In a paper published by the National Bureau for Economic Research, two researchers say they have found a startling connection between exposure to outdoor pollution and an increase in violent crime.

The study, released last week, presents “the first quasi-experimental evidence that air pollution affects violent criminal activity,” the researchers say. The authors are Evan Herrnstadt of Harvard’s Center for the Environment and Erich Muehlegger of the University of California, Davis.

They drew on a vast crime dataset from the Chicago Police Department — more than 2 million major crimes committed between 2001 and 2012 in the city. The data included the crimes’ dates and locations, and researchers were able to figure out how close they were to major interstates that cross the city of Chicago, like I-290, which runs west and east. Finally, they had meteorological data, and thus could know when winds were blowing tailpipe pollution from vehicles into neighborhoods south of I-290, or into neighborhoods north of it (to give one example).

The study design gets around a large problem that occurs if you simply try to correlate pollution with the occurrence of crime in a given location — namely, there are many counfounding factors such as income, Herrnstadt explains. By contrast, the study design of comparing neighborhoods with themselves, on days when they are and aren’t downwind, “allows us to drill down and really identify this causal effect of pollution on crime,” he says.

Through this method, the authors report, they find an estimated 2.2 percent higher prevalence of violent crime when a neighborhood is on “the downwind side” of these major roads or interstates. However, there was no effect on property crimes. The researchers say they cannot distinguish which car-related pollutants – such as carbon monoxide or nitrogen oxides – are actually responsible for the effect.

The authors also calculate that the economic cost of the crime caused by automotive pollution in this way could be $ 100 million to $ 200 million per year.

It isn’t clear precisely how the pollution would be affecting people in a way that promotes criminal behavior — especially violent criminal behavior — but the authors list a number of possibilities, including cognitive impairment and just greater irritation.

“We think the mechanism here is that you’re exposed to more pollution, either it’s an irritant, or it affects your impulse control in some other way, and basically results in you crossing lines that you wouldn’t otherwise cross,” says Herrnstadt. That’s why, he says, the data suggest more assault cases escalating into cases of battery in the presence of pollution.

The research is part of the NBER’s working paper series and papers in this series “have not undergone the review accorded official NBER publications,” although as Paul Krugman notes, economic journals can be so slow to publish that working papers provide a very good forum for researchers to share their results (which others can then criticize).

The result is actually not so surprising when considered in the context of prior research, suggests Josh Graff Zivin, an economist at the University of California-San Diego who commented on the paper at the Post’s request (but was not involved in the research).

“There is a body of epidemiologic health literature that shows that pollution at high levels can impair judgement, can increase aggression, can impair cognition,” says Zivin.

Indeed, a large body of prior research has tied another environmental factor — warmer temperatures — to crime and violence, and there has also been research suggesting that lead pollution drove a large amount of criminal activity that, once lead was phased out of gasoline, dropped off. There is also a small but growing body of research suggesting that poor indoor air quality can have significant cognitive effects.

What’s more surprising about the study, says Zivin, is that “if you think the crime that they see in their paper, violent crimes, if you think that those are in part about impulse control, then this is really novel. As far as I know, we don’t really have evidence that pollution can lead to increased impulsivity.”

Another economist familiar with the work, Reed Walker of the University of California-Berkeley, said the researchers did a good job of assuaging his skepticism. “There is an increasingly well developed literature studying the relationship between temperatures and crime, which made me initially somewhat skeptical of the findings in this paper (given the strong correlation between temperature and pollution),” said Walker by email. “However, the authors are careful to control very flexibly for temperature in their primary specifications.”

Walker concludes that “this is interesting and important work and, at the very least, begs for more research on the subject.”

The new study only looked at one type of air pollution — from vehicles — meaning that if the research is right, it could be the tip of the iceberg.

“If we think this extends to all pollution, it could be a lot bigger than this,” Herrnstadt says.

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