How does air pollution impact your brain?

Image credit: Ionut Stefan

We’ve known for some time that air pollution is bad for human health. However, the focus was mostly on the lungs, and to some extent, the heart. After all, airborne pollutants are inhaled through the lungs and can reach the heart through the bloodstream, so it’s not surprising these take the brunt of the damage. But somewhere along the way, it became somewhat mainstream to consider how air pollution might affects brains too.

What I found surprising was the recency of that acknowledgement. Isolated studies of air pollution effects on brain health go back earlier, but serious consideration of the matter started only around the 2010s. As an example, Deborah Cory-Slechta, now a prominent researcher in this field, began focusing on its impact on the brain only in 2012. And air pollution was added as a risk for dementia as late as 2020 (remember 2020?). Despite mounting evidence of widespread negative effects, air pollution is still everywhere, with the World Health Organization estimating that 99% of the world’s population is exposed to higher than recommended levels. I’ll say it again: 99% of the entire population of the world is exposed to levels of air pollution higher than the recommended limits.

But let’s start from the beginning. What do we mean when we say air pollution? Which brain disorders has it been linked to and why are there so many of them? And finally, what can we do other than read this article and feel upset?

Air pollution – a complex mixture

As is often the case, we’re dealing with a broad term. “Air pollution” covers emissions from cars, power plants, wildfires, gas stoves, and more. Major pollutants released into the atmosphere from all these sources include carbon monoxide, ammonia, nitrogen oxides, and particulate matter. The latter refers to tiny solid or liquid particles suspended in air, or in other words, anything that isn’t a gas. Particulate matter, or PM, is further classified according to size: PM10 refers to particles of 10 um or smaller, PM2.5 – 2.5 um or smaller, and ultrafine particles (UFPs) are those smaller than 0.1 um. This is important because, obviously, the smaller ones move around the body more easily, which makes them more dangerous for our health.

Some pollutants, like particulate matter and nitrogen oxides, are released directly into the atmosphere. These are called primary pollutants. Others form later through chemical reactions in the air. These are secondary pollutants. To make things even messier, heavy metals like lead and mercury can also be airborne and have been linked to harmful effects on the brain. For simplicity, we’ll talk about all of these under the same umbrella of air pollution, because they all contribute to negative health outcomes, including effects on the brain.

Still, you should keep in mind that this lumping isn’t how regulation and research typically work. For them, the complexity matters. Because regulation usually targets specific pollutants, studies have to isolate the effects of each one. In the real world though, the air is an everything-mixture. Primary pollutants contribute to the formation of secondary pollutants and we inhale a bunch of substances at once. That makes it even more difficult to say exactly which pollutant leads to what effect, but it might also mean that their combined effects might go underestimated.

Brain disorders associated with air pollution

It might’ve only been 10-15 years since the neuroscientific community turned its focus on how air pollution impacts the brain, but that doesn’t mean it hasn’t been a productive time. Researchers have combed the entire lifespan in search of effects and consequences, and this work has yielded impressive results.

Damage related to air pollution in the brain begins actually even before birth. Fetuses receive nutrients and oxygen through the placenta, a temporary organ that allows blood flow between the mother and the fetus. As you can imagine, a healthy placenta is important for a healthy fetus. Unfortunately, exposure to pollution can cause the placenta to shrink, reducing blood flow and nutrient delivery. Studies in mice and rats have demonstrated this effect quite clearly. In women exposed to UFPs at work, the placenta was also affected. Since the placenta contributes to the proper development of the fetal brain, these changes can have downstream effects in terms of brain development.

Perhaps the most examined neurodevelopmental condition in relation to air pollution is autism spectrum disorder (ASD). A meta-analysis of 28 studies, including 758,997 newborns, with 47,190 having an ASD diagnosis, found that exposure to 5 μg/m³ of PM2.5 either before conception, during pregnancy, or after birth was linked to an increase in autism diagnoses between 5 and 17% (depending on exposure timing and the model used). As a side note, the 5 μg/m³ value for PM2.5 is the recommended limit set by the WHO.

Other disorders have been linked too, although the evidence is a bit less overwhelming. Meta-analyses have found associations between air pollution and both ADHD and schizophrenia across multiple countries. The effects aren’t as strong or consistent as they are for ASD, but they still show up often enough to raise concern.

At the other end of the lifespan, chronic exposure to air pollution has been linked to neurodegenerative disorders, including Parkinson’s disease, Alzheimer’s disease and dementia (as we mentioned above, air pollution was added as a risk factor for dementia in 2020).

And it doesn’t end there. Some studies have also linked increases in anxiety and depressive symptoms with air pollution levels, although some gaps remain here.

Effects of air pollution on brain structure

Air pollution hasn’t been linked just to symptoms, but to structural brain changes as well. In children, exposure has been associated with decreases in both grey and white matter volume across multiple regions and circuits, including the cortico-striato-thalamo-cortical circuit, which in turn appears to be involved in multiple neurodevelopmental disorders.

Similarly, in adults, structural changes appear in both grey and white matter. Particularly interesting here is one study which found that vigorous physical activity mediates the effects of exposure to PM2.5, but not in the way you might expect at first glance. It seems that people who were exposed to this type of pollution and exercised more had worse decreases in white matter volume. While further studies need to be conducted before drawing final conclusions, it is reasonable to assume that working out in more polluted areas increases the amount of pollutants inhaled, and leads to worse outcomes.

Why so many targets?

Air pollutants are diverse, of course, but still the question remains: how can they be linked to so many brain disorders and how can they have such widespread influence on brain structure? After all, neurodevelopmental and neurodegenerative disorders come with different symptoms, affect different brain areas, and follow different timelines.

It’s because despite those differences, these disorders share some common threads. And air pollution seems to pull on all of them. Inflammation, for one, shows up across the board. Many of these conditions involve chronic activation of microglia, the brain’s resident immune cells. Air pollution is a known inflammation trigger.

Another shared vulnerability lies in the mitochondria. Mitochondrial dysfunction is increasingly being acknowledged across a range of neurodevelopmental and neurodegenerative disorders. On the other hand, preliminary studies link air pollution to the mitochondria. For example, UFPs have been found in the mitochondria of the brains of people chronically exposed to air pollution.

Then there’s glutamate, the brain’s primary excitatory neurotransmitter. We need it to function, but excess glutamate is toxic, leading to white matter damage, among others. And you guessed it, air pollution has been shown to alter glutamatergic function.

So when we ask why so many brain disorders seem to link back to pollution, the answer is that it taps into mechanisms that a lot of them already have in common.

What to do?

We’ve said this before, panic for the sake of panic is useless. First and foremost, we need to keep in mind that exposure to air pollution doesn’t lead to all-or-nothing effects. So even if you can’t completely avoid it, reducing exposure will still help.

Of course, what you can do depends a lot on your personal circumstances. But here are a few tips you can pick and choose from. Indoors (where many of us spend a lot of time), try to avoid ventilating when the air quality is bad. Usually, you can check online how that looks like for your region at any point in time, plus you can assume that ventilating during rush hour is a no-no. If you can, get an air purifier that can filter PM2.5 particles. Outdoors, you can try to avoid high-traffic areas. If the air is particularly bad, you can even consider wearing an N95 mask.

It goes without saying, but you can also take active action against pollution. It might not amount to much on an individual level, but it does add up.

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References
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Furlong, M. A., Alexander, G. E., Klimentidis, Y. C., & Raichlen, D. A. (2022). Association of Air Pollution and Physical Activity With Brain Volumes. Neurology, 98(4). https://doi.org/10.1212/wnl.0000000000013031

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