“Those nanoscale particles that are not contained within a matrix under normal temperature and pressure conditions that would reasonably be expected to prevent the particles from being separately mobile and a potential source of exposure. An engineered primary nanoscale particle dispersed and fixed within a polymer matrix, incapable as a practical matter of becoming airborne, would be ‘bound,’ while such a particle suspended as an aerosol would be ‘unbound’.”¹

1. ASTM International, “Standard Guide for Handling Unbound Engineered Nanoscale Particles in Occupational Settings,” E2535-07, § 3.2.12 (October 2007).