Wellness

Recent studies show air pollution heightens the risk of common mental disorders by 39 per cent. Is the Air You Breathe Messing with Your Head?

Scientists are learning more about the profound impact that pollution has on the brain. Clare Considine investigates.

By Clare Considine

3 March 2021
L

os Angeles, 26 July 1943. At the height of a long, hot and hazy summer, a thick, alien fog descends downtown, stinging the eyes of Angelenos and making their noses stream. The second world war is raging, and the panicked population brace themselves for a chemical attack. But in fact, this is a visual manifestation of what will come to be understood as air pollution.

It was the tipping point, the first sight of smog in LA. The result of an unprecedented influx of newcomers to an already populated city, located at the bottom of a valley with little breeze, which was at the time the world’s biggest market for motor vehicles.

Almost 80 years on, we don’t need apocalyptic scenes to alert us to the threat – we’ve lived with it. Ask any inner-city child of the ’80s, pre-congestion charge and pre-diesel filters, and they know what engine fumes smell and taste like. We’re more than clued-up now, and the air is cleaner in many parts of the country than it was (according to government data, since 2011 there has been an overall decrease in the number of days with moderate or higher pollution in urban areas, although the picture is very different in rural areas). But experts claim air pollution is still having a damaging effect on our bodies and, as we are just beginning to understand, our minds.

Image by Valou C

A review of air pollution studies from 16 countries was published by researchers at University College London in December 2019, and cited compelling evidence of a link between toxic air and increased risk of depression and suicide. At the end of 2020, another study carried out in south-east London and led by Dr Ioannis Bakolis – Senior Lecturer in Biostatistics and Epidemiology at King's College London – showed similar findings. The report followed 1,600 adults over a five-year period and concluded that heightened exposure to particulate matter and nitrogen dioxide (more on this in a second) increased the risk of common mental disorders by 39 per cent.

Depression and other forms of mental ill-health are of course endlessly complex, the result of myriad nuanced and coexisting factors, including inequalities in income (one 2018 analysis of the results of previous studies published in the journal World Psychiatry, for instance, found that income inequality was a key determinant of poor mental health), education, living conditions and more. But even when these variables were removed from the King’s College study data, the research still showed that almost 40 per cent increase in mental ill-health. “That’s the amazing thing, even when you control for all those other features [such as] socioeconomic factors, the air pollution signal remains,” says Dr Ian Mudway, a senior lecturer at Imperial College London and part of the research team alongside Bakolis. “It’s a very robust finding.”

Currently, the understanding of how air pollution causes damage to our brains is limited. But Isobel Braithwaite, Academic Clinical Fellow in Public Health at UCL – who led the groundbreaking 2019 study which linked toxic air with depression and suicide – says that we can look to a “set of best guesses that make sense biologically and are backed up by animal studies”.

She describes various pathways: “One of those is inflammation of nerve tissue in the brain, but also wider inflammation in the body that then affects the brain.”

Image by Valou C

For decades, scientists have been studying inflammation and the immune response when foreign particles – like pollen or, indeed those caused by air pollution – enter the body. By its very nature that immune response should be a good thing: our bodies’ means of fighting back against unwanted invasion. When sensing attack, our brains activate immune cells called microglia that defend against potentially dangerous organisms.

But microglia, when produced consistently over long periods, have the power to damage cells and biological structures in the brain. Though chronic inflammation has long been linked to the likes of arthritis, cancer and diabetes, it now looks as though it may also be a trigger for neurological problems. “Chronic inflammation of the brain is associated with some mental health disorders, as well as with faster ageing and increased risks of dementia,” Braithwaite explains. In 2009, researchers from Manchester University had begun to find links between inflammation and psychiatric disorders such as depression, anxiety and schizophrenia.

“Immune cells think a [pollution particle] is a bacteria, go after it and try to kill it by releasing enzymes and acids,” says Prof Dean Schraufnagel of the University of Illinois, who ran a 2019 global review which offered up convincing evidence that air pollution has the power to damage every organ in our body. “Those inflammatory proteins spread into the body, affecting the brain, the kidneys, the pancreas and so forth. Basically, our bodies have evolved to defend themselves against infections but not pollution.”

