AIR POLLUTION is nothing to sniff at. Perhaps a third of all deaths from strokes, lung cancer and respiratory diseases can be linked to toxic air. In some cities, breathing outdoors is as dangerous as smoking 25 cigarettes a day. But whereas the health problems associated with air pollution can take years to manifest themselves, new research highlights a much more immediate—and violent—risk.

Breathing dirty air is linked to aggressive behaviour, according to a new paper by Jesse Burkhardt and his colleagues at Colorado State University and the University of Minnesota. Using crime data from the Federal Bureau of Investigation and air-pollution data from the Environmental Protection Agency, the authors analyse the link between air pollution and violent crime in 397 American counties between 2006 and 2013.

They find that a 10% increase in same-day exposure to PM2.5 (particulate matter less than 2.5 microns in diameter) is associated with a 0.14% increase in violent crimes, such as assault. An equivalent increase in exposure to ozone, an air pollutant, is associated with a 0.3% jump in such crimes. Pollution levels can easily rise by much more than that. Last November, owing to wildfires, PM2.5 levels in San Francisco rose seven times higher than average. Correlation is not causation of course (there may, for example, be a third variable affecting both pollution and crime) and the authors are cautious not to speculate about the precise mechanism by which contaminated air might lead to more rapes or robberies.

This is not the first time researchers have identified a relationship between pollution and crime. In the 1970s America banned lead-based paint and began phasing out leaded petrol; two decades later, crime fell. Many researchers now argue that the two developments were linked. In a paper published in 2007, Jessica Wolpaw Reyes, an economist at Amherst College, estimated that the drop in lead exposure experienced by American children in the 1970s and 1980s may explain over half of the decline in violent crime in the 1990s.

The findings of Mr Burkhardt and his co-authors suggest that cleaner air could reduce violent crime still further. The benefits would be substantial. The authors estimate that a 10% reduction in daily PM2.5 and ozone exposure could save America $1.4bn a year through reduced assaults (the savings range from the cost of the immediate police response to lost productivity due to injuries). A lot of people get angry about pollution. Now it seems they may do so because of it, too.