There are few things in my everyday life that frustrate me more than cycling behind an old rickety van that is blowing black fumes into my face. I tried those face masks that filter pollutants as you breathe, but on a hot day, the sweat and discomfort is unbearable, and I must say, the mask makes me look rather like an unpopular supervillain.
I usually try not to complain about the world, and look for the good and bad in everything, and I think most of my previous articles have been relatively optimistic about life in general. The frequent headlines about new technologies either killing or curing our brains are misleadingly one-sided. The truth is usually somewhere in the middle. But when it comes to pollution, I don’t think I am exaggerating too much. It’s hard to see how the particles of poison entering my respiratory system could be good for my health in any way. As with cigarettes, sometimes the science is convincing enough that we can wholeheartedly say “smoking is bad for you”. So in the spirit of the anti-smoking lobbies that rightly worked so hard in the past, here is some of the recent evidence on how air pollution may be destroying your brain.
The first important question to ask is whether it’s even possible or feasible for external pollutants to enter the brain. The blood-brain barrier is good at keeping foreign particles in the blood from interfering with the brain, but pathogens can find other entry points. A research project published in 2016 looked for nanoparticles of magnetite (an iron ore) in brain tissue samples from people who died in fatal accidents while they lived in Manchester, UK, and Mexico City. Magnetite particles are produced by combustion and abundantly found in the air we breathe in major cities. Biologically produced magnetite is also naturally present in the brain, but the researchers could distinguish this natural magnetite from airborne magnetite by comparing their structures. Rounded nanoparticles like those found in air pollution outnumbered natural magnetite in the brain samples. The tiny size of the particles (< 200 nanometers) meant that they could enter the brain via the olfactory nerve, the nerve connecting the smell receptors in our nose to the brain. High amounts of brain magnetite have been linked to neurodegenerative diseases like Alzheimer’s, and the researchers did indeed find high concentrations in the brain samples from older people who had a history of symptoms. But scarier than that, some of the highest magnetite concentrations came from younger people living in Mexico City, especially in those who were exposed to the most polluted areas.
The research above shows that external pollutants can contaminate the brain. But the next question is whether there is good evidence of a link between pollution and brain abnormalities or cognitive deficits. I’ll describe the evidence of harm to different groups of people, in order of their age.
Let’s start with the unborn. Fetuses in the womb can be exposed to pollutants through the placenta, so researchers tested whether the mother’s air quality in the third trimester would predict the child’s brain development over its first 7–9 years of life. They found that greater prenatal exposure to pollutants was linked to smaller white matter volumes across the left hemisphere of the child’s brain in later life (at an average age of 8 years old). Sadly, this reduced white matter volume correlated with slower mental processing, more behavioral problems, and stronger ADHD symptoms in childhood.
Next up is air pollution at school. Researchers compared the cognitive development of children in schools exposed to high versus low levels of traffic-related pollution. They assessed memory and attention performance every three months for one year, for almost 3000 kids across 39 schools in Barcelona, Spain. As you might expect after the results of the last study, the kids in the more polluted schools showed less improvement in their working memory and attention performance over the course of the year. This deficit in cognitive performance may be linked to impaired connectivity across the brain in school children who are exposed to more environmental pollutants.
Tests on older women have also suggested a connection between brain structure and estimated exposure to air pollution based on where they have lived. Consistent with the placental pollutant effects I described earlier, these tests showed that women previously exposed to more airborne particulates had smaller white matter volumes, even after controlling for other demographics or relevant health issues. Given that air pollution is consistently a risk factor for dementia, it may be that these white matter deficiencies are related to the onset of neurodegenerative diseases. But we need to wait for more research to clarify the exact links between pollution, white matter damage, and cognitive declines in older age.
Most of these studies are about the long-term effects of toxic particulate matter, but there may also be immediate threats from sudden changes in pollution levels. A large systematic review of 6.2 million events across 28 countries looked at whether short-term increases in air pollutants resulted in increased hospital admissions for strokes or fatal health problems. Admissions did indeed increase with rises in airborne particulate concentrations and gases including carbon monoxide, sulphur dioxide, and nitrogen dioxide. So next time there’s a severe smog problem outside, it might be best to stay indoors, especially if you have existing health problems.
I’ll highlight one final study because it was published so recently and because the methods are both simple and smart. A group of researchers took the data from an existing national survey across China that included tests of verbal and mathematical ability. They then took the exact dates and locations of those surveys and matched them to daily air pollution data across China, while removing the effects of other county-level variables (e.g. GDP per capita and population density) and individual-level variables (e.g. household income and education). They found that verbal and mathematical performance was lower in areas with high pollution, and the effects were strongest when you averaged the pollution levels across longer time frames. Men appeared to be more vulnerable than women to the negative effects of air pollution on verbal test performance, and less educated men were the most vulnerable of all.
The evidence on how air pollution impacts our brain and cognitive performance is discomforting to say the least. There is still much to learn, so it’s worth keeping up to date with the science as it evolves. But in the meantime, I’ll be avoiding the busiest streets during bicycle rides to minimize the risk of the silent dangers lurking in our air.