There are a lot of explanations for the apparent gap in IQ test scores of white Americans and Americans of color.

Some experts point to things like bias in the exam itself, or the fact that white students tend to go to better-funded schools. Others tout racist myths. Take famed biologist James Watson, who said that the black-white IQ gap is genetic, or writer Jason Richwine, whose doctoral dissertation asserted that “the low average IQ of Hispanics is effectively permanent” — Harvard gave him a PhD for that gem.

One factor that is little considered, even by academics who rail against racist stereotypes, is pollution and its impact on the brain. Research shows that contamination from cars, planes, power plants, factories and landfills is eroding the bodies and minds of black and brown Americans. Two new books call attention to this invisible crisis.

“Intelligence in African Americans and other people of color — native Americans, Hispanics — is being assailed by the environment,” said Harriet Washington, a medical ethicist and author of A Terrible Thing to Waste: Environmental Racism and Its Assault on the American Mind, which investigates what pollution is doing to the brains of people of color.

“We don’t all breathe the same air, although we would like to think that we do,” she said. “African Americans are much more likely to live in fence-line communities where they share their neighborhoods with toxic factories. They’re spewing mercury. They’re spewing particulate matter. These things directly affect the brain, but they also cause hypoxia and asthma. And the hypoxia over time — that’s a lowered amount of oxygen delivered to the brain, which can cause a death of neurons and the eventual loss of cognition.”

Beth Gardiner, author of Choked: Life and Breath in the Age of Air Pollution, homed in on this point in her new book.

“One of the things that I found — and LA was a really good place to see it — is there’s kind of this double-stranded dynamic with air pollution, which is that it affects everyone who breathes it, but it affects some people more than others,” she said.

Gardiner described a long-term study on pollution in southern California. Researchers found that black and brown communities were more likely to be polluted. The kids who grew up in those communities were more likely to have lifelong health problem, such as asthma. And, as Washington mentioned, studies show that asthma can affect processing speed, attention, learning and memory.

The stark inequality in the air we breathe is part of a broad pattern of companies building factories, power plants and landfills in black and brown communities — and of public officials approving those plans.

“One fact that I point out in the book is that a recent report found that African Americans with an income of $50,000 to $60,000 a year — solidly middle class — are more exposed to environmental toxins than are white people with $10,000 a year — profoundly poor,” Washington said, describing the results of a 2008 study from the University of Colorado at Boulder. “So it’s race. It’s not poverty. Poverty is a risk factor. Race is a far larger one.”

Washington recalled when, in 1982, officials chose Afton, North Carolina, as the dumping site for thousands of tons of soil contaminated with PCBs, toxic chemicals linked with impaired brain function.

“They said [Afton] was the best site for [the landfill], but they didn’t cite any scientific studies,” Washington said, “And people in Afton said, ‘We know why you picked us — because we’re black. You think we won’t fight back.’”

Locals protested against the dumping of PCBs, which could seep into the air or ooze into the groundwater. Hundreds of people were arrested. They brought attention to the dumping, and in so doing, catalyzed the environmental justice movement. However, they also failed to stop the landfill, showing how hard it is to fight pollution. It can be difficult to prove that dumping was racially motivated and, further, that it harmed the brain of any one person.

“If I have a heart attack tomorrow, or if I lose someone I love, or if a child suffers some sort of a cognitive impairment, it’s often very difficult, except in the most extreme cases, to say [it was pollution],” Gardiner said.

Source: Little, Brown Spark (left), University of Chicago Press (right)

This fact helps protect polluters, and so polluters are working hard to make sure the problem stays invisible, she said. Gardiner explained that companies are working hard to discredit studies that show how air pollution undermines public health — a phenomenon she calls ‘air pollution denial.’

“I think we have all become pretty accustomed over the last few years or more to this idea of climate denial. But what we’ve really started to see even more recently is air pollution denial,” Gardiner said. “I think that the motivation for that is pretty clear. It’s because those studies justify regulation that costs money to corporate polluters. And if you can cast doubt on some of that science, it’s gonna make it easier for you to try to roll back some of that regulation.”

She said that Washington needs to pay attention to the research and use it to make good policy. Measures that cut pollution, for example, help keep people healthy and spare them costly hospital visits. She pointed to studies showing that the 1970 Clean Air Act has prevented tens of millions of asthma attacks and saved Americans trillions of dollars in health care costs.

“I think it’s one of the most consequential pieces of legislation in modern American history,” Gardiner said. “And I think it’s a little bit under-sung, given its achievements. And I think that is because of this idea of invisibility.”

She said that while the Clean Air Act radically improved air quality, a lot of work remains to be done. So long as lawmakers fail to heed the research on what pollution is doing to American brains, the country as a whole will feel it. Even a small amount of contamination can shave off IQ points — maybe just a couple of *points* here and there, but over years and across the country, it starts to add up.

“What does it mean nationwide to lose an average of five IQ points? It means that we cut in half the number of gifted children, that we gain millions and millions more children who are mentally retarded,” Washington said. “Your chance of having a child who’s going to be mentally retarded goes up astronomically with these small decrements because they’re cumulative over the entire lifespan. It’s not five IQ points and that’s it. It drags down the achievement of a child and the achievement of an entire nation over time.”

She added, “We need to act on what we know now. I think we have enough studies. It’s time to actually come up with policies that are going to address it.”

Jeremy Deaton writes for Nexus Media, a syndicated newswire covering climate, energy, policy, art and culture. You can follow him @deaton_jeremy.