Figure 1.

(A) Observation through scanning electron microscopy of the middle layer of surgical masks and filtering face piece type 2 (FFP2) respirators whether untreated or treated by moist-air heating at 70°C (75% humidity rate (HR) during 1 h) in the climate chamber. The inner panels show the correct structural integrity of polypropylene nanofibers at higher magnification. (B) Assessment of the molecular modifications of FFP2 respirators treated by moist-air heating at 70°C (75% HR during 1 h) in the climate chamber compared to untreated FFP2, as observed through Fourier-transform infrared attenduated total reflection (FTIR-ATR) (left panel) and thermal desorption–gas chromatography–mass spectrometry (TD-GC-MS) (right panel). The FFP2 respirator layers were confirmed as composed of polypropylene only, except for the mid-layer of treated/untreated FFP2 respirators that also presented four additional bands centered at 3,295, 1,640, 1,565, and 1,530 cm⁻¹, which probably correspond to a molecule of the amide family; this hypothesis is supported by the fact that this kind of molecule is known to be an effective process agent during the melt-blown process of the polypropylene fibers (22). The left panel shows only the internal and mid-layer as examples. The chromatogram displayed in the upper right panel showed the four layers of the untreated FFP2 respirators altogether. It indicated a low quantity of molecules: most of them were linear and branched alkanes. The two main peaks at 14 min represented butylated hydroxytoluene (BHT), i.e., a very well-known antioxidant, and 2-ethylhexanoic acid. In the lower right panel is superimposed the chromatogram of the treated FFP2 respirators to the untreated ones. Only two very weak new peaks were observed: the first one corresponds to benzoic acid, the second one being unidentified. a.u., arbitrary units; cm⁻¹, per centimeter; min, minute.