Researchers piece together the correlation between microplastics in the brain and neurodegenerative diseases, with both on the rise.
By Helen Petre
Ever-increasing numbers of studies on the effects of microplastics in human brains are being published, and talked about, and evidence continues to show rather large amounts of plastics in human brains. Plastics are everywhere, and although we seem concerned, the amount of plastics we are using and disposing of is increasing.
A recent article published in Molecular and Cellular Biochemistry reviews current research and identifies possible correlations between microplastics and the two most common neurodegenerative diseases, Alzheimer’s (AD) and Parkinson’s (PD). The researchers suggest that microplastics in the brain aggravate or accelerate the symptoms of these diseases. The lead researcher, Dr. Dua of the University of Technology Sydney, says that the average human ingests 250 grams of microplastics every year, which is enough to cover a dinner plate. In addition, the results of a 2024 study published in Nature Medicine, by Dr. Nihart and fellow researchers at the University of New Mexico, reported that the average brain contains about as much microplastic as a disposable spoon. This is cause for concern. Although the human body does a good job of clearing foreign particles, evidence shows that some remain, and microplastics that remain can lead to inflammation, neurodegenerative changes, and significant brain changes.
After a review of the literature, Dr. Dua’s team of researchers at the University of Technology Sydney, along with collaborators at Auburn University, identified five main pathways through which microplastics cause harm to the brain, including triggering immune cell activity, generating oxidative stress, disrupting the blood-brain barrier, impairing mitochondrial function, and damaging neurons.
History of microplastics
Microplastic particles have been around as long as plastics, but they first were discovered as a human health and environmental problem by Richard Thompson in 2004. Research into the biological and chemical implications for human health and detrimental effects on our environment has become more frequent as evidence of harm and persistence has increased. Microplastics are everywhere, from the bottom of the Mariana Trench to the top of Mount Everest. They are in our food, breastmilk, clothes, and brains. Most microplastics are removed from human bodies when the immune system recognizes them as foreign, but some remains, and what remains could lead to neurodegenerative diseases.
Where does microplastic come from? Everywhere. Microplastics come from plastic cutting boards, plastic water bottles, tires, construction debris, carpets, clothing, soil, seafood, agricultural plastics, plastic bags, and processed foods.
Types of plastic
Polyethylene (PE)
Most plastic in the environment is polyethylene (PE), the most commonly produced plastic on Earth. It is used in plastic bags, bottles, food containers, and cups. One hundred million tons are produced, and mostly thrown out, annually. Plastic is made of carbon and hydrogen, the same atoms we are made out of, but plastic is less biodegradable than we are. It breaks down into smaller physical pieces that we call microplastics, but the plastic retains its chemical composition.
Polypropylene (PP)
Polypropylene (PP) is the second most commonly used plastic. It is like PE, but stronger and harder. PE is used in flip top bottles, Tic Tac boxes, reusable plastic containers like Rubbermaid, plastic chairs, disposable bottles, plastic pails, car batteries, pitchers, carpets, rugs, and door mats. Fifty percent of the PP that is manufactured is diapers and sanitary products, which we throw out, so there is a lot of plastic in landfills, which is not a good thing.
Then, there are clothes. Look at the labels on your clothes, because they are actually plastic. The PP, Polypro, is made into long underwear and shirts and warm weather clothes, because it transports sweat away from the skin.
Polyethylene terephthalate (PET)
Polyethylene terephthalate (PET) is the fourth most common plastic, used for textiles and containers. The military has switched from PP to PET because PP holds odors and melts, which is not pleasant in some military applications. Since clothes are everywhere, and people really like those sweat-wicking clothes, PET and PP are everywhere, especially when you put those clothes in the dryer.
Polyvinyl chloride (PVC)
Polyvinyl chloride (PVC) is the world’s third most commonly produced plastic. It is used to make raincoats and building materials, bottles, and credit cards. Most PVC is made in China. There are lots of environmental issues with this product, but we still use it and it still ends up in our brains.
