Top panel: radial distribution of the surface density of hydrogen in the Galactic plane. We also show the contribution of the atomic and molecular hydrogen gas. Lower panel: radial distribution of the different ISM components, including the cold and warm neutral medium (atomic gas), the CO-dark H₂ component, and the H₂ gas that is traced by ¹²CO and ¹³CO. All surface densities have been corrected to account for the contribution of He.