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.
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