Styrene butadiene rubber (SBR)
Styrene butadiene rubber (SBR) is abrasion resistant and used in car tires, seals, chewing gum, latex gloves, and gaskets. It is a synthetic replacement for natural rubber. There is a whole lot of this on roads and airport runways. Even though it is abrasion resistant, tires take a lot of abuse, which means they wear down and the SBR becomes microplastics.
Polystyrene (PS)
Polystyrene (PS) is used to make packing peanuts, disposable silverware, plastic single-serve yogurt cups, smoke detectors, disposable razors, building insulation, and Styrofoam. PS is lightweight, waterproof, and virtually indestructible. It is not biodegradable. PS is a major component of litter, especially in oceans, because it is very light. It blows in the wind and floats on water. It gets into our brains through food containers and inhalation.
Neurodegenerative diseases
There are many studies on the effects of plastics on the environment and human health. Dr. Dua’s research team reviewed research on microplastics, and the relationship between microplastics in the human brain with the incidence of Alzheimer’s (AD) and Parkinson’s (PD) diseases. AD is the most common and PD is the second most common neurodegenerative disease in the US.
Alzheimer’s disease
Seven million Americans have AD. It is the leading cause of dementia in the world. The cause is unknown, but the researchers believe that microplastics aggravate and accelerate the progression of the disease. AD patients have a build-up of neurofibrillary tangles and amyloid plaques in their brains. This results in shrinking of the brain, as brain cells die. Patients have memory loss and lose the ability to do normal tasks. There is no cure.
Parkinson’s disease
PD is caused by a dopamine deficiency in the brain. This results in a rhythmic shaking of the hands and other motor pathologies. The cause of PD is unknown, but here again, the researchers suggest that microplastics accelerate the progression and aggravate the disease. There is no cure.
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Increased mortality due to AD and PD in the US
Scientists have found that people with AD and PD have microplastic in their brains. They also have found that certain physiological signs are present in the brains of people with AD and PD and these signs are correlated with microplastics in the brain. According to the CDC, deaths from AD have increased more than 50 percent between 1999 and 2014. Similarly, deaths from PD among adults 65 or older have increased more than 50 percent between 1999 and 2017. Something is happening here. There is a correlation between increased microplastics in human brains and increased deaths from AD and PD, but it could be other causes, such as sedentary lifestyle, obesity, other pollutants, pesticides, drugs, or diabetes that actually cause the diseases. Maybe all of those other pathologies cause the brain to take in and retain microplastics. We do not know. We know the amount of microplastics in human brains is increasing and we know death from AD and PD is increasing. We do not know why.
The researchers identified five pathophysiological signs that are common in AD, PD, and people with microplastics in their brains. These include increased immune cell activity, oxidative stress, disruption of the blood-brain barrier, impaired mitochondrial function, and damaged neural structures.
Blood-brain barrier
Blood vessels in the brain have a highly specialized single layer of squamous epithelial cells that regulate what can pass from the blood into the brain. In most of the body, stuff in the blood can pass easily into the body tissues. Not so in the brain, and for good reason. You really do not want a lot of stuff in your brain, like bacteria, viruses, and in some cases, drugs. There are specialized microglial cells, astrocytes, that line the brain capillaries and determine what can pass into the brain. The endothelial squamous cells have very tight junctions between them, which hopefully prevent stuff from seeping through in between cells. This is why you do not get brain infections as often as you get respiratory infections. This is also why, if you do get a brain infection, it is very difficult to treat.
The blood-brain barrier becomes damaged in patients with AD. No one knows how or why.
Microplastics cross the blood-brain barrier, and once in the brain, they trigger a cascade of molecular responses that harm the brain cells. When clumps of microplastics are in the brain, brain cells experience mitochondrial dysfunction, oxidative stress, impaired homeostasis, inflammation, and interference with neurotransmitters, or chemicals that transmit signals around the brain. This results in proteins not folding correctly and thus not functioning properly, brain cells dying, and neurotransmitters sending messages incorrectly, ultimately resulting in abnormal human behavior and cognitive impairments, like memory and motor function loss. These are all the same things that happen in AD and PD.
