Microplastics are tiny plastic fragments, generally defined as particles less than five millimeters in size, and they are now pervasive across the global environment. These ubiquitous particles originate from the breakdown of larger plastic debris or are manufactured to be small, such as microbeads or synthetic fibers from clothing. Microplastics are found in the air, water, and food sources worldwide, leading to unavoidable human exposure. The central question for human health is what happens to these foreign particles once they enter the body.
How Microplastics Enter the Human Body
The body absorbs microplastics primarily through ingestion and inhalation, two routes that involve daily environmental contact. Ingestion occurs when people consume contaminated tap or bottled water, seafood, or food that has been exposed to plastic packaging or dust. Inhaled microplastics often come from airborne synthetic fibers shed by textiles, tire wear, and general city dust suspended in the air.
Initial natural defenses, such as nasal hairs and saliva, attempt to filter out these particles before they can travel deeper into the respiratory or digestive systems. Despite these barriers, the smallest microplastics and nanoplastics—which are less than one micrometer—bypass the body’s initial filtration mechanisms. Once past these first lines of defense, the particles enter the gastrointestinal tract or the deep lung tissue.
The Body’s Primary Removal Mechanisms
Gastrointestinal clearance is the most effective process for ingested particles, where most microplastics pass through the digestive tract. Larger microplastics, particularly those greater than 10 to 20 micrometers, are typically encased within fecal matter and excreted.
The respiratory tract employs the mucociliary escalator, a system where inhaled particles are trapped in a layer of mucus. Tiny hair-like cilia then sweep this mucus and the trapped microplastics up toward the throat to be swallowed or expelled through coughing.
While the majority of larger microplastics are successfully removed, the smallest particles pose a greater challenge to the body’s clearance mechanisms. Nanoplastics and highly irregular fibers can evade these expulsion systems more readily than larger, more spherical fragments.
Where Microplastics Accumulate
Microplastics that are not successfully cleared by the digestive or respiratory systems can be retained within the body. Particles smaller than approximately 10 micrometers have the potential to cross biological barriers, such as the intestinal lining or the thin membrane of the lungs’ air sacs. Once they breach these barriers, the particles can enter the bloodstream.
From the circulatory system, microplastics are distributed throughout the body and can accumulate in various organs that filter blood or store foreign substances. Accumulation has been documented in the liver, which is the body’s main detoxification center, and the kidneys, which filter waste products from the blood. Evidence also suggests deposition in the spleen, a filtering organ, and even the placenta, demonstrating the ability of these particles to cross significant biological boundaries. Research has even found microplastics in brain tissue, indicating that the smallest fragments can potentially cross the blood-brain barrier.
Biological Impact of Internal Microplastics
Microplastics retained in human tissues can exert both physical and chemical harm. The presence of the plastic particles can trigger an inflammatory response as the immune system attempts to neutralize the foreign material. This localized inflammation can lead to oxidative stress, which is a cellular imbalance that causes damage to surrounding tissues.
Beyond the physical presence, microplastics act as carriers for chemical additives and environmental contaminants. Plastic polymers contain numerous chemicals, such as phthalates and Bisphenol A (BPA), which are added during manufacturing and can leach out into the body’s environment. These leached chemicals are often endocrine disruptors, meaning they can interfere with the body’s hormonal system, potentially impacting metabolism, development, and reproductive health. Studies suggest that this chemical leaching and the resulting oxidative stress are the primary mechanisms through which internal microplastics pose a risk to human health.