Microplastics (MPs) and nanoplastics are small plastic particles (1–5000 μm and 1–1000 nm, respectively) that are ubiquitous contaminants in our environment. Their extreme persistence makes them a particularly problematic contaminant of emerging concern (CEC). Studies over the past decade have reported MPs in all environmental compartments: air, water, soil, sediment, and biota. Due to the way these microscopic particles are extracted and analyzed, concentrations of MPs in the environment are almost exclusively reported as number of particles in a specific volume, such as particles/L, particles/m3, particles/g, etc. Concentrations as high as 102 000 particles/m3 in coastal waters, 175 particles/m3 in air, 42 960 particles/kg in soils, 4650 particles/m3 in freshwater, and 9597 particles/kg in freshwater sediment have been reported (reviewed in (1and2)). In parallel, a growing number of (eco)toxicity studies have focused on the adverse impacts from exposure of a wide range of biota to MPs, ranging from sublethal effects (e.g., oxidative stress, inflammation, diminished growth, reduced reproduction) to acute toxicity. Toxicity studies generally report effect concentrations in units of mass per volume, such as mg/L, because MPs are weighed and mixed in test water for accurate dosing. Most adverse effects to date have been reported in the mg/L or μg/L range for micro- and nanoplastics, respectively. For example, half of the exposed Daphnia magna subjects were immobilized after 48h of exposure to 66.97 mg/L or 660 μg/L of 1 or 0.11 μm polystyrene (PS) beads, respectively (reviewed in (3)). The difference in units (particles/L in the occurrence literature vs. mg/L in the toxicity literature) has made it particularly difficult to compare the two sets of data and properly assess the risk of MPs because of the uncertainty about how to convert from one to the other.