Microplastics on the Planet: Current Knowledge and Challenges

Microplastic (MP) pollution has become a global issue, attracting attention from stakeholders around the globe. Knowledge of MPs has been substantially advanced in recent years. Newly developed analytical technologies allow better characterization of MPs, but characterizing nanosized plastic particles remains challenging. Transport of plastic debris from land to oceans is a key driver for marine plastic pollution, and the relative contributions from riverine runoff and atmospheric deposition must be further quantified. Another lingering issue is whether MPs can bioaccumulate and biomagnify along a food web, which demands further investigations. Currently there is no health risk assessment for human exposure to MPs, probably due to the lack of reference threshold values. Hence, establishing reference threshold values for MPs should be a long-term and continuing task for the scientific community. Recent progress in quantifying the vertical settling of plastic debris in deep oceans has brightened the prospect of finding the "Missing Plastics", at least partially. Future efforts are urgently needed in developing novel analytical techniques, conducting additional laboratory and field measurements, and formulating robust models to better understand the occurrence, fate, and effects of MPs on the planet.