Flames tower over the redwoods. A thick blanket of dense, black smoke spirals upwards, leaving behind a pungent, acrid stench. A fierce wind howls, sucking at the flames. The wall of fire edges closer, slowly eating through the forest.
Wildfires are on the rise around the globe. Ever more frequent, severe and longer lasting fire hotspots are expanding to new territories, as the fingers of climate change reach farther around the planet.
But it’s not just the destructive power of the flames we need to worry about. As the landscape burns it releases a hazardous mix of pollutants into the atmosphere, including ozone, nitrogen oxide, and ultra fine particles.
The people living nearby aren’t the only ones choking on the smoke. If the wind is strong enough, toxic wildfire smoke can be detected hundreds of kilometres from the actual flames. You could be in New York, and breathe in the smoke from a wildfire raging in California without ever knowing.
This isn’t just hazardous to our physical health, it’s impacting people’s mental wellbeing as well.
But perhaps, the most insidious threat is only now being understood. New research has found that wherever the smoke goes, a rise in crime may not be far behind.
Smoke, and crime, are on the rise
Political scientist Lion Kircheis, from the University of Konstanz, grew up in southern Germany, but often holidayed in Italy, where he experienced wildfires first-hand.
He remembers the smoke scratching at his throat and making his eyes water, leaving him irritable. Were other people’s moods impacted by the smoke in the air?
Last year, Kircheis decided to investigate whether there could be a link between wildfires and behaviour and began poring over the US city of Seattle.
“Seattle is a good case study as it has above average air quality, because fresh air gets blown in from the Pacific. One day there is smoke from a wildfire. The next day the wind direction shifts, and the air is clean again,” says Kircheis.
This makes it much easier to compare days where smoke is polluting the air to days when the air is clear.
“Seattle also has lots of publicly available data from the Environmental Protection Agency [EPA], as well as quality data on traffic, crime and police use of force.”
Kircheis analysed crime report data from the Seattle Police Department from 2013 to 2023, and identified smoky days caused by wildfires on the basis of satellite data, analyses of wind direction and local monitoring stations.
What he found surprised him. For all 11 years, assaults were higher on days when nearby wildfires had released smoke and particulate matter into the Seattle air.
On average, when the particulate matter increased by 7.0 μg m−3, there were 3.6 per cent more assaults on these smoky days.
“That’s equivalent to around one additional assault per day in a city the size of Seattle. The statistical increase of 3.6 per cent is not high but, crucially, it is consistent,” says Kircheis.
“On those days, when air pollution is up by 7.0 μg m−3, it’s not as if the sun gets blocked by a thick haze of smoke or anything – the air still looks clear.
“But what is striking is that even a little bit of smoke in the air has an impact on people.”
Kircheis looked for any other likely causes for the increase in violence, such as more traffic jams or an increase in police response time. But neither of these were the case.
Interestingly, domestic violence call numbers didn’t rise when wildfire smoke was present. That suggests that outdoor exposure to wildfire smoke was the culprit – not indoor air pollution.
And, of course, socio-economic background then comes into play – although we might think we all breathe the same air, it’s not the case for those who work outdoors or do not have access to filtered indoor air.
This doesn’t mean the average person will become more violent just because they're inhaling invisible wildfire smoke. What is more likely is that aggressive conflicts will spill over into violent actions when there is wildfire-induced smoke in the air.
And it’s not just civilians who are susceptible. Police statistics suggest that officers in Seattle responded with more physical force on those smoky days.
The findings are supported by another study that used crime data from 21 major US cities from 2007 to 2020. This research showed that violent and drug-related crimes were particularly prevalent when wildfire smoke was in the air, with a 0.88 per cent increase in violent offences and a 1.35 per cent rise in drug-related crimes.
Bad air, bad day
It’s well established that air pollution is not good for our brains. Numerous studies have found that exposure to air pollution – whether from wildfires or some other source – is linked to an increase in adverse mental health.
One paper from the University of Oxford found exposure to air pollutants may lead to depression, anxiety and psychoses. Another study showed that people exposed to higher levels of air pollutants around their homes used mental healthcare services more frequently.
And, poor quality air has been linked to rises in crime. Jesse Burkhardt, an associate professor from Colorado State University (CSU) has been studying the link for years.
Back in 2019, Burkhardt analysed daily FBI crime data, air pollution maps produced by the CSU atmospheric science department from 2006 to 2013, and wildfire smoke plumes from satellite images created by the National Oceanic and Atmospheric Administration (NOAA) Hazard Mapping System.
“Our research showed that increased air pollution exposure is associated with higher levels of assaults, which is indicative of elevated aggressive behaviour,” says Burkhardt.
The researchers ensured external factors weren’t causing the link by using ‘fixed effects’, for example, only comparing data on a certain weekday in a particular city.