Microplastics passing through our bodies
We know microplastics cross the blood-brain barrier, but we do not know how or why. We know microplastics also cross the intestinal lining. It is possible that both of these happen because the particles are so small that they are undetected, and there could be interactions with chemicals and charges that trick the cells into letting the microplastic pass through. Studies have shown that smaller particles cross epithelial linings more frequently. Other studies found that the presence of cholesterol molecules enhances the movement of microplastics through the blood-brain barrier. All of our cells have cholesterol in the plasma membrane. Plastic is neutral, or not charged, and our epithelial cell plasma membranes are uncharged. Uncharged particles move more easily through the membrane than charged particles.
The researchers also found that mice fed PS had microplastic in their brains within two hours of ingestion. Scary. Another study showed that microplastics can move through the olfactory membrane lining the nasal cavities directly into the brain without passing through a blood-brain barrier. More scary.
The earlier University of New Mexico study by Dr. Nihart’s team found microplastics in the frontal lobes of the brain, the areas that function in personality, thinking, and reasoning. They did not study other brain areas. It is possible that microplastics are not uniformly distributed, but instead concentrate in regions critical for cognition. If that is true, that is not good, but we really do not know how microplastics are distributed in the brain.
There are few studies done on humans and no human studies that are double blind, or have control groups. It is simply not possible to feed plastics to a group of humans and not feed plastics to a control group. The only studies Dr. Dua’s research team reviewed were animal studies, or observational studies, most commonly done by autopsies on deceased patients with documented disease.
Neuroinflammation
Neuroinflammation is inflammation of the neural structures. It could be due to infection, brain injury, or toxins. In the brain, microglial cells are the immune cells. Once activated, the microglial cells stimulate an immune reaction, resulting in inflammation. Inflammation compromises the effectiveness of the blood-brain barrier, through little understood processes. Inflammation results in a leaky blood-brain barrier, which is less selective about what is allowed into the brain. Inflammation of the brain is not a good thing, and usually results in neurodegenerative disease.
There is some evidence that microplastics form clumps in the frontal cortex, which stimulates an immune response. Microplastics are recognized by microglial cells as foreign and this triggers an immune response. Once the brain realizes that the microplastics are foreign, they mount an immune response in the same way immune cells mount an immune response to bacteria, viruses, or any foreign agent. The immune response in the brain results in the accumulation of plaques and misfolded proteins, and if the microplastics accumulate in the areas of the brain that produce dopamine, the immune response results in damage to cells that produce dopamine and symptoms similar to PD.
Mechanisms of neuroinflammation
The researchers reviewed many studies that showed neuroinflammation through several mechanisms. Inflammation is stimulated through activation of astrocytes in the blood-brain barrier, oxidative stress, mitochondrial dysfunction, and physical damage to neurons. All of these pathways overlap as a response to foreign microplastics or other causes of inflammation, such as disease or injury. A neuroinflammatory response from whatever cause, coupled with neurodegenerative disease, results in increased negative symptoms.
One study the researchers reviewed showed that heavy metals, such as cadmium, iron, copper, arsenic, and lithium, adhere to microplastics and move into the brain attached to them. Heavy metals cause oxidative stress, mitochondrial dysfunction, and neuronal damage by themselves, and combined with microplastics, it is difficult to tell which is the causal agent. Heavy metals have been implicated as a cause of PD. Microplastics and heavy metals in the brain can both cause similar signs.
Neuroinflammation is common in patients with AD and PD. It accelerates brain aging and reduces the ability to form memories. Whatever the cause of inflammation, the results are negative.
Oxidative stress
Oxidative stress results from too many reactive oxygen species, which in turn results in the production of peroxides and free radicals that damage cells. This happens when there are not enough antioxidants, and occurs in AD, PD, and neurodegenerative diseases. Oxidative stress damages lipids and proteins, specifically plasma membranes of cells, fatty coverings (myelin sheaths) of nerve fibers, and protein signal molecules like neurotransmitters. Oxidative stress results in cell death, or apoptosis, which, when it occurs in the brain, results in dementia.
