Spectral evolution of the γ-ray emissivities relative to the local ISM q_LIS. The light grey crosses correspond to the optically thin spectrum and the black crosses to the 100 K case, favoured by the fit. The corresponding bands give the average emissivity and its errors. The q_CO coefficients are expressed in 10²⁰ cm⁻² K⁻¹ km⁻¹ s, q_H II in 10²⁰ cm⁻², q_DNM in 10²⁵ cm⁻². The other q coefficients are simple normalisation factors.