“If pollution went up on a Monday and crime went up on that same Monday, that was the correlation we were looking for,” says Burkhardt. “The most surprising thing about our results was that the correlation showed up in the data – the link was fairly robust.”
Something about bad air appears to be driving people towards crime.
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Crossing the barrier
The big question is why this might be happening.
One of the most hazardous pollutants produced by wildfires specifically are tiny smoke particles. These are smaller than a third the width of a human hair but can cause significant irritation when they interact with our bodies.
Kircheis suggests it could be as simple as a scratchy throat and watery eyes making people more volatile, and in some cases that is enough tinder to light the flame of aggression. But, both Kircheis and Burkhardt stress that they are not medical researchers and their studies do not provide evidence of a direct cause for the effects on the brain.
One expert whose research has looked at how nanoparticles interact with the brain is Professor Iseult Lynch – an environmental chemist, working at the interface of chemical and materials pollution and environmental policy.
Back in 2022, Lynch and her colleagues from the University of Birmingham collaborated with researchers in China to identify a possible direct pathway for inhaled particles through the bloodstream to the brain.
Plenty of evidence already exists to link air pollution and poor cardiovascular health. It’s also well established that air pollutants can pass from the nose through a very thin membrane to the brain. But what hasn’t been previously known is whether air pollutants could pass across the blood-brain barrier.
“The blood-brain barrier is almost a one way barrier – it allows only certain things to go in, and doesn't really let much back out,” says Lynch. “Hence, it protects the brain.”
Lynch and her colleagues at the University of Birmingham created an in vitro model of the blood-brain barrier – basically a replica of layers of a blood vessel and brain cells – to see whether nanoparticles could make it from the bloodstream to the brain through the blood-brain barrier.
They did – although in small numbers. The team then needed to find out whether those same particles ended up in the brain.
Colleagues in China had been looking at samples of cerebrospinal fluid (CSF), gathered from patients undergoing surgery at a local hospital. This fluid circulates around the whole brain, acting as a cushion to protect it from physical shock.
The researchers found evidence of the same air pollution particles found in the bloodstream in the CSF.
By combining their results, the two teams proved that these tiny particles of pollution can dodge the usual protective mechanisms in the body, such as immune cells and biological barriers, and make their way (in small numbers) into the brain.
“Not only did we find the same particles in blood samples as in the cerebrospinal fluid in the brain, proving they breached the blood-brain barrier, we also found agglomerates of two or more particles lumped together,” says Lynch.
“Usually, agglomerates can’t pass through the blood-brain barrier if it is intact. But their existence shows that these nanoparticles are physically damaging the barrier layer, and that's why the particles are getting through. It’s like they’re punching a hole through the barrier cells.”
The research also shows that, once in the brain, the particles are hard to clear, and are retained for longer in the brain than in other organs. The concern is that they might contribute to diseases such as dementia.
Certainly, new research by one of Lynch’s postdoctoral colleagues Dr Laura-Jayne Bradford at the University of Birmingham, published in March 2026, suggests that exposure to air pollution particles reduces cognitive performance and IQ, especially in children as a result of pre- and post-natal exposure to air pollution particles.
Indeed, the World Health Organization (WHO) recognises air pollution as the leading environmental risk factor to global human health, and ultra fine particles (100 nm or less) pose the greatest risk to human health. In 2021, an estimated 4.7 million deaths were linked to exposure to such particles alone.
Lynch and colleagues all emphasise that more research is needed to ascertain the actual mechanics of how these inhaled particles reach the brain, and the implications of the presence of particles on brain health and cognition.
However, there is a growing body of evidence pointing to the idea that air pollution, including wildfire smoke, is having a detrimental effect on mental health and causing an increase in violent behaviour.
The bigger picture
Earth’s warming climate is amplifying wildfire activity, particularly in northern and temperate forests. Droughts and extreme heat dries out more organic matter in forests, creating even more tinder for fires.
Last year was the most destructive wildfire season on record in Europe. According to the US Forest Service, fire seasons are starting earlier in the spring and extending later into autumn.
Parts of the western US, Mexico, Brazil and East Africa now have fire seasons which last more than a month longer than 35 years ago. And, according to the UN Office for Disaster Risk Reduction, wildfires now rank as the single largest driver of global forest loss.
“As a result of climate change, wildfire seasons are growing both longer and expanding geographically,” says Kircheis. “Areas like the tundra that never had wildfires before will become prone to them."
Burkhardt also sees the results of his study as another good reason to intensify climate protection activities: “Air pollution and wildfires will get worse with climate change. When Congress or the EPA wants to make the case for regulating airborne pollutants, they need to figure out the benefits of regulation and the cost of not doing so. That’s where my research comes in, as it adds up to the bigger picture.”
The message is clear – cleaner air can pay dividends not just for public health but also for public safety. Yet another reason to protect this warming planet.
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