Last year, Xi Chen, a public health researcher at Yale University, was working on a major research study investigating air pollution in China when he made an unexpected discovery. With his colleagues Xi Zhang and Xiaobo Zhang, he had been monitoring the performances of over 25,000 people on a test of math and language skills and correlating it with the levels of air pollution. Air pollution, of course, has been conclusively linked to heart and lung problems, but could it affect our brains as well?
When Chen looked at the results, he found that people who had been exposed to air pollution the longest were seeing the steepest drop in their cognitive test scores. The overall drop was roughly equivalent to a loss of one year of schooling. At first they thought this strong correlation might be driven by some other factors, but after running further statistical tests, the evidence that air pollution affected brain function was incontrovertible. “This research suggests that there is a hidden social cost of air pollution which is very large,” Chen says.
Their research, published in the Proceedings of the Natural Academy of Sciences, was the first study to link air pollution to cognitive decline. Similar research conducted in London found that air pollution may be linked to an increased link of developing dementia. That study, published in September 2018 and carried out by researchers from Imperial College London and King’s College London, followed 131,000 patients over an average of seven years.
“The fact that these studies have been able to detect a measurable, and clinically significant effect on cognitive function is extremely worrying indeed,” says Guddi Singh, a paediatrician at East London NHS Foundation Trust, who is involved in Doctors Against Diesel, a campaigning group made up of medical professionals. “This is not just of academic interest.”
Toxic air consists of a combination of particulates: minuscule (less than ten micrometres) particles released through burning fossil fuels, pollen, ozone, and gases such as sulphur dioxide and nitrogen oxide. Oxygen combines with sulphur particles released from exhaust fumes in cars, or nitrogen molecules from aerosols, to form a toxic mixture. The most dangerous particulates, PM2.5 (because it measures 2.5 micrometres) are released by wood burning and fossil fuel combustion. The finer the particulates, the more harmful they are to our bodies, because they are too small to be filtered by our nose and lungs (which often filter larger, coarse particles such as pollen).
However, the exact relationship between those particles and our actual brain function remains unclear. Singh says that air pollution is thought to affect processes which are related to inflammation. This can lead to a build up of proteins which associated with dementia, such as amyloid peptide.
Fecht, a medical researcher at Imperial College London who researches air pollution and public health, emphasises that we still do not know how a mixture of pollutants can affect our brains. She explains that people who breathe polluted air throughout the course of the day are more likely to suffer from oxygen deficiency, which impacts brain functions. When fine particulates (less than ten micrometres) enter our body, they corrode alveoli, small sacs in the lungs which are responsible for oxygen transport between the lungs and our blood, and stop them from working effectively, creating a knock-on effect. They can also interfere with our body’s ability to regulate neuro-inflammation.
Chen’s study also shows that, if you live in a city, the cognitive impact may be more pronounced. Densely packed traffic, combined with narrow streets and high-rise buildings, often trap pollutants at street level. In a city like London, for instance,95 per cent of the population live in areas with air pollution levels that exceed the WHO guidelines for the most dangerous toxic particles (PM 2.5). Sadiq Khan, the mayor of London, has made air pollution a priority, carrying out audits on air quality around primary schools, introducing Ultra Low Emission Zones, and re-fitting diesel buses.
“If [Sadiq] Khan does everything he says he will, then it will make a difference,” says Simon Alcock, head of UK public affairs at ClientEarth, which have taken the government to court (and won) over the levels of illegal air pollution in the air in 2017. ”But the national government needs to act too – we need a national network of clean air zones, changes to the tax system and mobility credits to help people buy electric vehicles, more clean air legislation – to show that they’re taking this seriously.”
Over 92 per cent of the world’s population are continually breathing in unsafe air, which has led air pollution to be designated a public health crisis. The Chinese study shows that the effect could be greater than previously thought, which should spur politicians into action. “It will require political courage. But when the rewards are a healthier population and healthier planet, it’s a win-win,” says Singh. “We’d be foolish and immoral not to.”
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