Microplastics are tiny plastic particles, generally defined as being between 1 nanometer and 5 millimeters in size. These pervasive contaminants enter the environment from various sources, sometimes intentionally manufactured in small forms, known as primary microplastics. These include microbeads in personal care products, microfibers shed from synthetic clothing, and plastic pellets used in manufacturing. Other microplastics, termed secondary microplastics, originate from the fragmentation of larger plastic items like bottles, bags, and fishing nets, which break down over time due to natural processes. Their widespread presence across the globe highlights a significant environmental challenge due to their remarkable persistence.
Factors Behind Microplastic Longevity
The longevity of microplastics stems from the inherent properties of plastics themselves. Plastics are polymers, characterized by strong covalent bonds within their chemical structures, which makes them highly stable and resistant to breakdown. Common types such as polyethylene (PE), polypropylene (PP), and polyethylene terephthalate (PET) are particularly unreactive under many environmental conditions. This intrinsic inertness means these materials do not easily decompose through natural biological or chemical pathways.
Environmental conditions also influence how long microplastics persist, often by accelerating fragmentation rather than complete disappearance. Ultraviolet (UV) radiation from sunlight can initiate photodegradation, causing plastics to discolor, crack, and become brittle, which leads to fragmentation. Temperature also plays a role, with higher temperatures accelerating degradation processes by causing loss of strength and toughness, while low temperatures can cause brittleness.
Oxygen availability facilitates oxidative degradation, where oxygen reacts with polymers to form free radicals that promote breakdown. Water can also contribute to hydrolysis, breaking polymer bonds, especially in humid or aquatic environments. Despite these environmental influences, most conventional plastics do not readily biodegrade, as microbial communities generally find it difficult to break down their complex polymer structures.
The Degradation Process
Microplastics undergo several degradation processes, though these mechanisms are typically slow and often result in fragmentation rather than complete disappearance. Photodegradation is a primary pathway, driven by sunlight’s ultraviolet (UV) radiation. UV energy causes chemical bonds within the plastic to break, leading to embrittlement and surface changes. This weakening makes the plastic more susceptible to physical forces like waves or wind, leading to further fragmentation into smaller particles.
Oxidative degradation involves the reaction of plastic polymers with oxygen, a process often accelerated by heat and light. This reaction generates free radicals, which then attack the polymer chains, resulting in changes to the plastic’s physical properties, such as reduced strength and malleability. While this process alters the material, it rarely leads to the full breakdown of plastic. Hydrolysis, the reaction of plastics with water, can also contribute to degradation, particularly for certain polymer types and in humid or aquatic environments.
A significant outcome of these degradation processes is the continuous fragmentation of plastics. Larger plastic items break down into microplastics, and these microplastics can further fragment into even smaller particles known as nanoplastics, which are less than 1 micrometer in size. While some “biodegradable” plastics are designed to break down under specific industrial composting conditions, most common microplastics persist. Conventional plastics biodegrade very slowly, meaning they do not disappear on observable timescales.
Estimating Lifespans
Determining the precise lifespan of microplastics is challenging due to varied compositions, diverse environmental conditions, and long timescales. Estimates are often based on laboratory experiments or models, which may not fully capture real-world complexities. Longevity can range from decades to thousands of years.
For common plastics like high-density polyethylene (HDPE), marine degradation estimates range from 58 years for bottles to 1200 years for pipes. Polyethylene terephthalate (PET) also degrades slowly in ambient and marine conditions, primarily through photo-oxidative degradation and hydrolytic cleavage. These long estimated lifespans highlight the persistence of these materials once they enter natural systems.
Even “biodegradable” plastics like polylactic acid (PLA) show varied degradation rates depending on the environment. While PLA degrades faster in industrial composting or soil (around 21 micrometers per year), its marine degradation rate can be comparable to conventional plastics like HDPE (e.g., 7.5 vs. 4.3 micrometers per year). Degradation rate is proportional to exposed surface area; smaller fragments might degrade faster per unit of mass, but complete breakdown still requires extensive periods.
Accumulation and Distribution
The prolonged persistence of microplastics leads to their widespread accumulation and distribution across various environmental compartments globally. Oceans serve as major sinks, with microplastics found throughout the water column, in seafloor sediments, and within marine organisms. They are ingested by creatures from zooplankton to apex predators.
Terrestrial ecosystems are also impacted, with agricultural soils showing high microplastic concentrations, sometimes 23 times greater than oceans. They enter agricultural lands through sources like plastic mulching films, organic fertilizers, sewage sludge, and contaminated irrigation water. In soil, microplastics can alter properties and be taken up by crops.
Freshwater systems, including rivers, lakes, and drinking water, increasingly contain microplastics. Wastewater treatment plants often contribute to microplastic release into aquatic environments. Microplastics also become airborne, detected indoors and outdoors, and transported long distances by wind. This atmospheric transport allows redistribution from oceanic sources to land, demonstrating pervasive mobility. Their enduring nature allows travel across vast distances and integration into food webs, raising concerns about ingestion and chemical transfer.