Although titanium dioxide (TiO2) is a suspected human carcinogen when inhaled, fiber-grade TiO2 (nano)particles were demonstrated in synthetic textile fibers of face masks intended for the general public. STEM-EDX analysis on sections of a variety of single use and reusable face masks visualized agglomerated near-spherical TiO2 particles in non-woven fabrics, polyester, polyamide and bi-component fibers. Median sizes of constituent particles ranged from 89 to 184 nm, implying an important fraction of nano-sized particles (< 100 nm). The total TiO2 mass determined by ICP-OES ranged from 791 to 152,345 µg per mask. The estimated TiO2 mass at the fiber surface ranged from 17 to 4394 µg, and systematically exceeded the acceptable exposure level to TiO2 by inhalation (3.6 µg), determined based on a scenario where face masks are worn intensively. No assumptions were made about the likelihood of the release of TiO2 particles itself, since direct measurement of release and inhalation uptake when face masks are worn could not be assessed. The importance of wearing face masks against COVID-19 is unquestionable. Even so, these results urge for in depth research of (nano)technology applications in textiles to avoid possible future consequences caused by a poorly regulated use and to implement regulatory standards phasing out or limiting the amount of TiO2 particles, following the safe-by-design principle.

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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.