Emission maps of I_[CII] and I_FIR, and maps of I_[CII]/I_FIR, for MC2-MHD, t_evol = 5 Myr (top), and MC2-HD, t_evol = 4 Myr (bottom). In the very young HII regions highlighted with a green rectangle, the low [CII]/FIR ratio is mostly due to the high I_FIR. Conversely, the black rectangle highlights a more developed HII region, where the [CII] deficit is mainly caused by the C+ → C²⁺ ionisation due to stellar radiation.