Currently monitored microplastics pose negligible ecological risk to the global ocean.

Given the rise in plastic production, microplastics (MP) dominate marine debris, and their impact on marine ecosystems will likely increase. However a global quantitative assessment of this risk is still lacking. We conducted an ecological risk assessment of MP in the global ocean by comparing the thresholds of biological effects with the probability of exposure to those concentrations, according to plastic density data adjusted to a log-normal distribution. Levels of MP from 100 to 5000 µm span from < 0.0001 to 1.89 mg/L, whereas the most conservative safe concentration is 13.8 mg/L, and probability of exposure is p = 0.00004. Therefore large MP pose negligible global risk. However, MP bioavailability, translocation and toxicity increase as size decreases, and particles < 10 µm are not identified by current monitoring methods. Future research should target the lowest size fractions of MP and nanoplastics, and use in toxicity testing environmental plastic particles rather than engineered materials.