Average Galactic H₂ surface densities of the diffuse (red, detected in ¹²CO, undetected in ¹³CO) and dense (green, detected in ¹²CO and ¹³CO) components as a function of Galactocentric radius (in bins of width 0.1 kpc), in logarithmic scale, combining all data sets. In the inner Galaxy, the pink line indicates the surface density of H₂ in molecular clouds identified in Roman-Duval et al. (2010).