Microplastics are plastic fragments smaller than five millimeters, including nanoplastics which are less than one micrometer in size. These particles are now ubiquitous, found in the air we breathe, the water we drink, and the food we eat. While the vast majority of microplastics that enter the body are quickly excreted, a small fraction can be retained. The exact residence time for these retained particles in humans remains under intense scientific investigation.
Routes of Exposure and Initial Processing
The two main pathways for microplastic entry into the human body are ingestion and inhalation. Ingestion occurs when we consume contaminated food, water, or dust, and the particles enter the gastrointestinal tract. Inhalation is the second major route, drawing microplastics suspended in air into the respiratory system. Larger particles settle in the upper airways, while smaller particles can reach the deeper regions of the lungs.
Primary Mechanisms for Clearing Microplastics
For the majority of ingested microplastics, the residence time is short, governed by the digestive process. These particles pass through the gastrointestinal tract and are eliminated via fecal excretion, typically within hours to a few days.
The body uses specialized defense mechanisms for inhaled particles. In the bronchial and upper airways, the primary clearance mechanism is the mucociliary escalator. This system uses a layer of mucus to trap particles, which is then swept upward toward the throat by cilia.
Once swept up, the particles are either coughed out or swallowed, directing them back into the digestive system for excretion. This mucociliary clearance is rapid, removing 80% to 90% of inhaled particles from the central lung within 24 hours. However, this defense is less effective against particles smaller than 10 micrometers, which can bypass the escalator and settle in the deeper lung regions.
Factors Determining Long-Term Retention
Long-term retention focuses on the small percentage of microplastics that are not quickly cleared. The most significant factor influencing retention is particle size. Particles smaller than 10 micrometers, especially nanoplastics, have the potential to cross biological barriers, a process known as translocation.
In the gut, these smaller plastics can cross the intestinal lining. Once they breach the intestinal or lung barrier, they enter the circulatory and lymphatic systems and are distributed throughout the body.
Studies have detected microplastics in tissues such as the liver, kidney, spleen, and brain. Their presence suggests a retention period much longer than gastrointestinal clearance, potentially lasting months or even years. The particle’s shape and composition also play a role; for example, irregular shapes may lodge more easily in lung tissue than spherical particles.
Current Understanding of Biological Impact
For the retained particles, current research suggests two main types of biological impact. The first is physical effect, where the particles cause localized inflammation and mechanical damage. Retained microplastics may trigger the release of inflammatory molecules in the gut lining or lung alveoli.
The second concern is chemical effects, which arise from the plastic itself. Plastics contain chemical additives, such as plasticizers, which can leach out into surrounding tissues. Microplastics can also act as carriers, adsorbing environmental pollutants onto their surfaces that are then released inside the body.
These leached chemicals or pollutants may interfere with endocrine function and other biological processes. Scientists are still working to establish the long-term health consequences of retained microplastics in humans.