Popular Science: Air pollution might be the new lead

Scientists now think it might put young brains at risk.

Sometimes air pollution is easy to see. It billows off the top of smoke stacks, and out the tailpipes of cars zooming down the highway. Misty smog hangs in the air in cities like Delhi, Beijing, and Los Angeles, fracturing sunlight into a muted haze.

Most of the time, though, dirty air just looks like air. About 92 percent of the world’s population, and more than half the people in the United States, live in areas with unhealthy air quality. The World Health Organization calls air pollution the world’s “largest single environmental health risk,” and it leads to the premature deaths of millions annually. It’s a major public health problem for reasons you might expect: breathing in dirty air isn’t good for your lungs, and the the connection between the lungs and the cardiovascular system means it puts pressure on your heart, too.

But it’s increasingly clear that the effects of air pollution aren’t constrained to body parts below the shoulders—they can hurt the brain in a whole host of ways, many of which researchers are still trying to understand. One major area of interest? The way exposure to polluted air can affect the cognitive development of babies and children. Researchers aren’t shocked to find that an environmental toxin could harm young brains, because they’ve seen it happen before.

“To me, air pollution is kind of the next lead, in a way,” says Deborah Cory-Slechta, a professor of environmental medicine at the University of Rochester.

Lead was everywhere throughout the start of the 20th century, readily used to make vacuums and paint and included as an ingredient in gasoline. It was known to be toxic, and concern over its health effects spurred fights for regulation, but it wasn’t until the 1980s that researchers linked even low levels of lead exposure to an increased risk for cognitive and behavioral problems in children—just as scientists are starting to do for air pollution now.

The parallel isn’t exact, but like lead, air pollution also disproportionately affects low income and minority communities. Like lead, air pollution is easy to put into the environment, and much harder to take out. “The more I do in this area, the bigger the problem seems to me,” Cory-Slechta says.

Pollution on the brain

Cory-Slechta actually started out studying the effects of lead exposure, and she was skeptical when she first heard air pollution might pose similar dangers. But when a research group at her university, which was studying air pollution and lung development, asked if she was interested in taking a look at the brains of the mice used in their studies, she figured she might as well take a look.

She was shocked to find evidence of inflammation and damage in pretty much every area of the mouse brains. “And this was a full two months after the exposure to air pollution had ended,” Cory-Slechta says.

Living in areas with high air pollution has been linked to poorer memory, attention and vocabulary; to below-average performance on intelligence tests; and to delinquent behavior. Air pollution has also been implicated in developmental disorders ranging from attention-deficit hyperactivity disorder to autism spectrum disorders.

Animal studies, where researchers can more strictly control the pollution exposure, back up the results from those human reports. They show that air pollution causes changes in behavior in rodents, and changes in their brains, like imbalances in the levels of certain molecules, hyperactivity in brain regions, and damage to neurons—many of which correspond to the way neurodevelopmental diseases look in these animals. The widespread inflammation seen in mouse brains after air pollution exposure, like Cory-Slechta observed in her initial studies, can damage neurons, and, during development, prevent the brain from organizing itself properly.

Although the research isn’t far enough along to draw an explicit, causative link between air pollution and developmental changes in humans, there’s a strong association between the two, strengthened by the accompanying research on animals. “We have a pretty good correspondence between the epidemiology studies in humans and the animal studies,” Cory-Slechta says.

The particles in the air get into the body and into the brain through a few different pathways: they can pass through the lungs and into the bloodstream, where they can travel up to the brain directly, or cause changes in the body’s immune response that trigger damaging inflammation. There’s also only a thin barrier between the nasal cavity and the brain, and tiny particles of air pollution can pass directly through.

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