Commuters wear masks as they walk along a road amid heavy smog in New Delhi, India, on November 9th, 2017. (Photo: Sajjad Hussain/AFP/Getty Images)

Looking for an excuse the next time you get caught doing something unethical? If you live in a highly polluted city, you may be in luck.

New research offers evidence that air pollution inspires unethical behavior, ranging from low-stakes cheating to criminal activity. It reports this is likely due to polluted air increasing personal anxiety, which can throw people's moral compasses out of whack.

"This research reveals that air pollution may have potential ethical costs that go beyond its well-known toll on health and the environment," lead author Jackson Lu of Columbia Business School said in announcing the results. "Our findings suggest that air pollution not only corrupts people's health, but can contaminate their morality."

Evidence of a link between pollution and crime has been growing for several years. A 2015 study found violent crimes were 2.2 percent higher in neighborhoods downwind of air pollution. Another released in December linked higher levels of particulate matter with higher rates of juvenile delinquency.

In the first of their four studies, Lu and his colleagues add to this evidence. Using data from the Federal Bureau of Investigation and the Environmental Protection Agency covering the years 1999 to 2009, they compared air pollution levels and crime rates in 9,360 American cities.

They found that, even after taking a variety of factors into account, including the poverty and unemployment rates and the numerical strength of the local police force, cities with higher levels of air pollution also had higher rates of seven major categories of crime, including murder, aggravated assault, and robbery.

But was smog really a catalyst for these crimes? To attempt to find a causal link, the researchers conducted an online study featuring 256 participants. They viewed a photograph of an outdoor scene featuring either clean or polluted air, and were asked to imagine it was where they lived. They were then instructed to "reflect on how they would feel if they walked around in this area and breathed the air."

Afterwards, participants completed a purportedly unrelated test. At the beginning, they were told that, due to a "computer glitch," they could easily find the correct answers, but were told not to do so. Not surprisingly, many ignored that instruction.

"On average, participants cheated on 2.77 out of five trials," the researchers report. More importantly, those who had been contemplating living in a polluted place "cheated significantly more times" than their clean-air counterparts.

For the third study, 129 American university students looked at 15 photos of contemporary Beijing. Half saw photos taken on a clear day, while the others looked at the same locations on a day marked by heavy air pollution.

All were asked to imagine they lived in the Chinese capital and write a diary entry about their experience. Afterwards, as in the previous experiment, they performed a task that allowed for cheating.

Research assistants read through their faux diaries and rated the extent to which the entries reflected distress, irritability, and nervousness, as well as happiness and excitement. They found the descriptions expressed significantly more anxiety, and significantly less positivity, if the participants had viewed the city on a polluted day.

Scores on the final task indicated those participants were also more likely to cheat. The fourth study, which featured 141 participants from India, one of the world's most polluted nations, produced similar results.

"The psychological experience of air pollution increased anxiety, which in turn increased people's tendency to behave unethically," the researchers write in the journal Psychological Science.

The results "provide another compelling reason for policymakers to combat air pollution," they conclude. "A less-polluted environment is not only a healthier one, but a safer one."