Does air pollution affect human behaviour? A case study of Seattle conducted by the University of Konstanz shows that higher numbers of violent crimes take place on days when smoke from wildfires is present.

Is there a connection between air pollution and aggressive human behaviour? A recent study by political scientist Lion Kircheis from the University of Konstanz provides potential indications for such a link. In a case study of the US city of Seattle between 2013 and 2023, he examined the violent crime rate on days when nearby wildfires put smoke and particulate matter into the air. For all eleven years studied, Kircheis was able to document a consistently elevated criminal statistic: On average, there were 3.6 percent more cases of assault on smoky days than on regular ones. The research results were recently published in Environmental Research Letters.

A notable statistic
"I would especially like to emphasize that our study does not provide evidence of a direct causality for the effects of smoke on the human body", says Lion Kircheis, who cautions against jumping to quick conclusions. His work is expressly not a medical study documenting the biological effects of smoke particles on the human organism. Instead, it is a purely statistical analysis of a potential association between air pollution and violence.

This said, the results are notable, nonetheless: Over the entire eleven-year period studied, the city's police statistics recorded an average of 3.6 percent more violent crimes on days with wildfires. It is significant that this higher rate of violent crime appears to be driven by crimes committed outdoors – where people were exposed to smoke. The statistics on violence committed indoors (as evidenced by the number of emergency calls resulting from domestic violence), however, showed no statistical difference between days with smoke and those without.

Using police statistics, Kircheis also documented that officers in Seattle responded with more physical force on smoky days: There were more reported uses of force by police. The data show that, on such days, people in the city seem to feel testier.

Explaining the numbers
Is 3.6% a lot? "The statistical increase is not high, but it is consistent", Kircheis explains the results. In a city the size of Seattle, this is the equivalent of about one additional case of violent crime per day. Though this may not sound like much, the effect was documented consistently over the course of the eleven years studied.

So, does this mean that people become violent as a result of breathing smoke? "No. On the whole, the vast majority of people in Seattle does not act violently, even on smoky days", Kircheis says, "but the probability that potentially violent conflicts actually escalate to violence is greater on days with wildfires."

"We don't all breathe the same air", Lion Kircheis reflects. People are exposed to different levels of particulate matter – for example, some people work outdoors or do not have access to filtered indoor air. This can affect their health and potentially also their behaviour. Kircheis had the idea to investigate the connection between air pollution and aggressive behaviour after experiencing a wildfire for himself: "It feels uncomfortable when there is smoke in the air. It is really unpleasant: Your throat is scratchy and your eyes tear up. You feel more irritable as a result. So, I asked myself if it would be possible to document a statistical effect that would be observable at the societal level."

For his study, Kircheis used publicly available police and traffic statistics for the city of Seattle. He identified smoky days on the basis of satellite data, analyses of wind direction and local monitoring stations. Seattle was chosen because it is a large city with comparatively high air quality and well-documented, publicly available data (e.g. police statistics). There were wildfires on a total of 447 days during the eleven-year period in the Seattle region – that is 11.1 percent of the time period studied.

The results are also relevant beyond Seattle, Kircheis says: "As a result of climate change, fire seasons are both expanding geographically and growing in length. In some cases, smoke can spread hundreds of kilometres. Just last year (2025), Europe experienced a record year of wildfires." Kircheis sees the results of his study as "another good reason to intensify climate protection activities."

Key facts: Original publication: Lion Kircheis, Wildfire smoke increases assaults: evidence from Seattle, 2026 Environ. Res. Lett. 21 044020 Link: https://iopscience.iop.org/article/10.1088/1748-9326/ae436c DOI 10.1088/1748-9326/ae436c Lion Kircheis is a doctoral researcher in the research team "International Politics and Conflict Research" led by Professor Nina von Uexkull at the University of Konstanz. He studies the causes and consequences of violence and conflicts, with particular attention to environmental factors and climate change.