Because of the possible health risks related to the use of nanoparticle technologies in face masks, a general methodology is developed for identification, localization and particle size measurement of nanoparticles in situ in face masks by conventional and analytical transmission electron microscopy (TEM). The methodology can be applied by following this protocol which includes (i) preparation of resin-embedded ultra-thin sections of face masks suitable for TEM analysis, a procedure that can be applied to any type of textile, (ii) visualization and identification of the (nano)particles inside the cross section of the textile fibres by scanning TEM (STEM) combined with energy dispersive X-ray spectroscopy (EDX), (iii) measurement of the number-based particle size distribution based on a (semi-)automatic quantitative analysis using ImageJ, and (iv) in view of risk assessment, a calculation to estimate the amount of particles available for release from the mask.