says Nelson. "Maybe we shouldn't."


Kirsten Weir is a writer in Minneapolis.

Watch the Video

How does air pollution affect our brains? It depends on the size of the particle we inhale, says Jennifer Weuve, MPH, ScD, of Rush Medical College. Fine particulate matter, which includes smoke, car exhaust and pollen, can interact directly with the brain. Coarse particulate matter, however, is more of a mystery that researchers are only now beginning to study.


U.S. cities with the worst air pollution by year-round particle pollution, 2012

U.S. cities with the worst air pollution

1. Bakersfield-Delano, Calif.
2. Hanford-Corcoran, Calif.
3. Los Angeles-Long Beach-Riverside, Calif.
4. Visalia-Porterville, Calif.
5. Fresno-Madera, Calif.
6. Pittsburgh-New Castle, Pa.
7. Phoenix-Mesa-Glendale, Ariz.
8. Cincinnati-Middletown-Wilmington, Ohio-Ky.-Ind.
9. Louisville-Jefferson County-Elizabethtown-Scottsburg, Ky.-Ind.
10. (tie) Philadelphia-Camden-Vineland, Pa.-N.J.-Del.-Md. St. Louis-St. Charles-Farmington, Mo.-Ill.

Worst air pollution Source: American Lung Association 2012 State of the Air report.