Heavy traffic on Interstate 90, the Kennedy Expressway, is seen in Chicago. AP Photo/Kiichiro Sato

Neighborhood assaults increase when gas and particles blow in from the interstates, a new study finds.

A growing body of scientific literature tells us that air pollution is bad for the brain. Fine particles and ozone are neurological irritants, reducing productivity, weakening cognitive skills, and encouraging anti-social behavior as they enter the body. And as with noise pollution, the physical discomfort induced by breathing air layered with carbon monoxide and nitrous oxide can lead to more aggressive actions, too.

One implication about this research is that air pollution could factor into the one of the worst expressions of a hobbled brain: Violent crime. Like the old chestnut that homicide rates rise with the heat, might poor air quality have a similar psycho-neurological effect?

A new working paper published by the National Bureau of Economic Research says yes: In Chicago, a city trellised by smoggy highways, car pollution has a measurable effect on criminal activity.

Downwind effects

Evan Herrnstadt, a postdoctoral fellow at Harvard University Center for the Environment, and the co-author Erich Muehlegger, assistant professor of economics at UC Davis, examined data from the Chicago Police Department that accounted for more than two million crimes committed between 2001 to 2012. They zeroed in on criminal activity in neighborhoods that border the interstates that cut through the city: I-90, I-94, I-290, I-55 and I-57, which are major sources of local air pollution. They coupled this crime data with daily NOAA wind direction measurements, taken from weather stations along the highways and in the neighborhoods themselves.

(Lake County, Illinois Convention & Visitors Bureau)

Why look at wind direction? Because pollution wafts with the breeze. For example, “I-290 runs due west from the Chicago city center to the suburbs of Oak Park and Berwyn,” the authors write. “On days when the wind blows from the south, the pollution from the interstate impacts on the north side of the interstate”—and vice versa.

Studying wind direction was also important to their methodology: Herrnstadt says that this allowed them to isolate the causal effect of automobile pollution without too many confounding factors. The researchers could have approached their question by looking at neighborhoods that have become more or less polluted over time, and seeing how crime levels matched up to that trend. But that would introduce lots of factors they would have had to control for, such as local economic conditions and weather effects.

Instead, the researchers looked at how crime related to wind direction in pairs of neighborhoods across the interstate from one another, on the same day. “We use the partner neighborhood as a control group for the other one, depending on which direction the wind is blowing,” says Herrnstadt. Overall crime rates (and their various fluctuations), ambient pollution, and neighborhood economic activity were all factors for which the “upwind” side could act as a control.

Offenders cross a line

(NBER)

The conclusion: On days when they were on the downwind side of the interstate, neighborhoods saw roughly 2.2 percent more violent crimes—homicide, rape, assault, and battery—than they did on upwind days. There was no effect on property crime. What’s more, the increase in violent crime was driven mostly by arrests for aggravated battery, while arrests for aggravated assault actually decrease. That is to say, offenders become more physical engaged with victims.

“We think that’s suggestive of the idea that people are more irritable, more likely to cross a line that they wouldn’t have otherwise crossed,” Herrnstadt says.

Herrnstadt cautions that these findings aren’t predictive; air pollution doesn’t necessarily lead to more crime. “It’s an average effect over time,” he says. There are also caveats about the study to consider: Police data, for example, only reflects crimes that were reported, and can contain inaccuracies about time and location. And the results can’t be directly extrapolated to the rest of the country, since they are specific to the shape and density of the city of Chicago.

A $200 million problem

Still, the study offers environmental policy-makers food for thought. In their conclusion, the researchers make a rough-sketch calculation as to how much pollution-induced crime is costing the U.S., assuming that the criminological effects of air pollution scales with population. Adding up all the tangibles—medical expenses, cash losses, property theft or damage, lost earnings, even the EPA’s statistical value of a life—they estimate (conservatively) that the country loses $100-200 million annually to pollution-induced crime.

For a relatively modest effect on crime (that 2.2 percent uptick), car pollution has significant aggregate costs. And that’s not even counting respiratory disease, cardiovascular inflammation, and all the other long-term outcomes of a brain that can’t quite cut through the smog.

The good news? U.S. cities are becoming way less polluted, on the whole. But for places like Los Angeles, Fresno, and Pittsburgh—which consistently rank as some of the nation’s worst places to breathe—the benefits of cleaner air just keep stacking up.

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