Biological Effects of Sub-Micrometer Plastics
Nano- and microplastics often come from unintentional anthropogenic rather than engineered processes. They are intended to be biologically active, even when intentionally produced. For both engineered nanomaterials and nano- and microplastics, it is therefore useful to consider their intended use and properties when evaluating their potential environmental risk .
The smaller-sized polymer particles, with a larger fraction of the molecules, will be present on the surface of the particle, which can lead to an increase in chemical reactions and biological interactions. For example, smaller particles (on a mass basis) have a larger adsorption capacity as compared to larger particles , resulting in relevant vector effects. Also, they have potential to cross biological barriers. Engineered nanoparticles (ENPs) are able to cross cell membranes and become internalized, although the uptake of ENPs is size dependent, with uptake occurring by endocytosis or phagocytosis . They are stored inside vesicles and mitochondria . Cellular responses include oxidative stress, antioxidant activity and cytotoxicity .
The leaching of molecules from particles and the transformation processes, such as oxidation/reduction, interaction with macromolecules, light exposure and biological transformation, can significantly influence the integrity, behavior and persistence of nanomaterials in aquatic media .
Desai et al.  showed that polylactic (polyglycolic) acid nanoparticles of 100 nm copolymer had a tenfold higher intracellular uptake in an in vitro cell culture when compared to 10 pm particles made of the same material. Several cytotoxic, genotoxic, inflammatory and oxidative-stress responses in mammalian and fish systems have been reported. Therefore, there is a need to understand the molecular and cellular pathways and the kinetics of absorption, distribution, metabolism and excretion mechanisms that may be unique to MPs in the nano-size range. Aging is currently not incorporated in standard ecotoxicity test protocols, but has been proposed for engineered nanomaterials .