Plastic particles are rapidly accumulating in the environment and making their way into the food chain, raising concerns over potential impacts on human health.
A new study provides alarming evidence that tiny plastic particles can cross into the brain shortly after being ingested.
Key findings:
- Polystyrene micro- and nanoparticles ranging from 0.3 to 10 microns were administered orally to mice
- Within just 2 hours, 0.3 micron particles crossed the blood-brain barrier and accumulated in the brain
- Larger particles did not appear to cross the barrier, suggesting size is a key factor
- Computer simulations showed a cholesterol coating enabled nanoparticles to pass into a model lipid membrane
- A protein coating inhibited nanoparticle transfer, indicating the particle’s surface chemistry affects crossing ability
Source: Nanomaterials (Basel)
Microplastics Permeate the Planet…and Our Bodies
Over the past several decades, the mass production and disposal of plastics has led to staggering environmental contamination.
Microplastics less than 5 mm in size are now found in all corners of the globe – from the deepest ocean trenches to pristine mountaintops.
The tiny plastic fragments making their way into oceans, lakes, and soils ultimately find their way back to humans through seafood, drinking water, and produce.
Multiple studies have now confirmed that micro- and nanoplastics accumulate in animal and human tissues after being ingested.
These minuscule plastic particles have been found in blood, lung, liver, spleen, and even placental and fetal tissues.
The potential health implications of plastic particle accumulation are only starting to be explored, but early evidence suggests impacts on immunity, metabolism, cancer risk, and more.
New Research: Polystyrene Nanoplastics Infiltrate the Brain Within Hours
While past studies have shown ingested microplastics can cross the intestinal barrier and accumulate systemically, few have assessed whether they can cross the blood-brain barrier.
The blood-brain barrier strictly regulates transport of molecules and particles into the brain, blocking entry of most foreign substances while allowing passage of essential nutrients.
A new study led by scientists at the Medical University of Vienna provides alarming evidence that polystyrene plastic nanoparticles can rapidly cross this critical barrier and enter the brain after ingestion.
The researchers orally administered a mixture of differently sized polystyrene micro- and nanoparticles to mice and examined their brains after 2 and 4 hours.
Within just 2 hours, abundant fluorescent signals from 0.3 micron nanoparticles were visible in brain tissue, indicating these tiny plastic particles had already crossed the blood-brain barrier.
Signals were lower by 4 hours after administration.
In contrast, no signal was detected from larger 1.1 and 10 micron particles, even after 4 hours.
Computer Simulations Suggest a Cholesterol Coating Enables Nanoparticle Transfer
To investigate how the plastic nanoparticles could cross the blood-brain barrier, the researchers performed computer simulations modeling the interaction between polystyrene particles and a lipid bilayer serving as a simplified model of the barrier.
They compared how pristine polystyrene particles versus particles coated with cholesterol or protein interacted with the model lipid membrane.
The simulations revealed a striking difference:
- Cholesterol-coated nanoparticles spontaneously transferred into the hydrophobic core of the membrane
- Pristine nanoparticles entered the membrane slowly
- Protein-coated particles failed to enter the membrane
The researchers propose that the cholesterol forms favorable interactions with the membrane lipids, enabling it to lodge tightly to the membrane surface.
This induces local deformation in the membrane, loosening the nearby lipid packing and creating sufficient space for the particle to diffuse through.
Once inside, the cholesterol molecules dissociate and disperse within the membrane interior while the polystyrene chains disentangle.
Although this causes disruption to normal membrane structure, overall integrity is maintained.
Size Matters: Crossing Ability Depends Strongly on Diameter
A key finding from the in vivo experiments was that 0.3 micron particles readily crossed into the brain, while larger microparticles did not.
This size dependence makes sense in light of the computer simulation results.
The simulations suggest a mechanism of passive transfer directly through the lipid membrane, rather than endocytosis or transcytosis through cells.
