Spectra towards the H ii region complex W42 (red) and a diffuse region in the Galactic plane centred at (blue). The circles show the total intensity, with their corresponding uncertainties. The dotted lines represent the modified blackbody model, where one single opacity spectral index is fitted to IRAS 100 μm and HFI 857, 545, and 353 GHz data. The solid lines represent the total emission, including the contribution from metallic dust particles (Draine & Hensley 2013), at the same temperature T_d. This contribution to the total emission at 100 GHz is 41% and 63% for the H ii and diffuse regions, respectively.