A new report by researchers at the London School of Economics (LSE) suggests that crime in the capital is being driven by air pollution.

Their results show more polluted areas will see spikes in crime, particularly for less serious offences like shoplifting and pickpocketing.

This phenomenon appeared to strike London indiscriminately, affecting both rich and poor areas, and the authors of the paper say tackling air pollution could help the city tackle crime as well.

Overall, the researchers’ findings show the crime rate in London is over 8 per cent higher on the most polluted days than the least.

While the study relies on observational data and therefore cannot make definitive conclusions, it adds to a small but growing body of evidence linking pollution and crime.

“Higher levels of air pollution cause an increase in crime,” Dr Sefi Roth, an expert in environmental economics at LSE who co-authored the report, stated to The Independent.

“We analysed data on air pollution and more than 1.8 million crimes over two years in London – we link that with data on air pollution, and we use various statistical techniques to ensure our results are robust.”

Previous experiments have shown that increased levels of particulate matter in the air lead to increased blood levels of stress hormones such as cortisol.

The authors therefore suggest that behavioural changes resulting from increased stress hormone levels may in turn lead to an increased likelihood a person will commit a crime.

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“What this means is that pollution can have a negative effect on people’s ways of thinking, including decision making and the way they think about future punishment,” said Dr Roth.

“Higher levels of pollution mean higher levels of cortisol. Higher levels of cortisol affect the way that punishment is being perceived by criminals.”

The results come as new figures released by the Office for National Statistics (ONS) revealed a spike in knife crime across England and Wales in 2017.

However, the researchers noted they saw no link between air pollution levels and the most violent forms of crime such as rape and murder.

“We did not find that London’s on-going spate of knife crime would be affected by improved air quality,” said Dr Roth.

“However, if the number of less serious crimes could be reduced, the police could potentially be freed up to allocate more resources to these types of very serious incidents.”

Dr Roth and his LSE colleagues used various statistical techniques to examine the links between air pollution in specific areas and crime.

Their findings took factors such as temperature, humidity and rainfall into account, and compared crime rates and pollution data within boroughs to ensure like was being compared with like.

The scientists also made use of a “natural experiment” involving the wind to establish a causal link between the two.

“Imagine there is a cloud of air pollution in London that happens every day, and then the wind reshuffles this cloud to different parts of the capital,” explained Dr Roth.

“What we do as researchers is essentially follow this cloud of pollution and see what happens to crime on a daily level where we see this cloud.

“Sometimes it will go into poorer areas, sometimes into rich areas and we found wherever it goes we see an increase in crime.”

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Their results were published in a discussion paper for the IZA Institute of Labour Economics, an independent research organisation.

Though the paper has yet to be peer reviewed and published in an academic journal, it has undergone internal peer review at LSE’s Grantham Research Institute on Climate Change and the Environment.

The research is not the first to explore links between air pollution levels and crime.

A study published in the journal Psychological Science also claimed stress related to particulate matter in the atmosphere was leading to increased crime levels.

Professor Julia Lee, one of the co-authors of that study, concluded “air pollution not only corrupts people’s health, but also can contaminate their morality”.

However, not all scientists are convinced there is a definitive cause and effect relationship between air pollution and crime.

“This is an interesting paper, but there are several points to bear in mind,” said Professor Kevin McConway, a statistician at The Open University.

“The researchers’ suggestion that reducing air pollution could reduce crime makes sense only if the increased crime rate that they observed at time of high pollution is actually caused by the pollution.

“The observational design of the study makes it impossible to be sure about that.”

Although Professor McConway acknowledged the “natural experiment” used by Dr Roth and his collaborators, he said this did not comprehensively prove air pollution was causing crime.

“For that to be valid, as the researchers make clear, one has to be confident that, after allowing for variables such as other aspects of the weather, the only way that the wind direction could affect crime rates is through its effect on air pollution levels,” he said.

“That’s quite a strong assumption and one that can’t be tested directly with the data that they have.”

Despite these reservations, Dr Roth said that similar effects had been seen in American cities including Chicago, and emphasised that he thinks this phenomenon is likely universal.

“We did the study in London but it can be applicable to many other cities in the UK and elsewhere,” he said.

“Places like Glasgow, Bristol and Leeds also have very high levels of air pollution – it’s not just for Londoners, it’s for people all over the world.”

While this sounds like a bad thing, Dr Roth said the fact that air pollution is theoretically a manageable problem means there is an opportunity to kill two birds with one stone.