The energy required for a particle to transfer into the hydrophobic membrane core depends on its diameter – smaller particles can diffuse in more easily.
There appears to be a cutoff between 0.3 and 1 micron where the energy barrier for passive transfer becomes prohibitively high.
Nanoparticles below this size threshold can access this direct membrane crossing route, while microparticles above this size are blocked.
Implications for Human Health – Evidence of Harm is Growing
The immediate detection of ingested polystyrene nanoparticles in the brain raises concerns over potential health impacts.
Studies suggest micro- and nanoplastics can cause oxidative stress, inflammation, cell death, and disruptions to metabolism and neurotransmission in the brain.
In the longer term, plastic particle accumulation may contribute to cognitive decline, impaired memory and learning, and risk of Alzheimer’s disease and other neurodegenerative disorders.
However, research on effects in humans is scarce.
While this small study provides proof-of-concept that nanoparticles access the brain soon after ingestion, many questions remain regarding the time course of accumulation and associated health risks.
Tracking long-term buildup following chronic lower level exposures remains an important area for further research.
Reducing Plastic Pollution is Key to Limiting Exposure
In light of the ubiquitous presence of microplastics in the modern environment and food system, completely avoiding exposure is unrealistic.
However, reducing reliance on disposable plastics and curbing additional environmental releases must be priorities to decrease contamination.
In particular, stemming the accumulation of the smallest nanoplastic particles will be critical, as these can penetrate barriers and tissues throughout the body.
Though an enormous challenge, stemming the tide of plastic pollution leaching into the environment is paramount to protecting human health.
Along with plastic reduction efforts, advanced filtration technologies capable of removing tiny micro- and nanoplastics from water supplies merit continued development and implementation.
Such measures to limit exposure will provide time to better understand health risks as the science continues to evolve.
The Brain’s Blood-Brain Barrier Compromised
The blood-brain barrier provides essential protection for the brain’s sensitive neural networks,blocking entry of toxins and pathogens while allowing passage of nutrients and beneficial molecules.
This rigorous security system relies on tightly packed endothelial cells that line cerebral blood vessels to prevent uncontrolled transport.
The finding that tiny plastic nanoparticles can rapidly breech this barrier raises alarm.
It provides compelling evidence that micro- and nanoplastics are not inert, but interact with and perturb biological systems in ways we are only beginning to grasp.
While this study involved only short-term exposures in mice, the results highlight the possibility such exposures could also compromise the human brain’s critical security system.
The implications for brain health as plastic pollution climbs will be important to unravel in coming years.
Surface Chemistry Dictates Environmental Interactions
Plastics become contaminated with complex mixtures of chemicals as they weather in the environment.
This creates a so-called eco-corona that endows plastic particles with a suite of surface properties that dictate their interactions with living tissues.
The simulations in this study underscore this phenomenon.
Simply coating polystyrene nanoparticles with cholesterol or protein dramatically changed their ability to cross the membrane barrier.
The brain accumulates hydrophobic chemicals with an affinity for lipids and cholesterol, giving it a high vulnerability to plastic particles coated with similar substances.
Further research is urgently needed to understand interactions between plastic surface chemistry and brain lipids.
A Future of Plastic Accountability
Plastics deliver many benefits essential to society, but the unchecked lifecycle of plastic from production to breakdown has created a global pollution crisis.
Governments and industry must dramatically improve accountability throughout the plastic economy.
To protect public health, we can no longer accept the unconstrained dissemination of tiny plastic particles into the environment and food supply.
Improved characterization and monitoring of micro- and nanoplastics from source to fate is essential to inform evidence-based policies.
With the genie already uncapped from the plastic bottle, halting further harm requires on an honest reckoning of plastic’s true costs.
Though difficult, reversing the rising tide of plastic pollution is within our control if we act decisively.
References
- Study: Micro- and nanoplastics breach the blood-brain barrier (BBB): biomolecular corona’s role revealed
- Authors: Verena Kopatz et al. (2023)
