Air pollution is considered a severe health problem across the globe, causing millions of deaths every year due to exposure to a mix of particles and gases. But breathing dirty air doesn’t only make you sick but also more aggressive, according to research.

A set of studies by researchers at Colorado State University found strong links between short-term exposure to air pollution and aggressive behavior, in the form of aggravated assaults and other violent crimes across the continental United States.

The team cross-analyzed 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 U.S. Environmental Protection Agency monitors; and daily data on wildfire smoke plumes from satellite imagery.

Rates of pollution are usually measured 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 research showed 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.

“We’re talking about crimes that might not even be physical – you can assault someone verbally,” co-author Jude 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. The results only show a strong correlative relationship between such crimes and levels of air pollution, not looking at other possible explanations.

“The results are fascinating, and also scary,” Jeff Pierce said. “When you have more air pollution, this specific type of crime, domestic violent crime in particular, increases quite significantly.”

Air pollution

The combined effects of ambient (outdoor) and household air pollution cause about 7 million premature deaths every year, largely as a result of increased mortality from stroke, heart disease, chronic obstructive pulmonary disease, lung cancer, and acute respiratory infections.

More than 80% of people living in urban areas that monitor air pollution are exposed to air quality levels that exceed the World Health Organization guideline level of 10µg/m3, with low- and middle-income countries suffering from the highest exposures.

The major outdoor pollution sources include vehicles, power generation, building heating systems, agriculture/waste incineration and industry. In addition, more than 3 billion people worldwide rely on polluting technologies and fuels (including biomass, coal, and kerosene) for household cooking, heating and lighting, releasing smoke into the home and leaching pollutants outdoors.