Free radicals are normal products of life, but they are usually controlled because of antioxidants. Free radicals are unstable and take electrons from other molecules. If there are too many free radicals, the result is damaged cells. Antioxidants donate electrons to free radicals, so everything is balanced. Citrus fruits, bell peppers, and cantaloupe have lots of vitamin C, which is an antioxidant.
A study the researchers reviewed showed that microplastics impaired antioxidants and increased reactive oxygen species. Microplastics also enter the mitochondria and prevent it from making energy efficiently. All of this can stimulate an immune response.
Mitochondrial dysfunction
Mitochondria make energy in cells. If cells do not have energy, they do not function properly or they die. Studies reviewed by the researchers showed that microplastics enter the mitochondria and impair energy production. Mitochondria use oxidative phosphorylation to produce ATP. When the neurons do not have enough ATP, they die. Damaged neurons induce immune responses, which result in further neurodegeneration.
Damage to neural structures
Microplastics physically damage neuron structures, including the fatty myelin sheaths that cover nerve fibers, microtubules in nerve fibers that carry nerve impulses, and plasma membranes of neurons. Damage triggers neuroinflammation.
Conclusion: more microplastics in the body
The incidence of microplastics in human brains is increasing. The earlier University of New Mexico research team (Nihart et al.) reported that there was a 50 percent increase in microplastics in human brains between 2016 and 2024 samples. That is not a good thing. The microplastics are recognized as foreign and start an immune response in healthy people. In people who are already compromised due to AD, PD, or other neurodegenerative conditions, the result is aggravated and accelerated neurodegenerative disease.
Besides neurodegenerative disease, microplastics damage the intestinal lining, resulting in digestive disturbances and altered bacterial communities.
Limitations of the recent study
Most of the studies this research group reviewed focused on PS. Besides the more common environmental plastics, like PP, PE, and PET, other environmental toxins could also influence neurodegenerative diseases. Bisphenol A (BPA) and phthalates have been shown to cause oxidative stress, mitochondrial dysfunction, cell death, and impaired cognition. While the reviewed studies did not account for these chemicals, they were likely there.
In addition, none of the studies showed a causal relationship. There is ample evidence that there is a correlation between microplastics and neurodegenerative disease, but the only conclusion thus far is that there are microplastics in the brains of patients who have AD and PD. The researchers concluded that microplastics contribute to neurodegenerative diseases through several interconnected pathways, including activation of immune cells, oxidative stress, disruption of the blood-brain barrier, mitochondrial dysfunction, and neural damage. How all of this happens is not understood. Given the growing human population and the increasing use of plastics, there is a need for research to determine the significance of plastics in human brains. We know microplastics are in our brains, in other parts of our bodies, and in the environment. They were not there before we manufactured them. We do not know what the conclusion will be.
Moving forward
To decrease the impact of microplastics, we could decrease plastic use, improve waste management, and mitigate increasing health risks that result from other contributing factors, such as obesity, lack of exercise, poor diets, and lack of educational and social activities.
A thorough review of the literature has shown that there are microplastics in our brains and they are contributing to increased incidence of AD and PD.
This study was published in the peer-reviewed journal Molecular and Cellular Biochemistry.
References
CDC. (2019, September 6). QuickStats: Age-Adjusted Death Rates for Parkinson Disease Among Adults Aged 65 Years or Older. National Center for Health Statistics. https://blogs.cdc.gov/nchs/2019/09/06/4576/
Nihart, A. J., Garcia, M. A., El Hayek, E. et al. (2025). Bioaccumulation of microplastics in decedent human brains. Nature Medicine, 31, 1114–1119. https://doi.org/10.1038/s41591-024-03453-1
Taylor, C. A., Greenlund, S. F., McGuire, L.C., Lu, H., Croft, J. B.; CDC. (2017, May 26). Deaths from Alzheimer’s Disease—United States, 1999–2014. Morbidity and Mortality Weekly Report, 66(20), 521–526. http://dx.doi.org/10.15585/mmwr.mm6620a1
Featured image “Microplastics – P061337-299845” by European Commission (Lukasz Kobus) is licensed under CC BY 4.0.
About the Author
Helen Petre is a retired biologist. She spends her time volunteering, teaching, and learning.
