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It was the dirty air that made me do it…

A cross-disciplinary study by Colorado State University researchers claims "strong links" between short-term exposure to air pollution and aggressive behaviour, in the form of aggravated assaults and other violent crimes across the continental US.

The results were derived from daily Federal Bureau of Investigation crime statistics and an eight-year, detailed map of daily US air pollution. The paper's lead author is Jesse Burkhardt, assistant professor in the department of agricultural and resource economics, who teamed up with fellow economist Jude Bayham in the same department; Ander Wilson in the department of statistics; and several air pollution experts in civil engineering and atmospheric science.

The CSU researchers cross-analysed three highly detailed datasets: daily criminal activity from the National Incident-Based Reporting System managed by the FBI; daily, county-level air pollution from 2006-2013 collected by US Environmental Protection Agency monitors; and daily data on wildfire smoke plumes from satellite imagery provided by the National Oceanic and Atmospheric Administration's Hazard Mapping System.

Air pollution scientists typically measure rates of pollution through concentrations of ozone, as well as of "PM2.5," or breathable particulate matter 2.5 microns in diameter or smaller, which has documented associations with health effects.

The study said 83% of crimes considered "violent" by the FBI are categorised as assaults in crime databases. In the researchers' study, they observed whether crimes occurred inside or outside the home; they found that 56% of violent crimes and 60% of assaults occurred within the home, which is an indication that many such crimes are tied to domestic violence.

The research results show a 10 microgram-per-cubic-meter increase in same-day exposure to PM2.5 is associated with a 1.4% increase in violent crimes, nearly all of which is driven by crimes categorized as assaults. Researchers also found that a 0.01 parts-per-million increase in same-day exposure to ozone is associated with a 0.97% increase in violent crime, or a 1.15% increase in assaults. Changes in these air pollution measures had no statistically significant effect on any other category of crime.

"We're talking about crimes that might not even be physical – you can assault someone verbally," co-author Bayham said. "The story is, when you're exposed to more pollution, you become marginally more aggressive, so those altercations – some things that may not have escalated – do escalate."

The researchers made no claims on the physiological, mechanistic relationship of how exposure to pollution leads someone to become more aggressive; their results only show a strong correlative relationship between such crimes and levels of air pollution. The researchers were careful to correct for other possible explanations, including weather, heat waves, precipitation, or more general, county-specific confounding factors.

The tool that allowed the team to overlay crime data with pollution data was originally used in collaboration with CSU epidemiologist Sheryl Magazmen to study health effects from air pollution, explained co-author Jeff Pierce, associate professor in the department of atmospheric science and a Monfort Professor. Pierce, associate professor Emily Fischer and researchers Kate O'Dell and Bonne Ford, had previously worked with Magzamen to detail how smoke and particulate matter exposure correlated with things like hospitalisations and asthma inhaler refills.

Burkhardt had been wanting to study whether breathing smoke could enact behavioural change when he met atmospheric scientist Pierce.

"Several years ago, Fort Collins experienced a fairly severe wildfire season," Burkhardt said. "The smoke was so bad that after a few days, I started to get frustrated, and I wondered if frustration and aggression would show up in aggregate crime data."

Pierce recognised that the pollution-concentration product he and colleagues had designed, which provided detailed concentrations of total particulate matter and the fraction from smoke, would be useful for Burkhardt's desired application. "The results are fascinating, and also scary," Pierce said. "When you have more air pollution, this specific type of crime, domestic violent crime in particular, increases quite significantly."

The economists calculated that a 10% reduction in daily PM2.5 could save $1.1m in crime costs per year, which they called a "previously overlooked cost associated with pollution."

The authors remain interested in the relationships between pollution and cognitive outcomes, Burkhardt said. They are now working with a large online chess platform to determine if increased pollution exposure is correlated with worse chess performance.

The results are just one outcome of CSU's philosophy around "cluster hiring" faculty from disparate fields to study interdisciplinary problems. In this case, several of the researchers came to CSU under the Partnership for Air Quality, Climate and Health initiative launched several years ago by the Office of the Vice President for Research.

We estimate the effect of short-term air pollution exposure (PM2.5 and ozone) on several categories of crime, with a particular emphasis on aggressive behavior. To identify this relationship, we combine detailed daily data on crime, air pollution, and weather for an eight-year period across the United States. Our primary identification strategy employs extremely high dimensional fixed effects and we perform a series of robustness checks to address confounding variation between temperature and air pollution. We find a robust positive effect of increased air pollution on violent crimes, and specifically assaults, but no relationship between increases in air pollution and property crimes. The effects are present in and out of the home, at levels well below Ambient Air Pollution Standards, and PM2.5 effects are strongest at lower temperatures. The results suggest that a 10% reduction in daily PM2.5 and ozone could save $1.4 billion in crime costs per year, a previously overlooked cost associated with pollution.

Jesse Burkhardt, Jude Bayham, Ander Wilson, Ellison Carter, Jesse D Berman, Katelyn O'Dell, Bonne Ford, Emily V Fischer, Jeffrey R Pierce

[link url=""]Colorado State University material[/link]

[link url=""]Journal of Environmental Economics and Management abstract[/link]

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