Background emissivity (K pc⁻¹) as a function of path length (kpc) from the considered HII region to the far boundary of the Galaxy. The P19 data are indicated in blue and the updated catalog data are shown in smaller orange dots. The error bars include propagation of the brightness temperature error and the distance error. The x-axis is reversed with respect to path length; short path lengths are on the right and long path lengths on the left.