Fig. 5

An overview of the different metrics for characterizing the particle size of microplastics. A The Martin’s diameter, x_M, is the length of the chord that intersects the projection area into two equally large parts. B For Feret’s diameter, d_F, a multitude of different types can be defined, e.g., the minimum and maximum Feret’s diameter, x_F,min and x_F,max. Additionally, the largest axis perpendicular to these two size metrics is frequently measured as a Feret’s diameter (x_F,min90 and x_F,max90). The figures of A Martin’s diameter and B Feret’s diameter are a function of particle orientation and, consequently, distribute around a central value. That is why they are called statistical diameters. C The area equivalent sphere diameter, x_A, is the diameter of a sphere with the same area as the particle under consideration. In the same way an area equivalent square diameter can be measured (not shown). All of the aforementioned size metrics have different figures for a given particle. D However, when measuring the size of fibers, they are inappropriate. Instead, the geodesic length, x_GL, and width, x_GW, should be measured by skeletonizing the two-dimensional projection of microplastics. Overall, the selection of the size metric depends on the research question. Created with biorender.com