Image by Carl Abiador

One 2020 study led by Lilian Calderón-Garcidueñas at the University of Montana discovered polluted air particles in the brain stems of 186 young people in Mexico City who died suddenly between the ages of 11 months and 27 years. These particles are likely to have reached the brain after being inhaled into the bloodstream, and have been thought to trigger Alzheimer’s and Parkinson’s. Findings published by the American Heart Association, meanwhile, suggest that breathing dirty air causes stress hormones to rise. Released from our brains’ hypothalamus, hormones like cortisol are there to aid our fight-or-flight response, but when released systematically over long periods, the central nervous system has no opportunity to return to normal. “Chronically high levels, which are associated with regularly being on high alert or feeling stressed, can be harmful in various ways, including anxiety, depression, weight gain and difficulty sleeping and concentrating,” says Braithwaite.

The question, of course, is what can be done about it? Prof Alastair Lewis, chair of the UK government’s Air Quality Expert Group, says that it is important to distinguish between two major components of the harmful air we breathe. “Nitrogen dioxide is the gas that we associate with diesel cars,” he explains. “Particulate matter is a more general group of tiny particles coming from a range of sources including cars, combustion, cooking, fuel use and agriculture”.

The two elements affect us differently, he continues. “Nitrogen dioxide lives in the atmosphere for 30 minutes to a couple of hours and is highest at the roadside. If you don’t live by a main road, it’s possible to try to avoid it. Particles are much harder. They exist in the atmosphere for one to two days and are spread out across the whole of the UK.”

Image by Tom Smith

Despite what seem to be a robust set of theories around why these polluting particles have such a profound impact on our mental health, Bakolis, lead researcher of the south-east London study carried out by King’s College, talks about three “big questions” that remain unanswered: “Which specific component of air pollution is the most relevant? How do inhaled pollution particles gain access to the brain? And which pollutants can affect the brain?”

To answer these, studies are likely to become increasingly long-term, and more precise in terms of how exposure to air pollution is measured. Joanne Newbury, a psychological researcher and fellow in psychiatric epidemiology at Bristol University, points to exciting new “wearable monitors – little backpacks that measure air pollution that people are exposed to throughout the day”.

But the bottom line is that we already know enough to know that changes need to be made on a global scale and at breakneck speed. Here in the UK, congestion charges, ultra-low emission zones and an increasing embrace of electric cars have all contributed to cleaner air. “Nitrogen dioxide is reducing in concentration in the UK. We won’t be able to buy a diesel car in 2030,” says Lewis. “By then we will hopefully look back on nitrogen dioxide as a problem that we’ve largely fixed.”

In 2018, the last time this data was collected, London reached the EU’s annual legal air limit before the end of January. Worryingly, despite this, we are still on a comparatively good footing globally. “If you take London as the only really large mega-city in Europe, its air quality is actually pretty good.” says Lewis. He cites the likes of Lagos, Delhi and Beijing as cities in much worse shape. “The UK starts with some competitive advantages due to the fact that we’re on the western side of Europe. We have clean air that comes off the Atlantic. Plus, London has very little heavy industry surrounding the city now.”

But we must not be complacent, because new environmental problems arise all the time. Particulate matter is a byproduct of a bevy of sources, including intensive meat farming, gas boilers, wood-burning stoves and even electric cars, where pollution comes not from a tailpipe but from the friction between the wheels and the road. For this, Lewis suggests, we need a collective adjustment in our thinking. “The key is that everybody should educate themselves about what the sources of pollution actually are.”

As with all discussions surrounding mental health, the most important thing to remember is: be kind to yourself. “If you take a lot of the stressors on public health, like obesity or alcohol consumption, we can do something about it. But for air quality there’s only so much that the individual can do,” says Lewis. Things you can try include avoiding using cars where you can, being mindful of your walking or cycling routes and continuing to exercise, even if you live in a polluted area.

'Things you can try include avoiding using cars where you can, being mindful of your walking or cycling routes and continuing to exercise, even if you live in a polluted area'

Plus, there is a sense of empowerment to be had by lobbying the policymakers. Have lockdown food deliveries turned your road into a rat run? Write to your MP about it.

“We already know that there’s this big physical effect to health from air pollution, and yet we still haven’t adopted the World Health Organization’s air-quality guidelines,” says Braithwaite. “I don’t know if research really has all the answers. Maybe it’s time for coordinated action: collaboration between researchers and citizens to really drive policy change.”

The Short Stack

The evidence is building, that air pollution affects our mental health as well as our physical health – and not enough is being done to tackle it.

By Clare Considine

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