What Are Microplastics?

Microplastics are plastic particles smaller than 5 millimeters in their longest dimension, a size ceiling adopted by the NOAA National Ocean Service and now used across most environmental monitoring programs. They occur as fragments, fibers, films, foams, beads, and pellets, and they are conventionally divided into primary microplastics, which were manufactured at that size for use in abrasives, industrial feedstock pellets, or personal care products, and secondary microplastics, which form when larger polymer items weather and break apart under ultraviolet exposure, mechanical abrasion, and thermal cycling. Particles below one micrometer are usually treated as a separate class, nanoplastics, because their transport behavior and detection requirements differ.

As a materials topic, microplastics span the common commodity polymers: polyethylene, polypropylene, polystyrene, polyethylene terephthalate, polyvinyl chloride, and polyamide. Density largely determines where a particle ends up, since polyethylene and polypropylene float in seawater while polyethylene terephthalate and polyvinyl chloride sink. Biofouling, the accumulation of microbial films and organisms on a particle surface, changes effective density over time and moves buoyant particles into the water column and sediment.

Sources and Environmental Transport

Tire wear, synthetic textile laundering, paint, and the fragmentation of mismanaged plastic waste account for the largest measured inputs. Wastewater treatment plants capture a substantial fraction of incoming fibers in sludge, which is often applied to agricultural land, so removal from water can become an input to soil. Once in the environment, particles move with wind, river discharge, and ocean currents, and they concentrate in convergence zones, coastal sediments, and the deep sea floor. Atmospheric transport carries fibers to remote regions with no local plastic source. The NOAA National Centers for Environmental Information microplastics database aggregates global concentration measurements to support this kind of transport and inventory work.

Sampling and Measurement

Measurement is the limiting factor in the field, because concentration values depend heavily on mesh size, sample volume, and identification method. Surface water is commonly sampled with a manta or neuston net, historically with 333 micrometer mesh, which by construction misses smaller particles that dominate by count. Sediment and biota samples require density separation and chemical digestion of organic matter before analysis. Polymer identification relies on vibrational spectroscopy, chiefly Fourier transform infrared and Raman microspectroscopy, or on pyrolysis gas chromatography mass spectrometry when mass rather than particle count is the target. Standardization efforts, including the NOAA Marine Debris Program manual of laboratory methods for the analysis of microplastics, aim to make results from different laboratories comparable. Reference materials and instrument calibration for these methods are an active measurement science problem, and the NIST marine science program has produced a reference kit of 22 environmental polymers with the Center for Marine Debris Research and the American Chemistry Council.

Exposure and Effects

Microplastics have been reported in drinking water, seafood, salt, and human tissue samples, and the analytical difficulty of distinguishing genuine particles from laboratory contamination makes many early results hard to reproduce. Effects research focuses on physical blockage and abrasion in the digestive tracts of small organisms, on the leaching of additives such as plasticizers and flame retardants, and on the capacity of particle surfaces to sorb hydrophobic contaminants and metals. Dose relevance remains contested, since laboratory exposures frequently exceed measured environmental concentrations by several orders of magnitude.

Applications

Microplastics research has applications in a range of fields, including:

• Environmental monitoring programs that track marine and freshwater debris loads
• Water and wastewater treatment engineering, including filtration and sludge management
• Analytical instrumentation development for spectroscopy and particle counting
• Polymer design aimed at reducing fragmentation and additive leaching
• Remote sensing and modeling of ocean surface transport
• Regulatory and standards work on measurement protocols and product restrictions