For most geometries (those that retain the same morphology as they degrade), Equation 5 can be approximated by Equation 6 where 𝑚 0 is an item's initial mass, and 𝑛 is a dimensionless 'shape factor' (𝑛 is 1 for films, 2 for solid cylinders and beams, and 3 for spheres) (Section S3).