Wildfires and Alzheimer’s: Your Brain on Particulate Matter
Ethan Lee ’25
Victoria Kenton '25
In June of 2023, my friend sent me a picture of the outside of his apartment in Manhattan. All you could see was orange smoke: not even the neighboring apartment buildings were visible. At the time, wildfires were raging across all 13 provinces of Canada, and the smoke from these fires was being carried over by winds to parts of the American midwest and northeast. The governments of affected states issued warnings to evacuate or shelter in place, and for good reason. Smoke from wildfires contains particulate matter that is less than 2.5 micrometers wide in diameter, otherwise known as PM2.5. Aside from causing short-term symptoms like itchy eyes and throat, it can also cause significant damage to the lungs. This information comes intuitively to most people, since smoke from normal fires usually causes coughing and lung issues. But what if I told you that PM2.5 – whether it be from wildfire smoke, everyday air pollution, or some other source – could also trigger stroke, dementia, and other neurological conditions? This article aims to examine two major consequences of PM2.5: stroke and nerve death (Li et al., 2022).
Ischemic stroke is a condition that occurs when blood flow to the brain is inhibited, either by plaque buildup or a blood clot. PM2.5 often contains reactive oxygen species (ROS) in the form of metals as well as other compounds, and introduces them into the body. An excess amount of ROS causes something called oxidative stress. This results in two major problems. Firstly, the cells that line the inner surface of intracranial arteries (major blood vessels in the brain) experience oxidative stress. This oxidative stress triggers an immune response by the body, and it is this immune response that promotes the formation of plaques along the inner-lining of the intracranial arteries, thereby increasing the risk of ischemic stroke. This oxidative stress also causes a decrease in Nitric Oxide (NO). NO enables blood vessels to relax (AKA dilate) which increases blood flow and decreases the risk for stroke. An increase in reactive oxygen species therefore means a decrease in NO and, consequently, a decrease in blood flow to the brain (Li et al., 2022).
Nerve damage and death are major consequences of PM2.5 exposure and lead to a plethora of brain conditions. A study by Nephew et al. (2020) found that “long-term exposure to traffic-related PM in rats and their offspring during pregnancy and lactation reduces social behavior, increases anxiety, impairs cognition … and undermines the neuro integrity of young male offspring.” This study clearly highlights the connection between PM2.5 and conditions like Alzheimer’s disease, Parkinson’s Disease, and mental disorders like depression, anxiety, and autism. Although the study focuses on “traffic-related” PM2.5, the results apply to all types of PM2.5, including those from wildfires. As seen in the intracranial arteries, the reactive oxygen species introduced by PM2.5 cause inflammation, but this time in neurons rather than blood vessels. This inflammation results in neuronal injury and death, which are the basis of cognitive impairment associated with diseases like Alzheimer’s. Another study by Chao et al. (2017) found that fetal mice exposed to PM2.5 showed increased levels of certain microRNAs (miRNA). miRNAs are short, single-stranded strings of nucleotides (the things that DNA and RNA are made of) that reduce the expression of genes. PM2.5 seems to increase the amount of miRNAs that inhibit the expression of genes associated with brain development, cognitive function, and neuronal integrity. Another important mechanism by which PM2.5 causes nerve damage is through the gut-microbiota-brain axis. This axis can be understood as the relationship between the bacteria in your gut (much of which is “good” bacteria) and the functioning of your brain. The good bacteria in your gut are important for keeping harmful bacteria and other pathogens from crossing the intestinal barrier, entering the circulatory system (blood vessels), and then potentially reaching the brain. PM2.5 can enter the gastrointestinal tract and can change the kind of bacteria present in your gut, thereby weakening these defenses. If harmful bacteria were able to reach the brain, they would likely cause neuroinflammation and death, potentially resulting in neurological conditions like Alzheimer’s (Li et al., 2022)
All in all, particulate matter less than 2.5 micrometers in diameter can cause stroke and nerve death via reactive oxygen species, inflammation, miRNA imbalance, and the gut-microbiota-brain axis. The exact amount of PM2.5 one must intake to cause any noticeable effect is unknown, but either short-term exposure to high concentrations or long-term exposure to particulate matter is dangerous. With all of this information in mind, it then makes sense that residents of highly polluted neighborhoods – usually poorer neighborhoods near industrial plants – are more susceptible to the effects of PM2.5. PM2.5 is just one of the many environmental dangers that disadvantaged members of society have to face, and it is important that society appreciates PM2.5’s potential to act as an instrument of environmental injustice.
References
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Li, W., Lin, G., Xiao, Z., Zhang, Y., Li, B., Zhou, Y., … Chai, E. (2022). A review of respirable fine particulate matter (PM2.5)-
induced brain damage. Frontiers in Molecular Neuroscience, 15. https://doi.org/10.3389/fnmol.2022.967174
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United States Environmental Protection Agency. (2023, July 11). Particulate Matter (PM) Basics. Retrieved from US EPA website:
https://www.epa.gov/pm-pollution/particulate-matter-pm-basics
Wikipedia. (2023, June 13). 2023 Canadian wildfires. Retrieved from Wikipedia website:
