Microplastics (MPs) are tiny fragments of plastic material smaller than five millimeters in length. They originate from the breakdown of larger plastic debris or are manufactured intentionally, such as microbeads in cosmetics. Microplastics are now ubiquitous in the environment, leading to constant human exposure through the air, food, and water we consume. This raises a fundamental question about the body’s ability to process and eliminate these foreign, non-biodegradable particles.
How Microplastics Enter and Spread in the Body
The two primary routes of human exposure are ingestion and inhalation. Ingestion occurs through contaminated drinking water, seafood, and food stored in plastic containers. While the vast majority of ingested microplastics pass through the gastrointestinal (GI) tract and are expelled in feces, the smallest particles can breach this barrier.
Inhalation exposes the respiratory system to airborne microplastics, such as fibers from synthetic clothing and tire dust. Particles larger than ten micrometers are generally trapped in the upper respiratory tract, but smaller fragments, especially those less than 2.5 micrometers, can penetrate deep into the alveoli.
Once inhaled or ingested, the smallest particles, particularly nanoplastics (less than 0.1 micrometers), can cross the GI lining or lung tissue and enter the bloodstream or lymphatic system. This translocation allows for wider distribution, with particles subsequently detected in organs such as the liver, kidneys, and lungs.
Biological Factors Governing Microplastic Clearance
Particle size is the most important characteristic determining whether a microplastic is eliminated or retained. Larger microplastics, typically greater than 150 micrometers, are largely confined to the digestive tract and quickly pass through the system without absorption. Conversely, particles less than a few micrometers are far more likely to be translocated into the systemic circulation.
Particle shape also plays a role; fibrous microplastics, common in textiles, interact differently with biological surfaces than spherical fragments. In the lungs, particles larger than six micrometers are efficiently removed by physical mechanisms, while smaller particles penetrate deeper and are retained longer. The particle’s surface chemistry, including its charge and interaction with biological molecules, influences cellular uptake and clearance. For instance, a positive surface charge can reduce mucociliary clearance efficiency in the airways by increasing adhesion to the mucus layer.
Primary Pathways for Eliminating Microplastics
The body employs several distinct pathways to remove the majority of the microplastics it encounters, with the efficiency of these mechanisms depending heavily on particle size.
Fecal Elimination
Fecal elimination is the most significant route for clearing ingested particles. Most microplastics that are not absorbed through the gut wall are packaged in the digestive waste and excreted, often within 48 hours of ingestion. Smaller microplastics between 30 and 50 micrometers are excreted at a significantly lower rate than larger 200 micrometer fragments, indicating a degree of retention even within the gut environment.
Renal Clearance
For the fraction of particles that successfully cross the gut or lung barriers and enter the bloodstream, the renal system provides a secondary route of clearance. Studies suggest that microplastics up to three micrometers in size may be excreted through the urine. Larger particles can potentially bypass the kidney’s primary filtration barrier through active secretion by tubular cells or if the barrier permeability is altered.
Mucociliary Clearance
In the respiratory tract, mucociliary clearance is a robust defense mechanism for inhaled particles. This process involves a layer of mucus lining the airways that continuously traps foreign material, which is then swept upward by hair-like cilia toward the throat to be swallowed or expelled. This rapid, non-specific clearance mechanism removes 80 to 90 percent of deposited particles from the central airways within a single day.
Cellular and Tissue Response to Retained Particles
A fraction of microplastics, particularly nanoplastics and the smallest microplastics, are retained in various tissues and organs despite clearance mechanisms. The biological management of this persistent foreign material begins with the immune system’s phagocytic cells, primarily macrophages. These cells, which are resident in tissues like the lungs (alveolar macrophages) and liver (Kupffer cells), attempt to engulf and isolate the particles.
Once microplastics are taken up via phagocytosis, the cellular response can vary depending on the particle’s size and chemical composition. This engulfment can provoke a localized inflammatory reaction, marked by the release of immune signaling molecules like Interleukin-6 (IL-6). For particles that cannot be degraded, macrophages may transport them to local lymph nodes, sequestering the foreign material. This cellular isolation and inflammatory response manages the long-term presence of retained microplastics.