Dust opacity spectral index β of warm and cold aperture groups. Shown are the distributions for the apertures in the cold group (“Filament” and “IRDC” classifications) in light gray and the apertures in the warm group (“Entire”, “Other H II”, and “PDR” classifications) in dark gray. Where the two distributions overlap, we show light gray with dark gray hatching. The two aperture groups have only slightly different dust emissivity index distributions. The typical error in β for individual apertures is  ~0.3.