Between 1 and 8 January 2017, London breached its annual air pollution limits. In just a week, the city broke EU regulations that limit nitrogen dioxide (NO2) emissions – which are produced by diesel vehicles. It is a publishable, verifiable, undeniable scientific fact that this gas is connected to heart and breathing problems.

Depression; anxiety; Alzheimer’s; poor academic performance – these are just some of the things that scientists have connected to air pollution in recent years. Research is relatively young, and it is dangerous to establish cause and effect too freely, but it now seems apparent that the smog affecting our bodies could also be affecting our brains.

“Our study found that those with higher exposures to fine particulate matter, a type of air pollution, were more likely to experience high anxiety symptom levels,” says Dr Melinda Power, a professor at George Washington University who warns against establishing causality too early. In 2015, Power published her research, which used data from 70,000 women in the Nurses’ Health Study who then filled out a survey on their anxiety levels. She discovered that fine particles in the air (which come from, among other things, cars and factories) were connected to increased anxiety levels, and that the more recent the exposure, the higher the level of anxiety experienced.

“As relatively little research has been done on the relationship between air pollution and mental health, further research is needed to confirm our findings,” she says, noting that women in more polluted areas may experience other stresses that caused their anxiety.

When it comes to identifying a cause for the recent epidemic in mental health disorders, such as anxiety and depression, often simpler explanations get precedence in the media. The idea that social networks like Facebook “make us” depressed has been flying around for years. This is easy for individuals to identify as an affect on their mental health, if it is affecting them that way. But not many of us stop to consider how the invisible air around us might be affecting our mental health, and it is much harder to find any anecdotal evidence of whether this is the case. For more answers, we must turn to science’s most faithful research assistant: mice.

“We got into this research by accident,” says Dr Randy Nelson, a professor at the Ohio State University Wexner Medical Center. “I was walking across campus and saw a trailer that was being used to expose mice to particulate matter. The work was being directed by a cardiologist who had demonstrated that exposure to fine airborne particulate matter caused inflammation in the heart.

“It seemed reasonable to hypothesize that exposure to this type of air pollution would also cause inflammation in the brain and that is often associated with depression and cognitive impairments.”

Nelson and his team exposed mice to fine particle air pollution in the same high levels that are found in urban areas. They discovered that after ten months, mice exposed to polluted air took longer to complete a maze task than mice exposed to filtered air. More incredibly, the “polluted” mice also exhibited depressive symptoms and “behaviour despair”, such as an unwillingness to swim when placed in water.

Researchers at Duke University also found pregnant mice exposed to diesel exhaust had offspring who exhibited increased anxiety.

In 2015, scientists at the University of Utah found a link between air pollution and suicide in middle-aged men. It is also already proven that air pollution affects our physical health, and Power notes that this, in turn, can affect us mentally. “Air pollution may be related to mental health, particularly anxiety, through effects on oxidative stress and systemic inflammation or through promotion or aggravation of chronic diseases,” she says. Put simply, being sick can make us depressed.

And yet while research about how air pollution affects mental health is in its infancy, there is significantly more information about the link between air pollution and cognitive health. Power has found that men with higher past exposures to traffic-related air pollution had worse cognitive functions. An extensive 2012 article by the American Psychological Association outlines the many studies in this area.

A spokesperson for the European Commission, which sets our air quality targets, says the World Health Organisation is now reviewing evidence about mental health, and new targets will take this into account. Power says more “big, high-quality, longitudinal studies are needed”, yet Andrea Lee, a healthy air campaigner for ClientEarth, says we need to act sooner rather than later. “As research continues in all of these areas, what is beyond doubt is that air pollution in the UK is above legal limits,” she says.

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