Upper left panel: [CII]/FIR ratio (defined as in Helou et al. 1988) as a function of FIR luminosity for the LBA sample, a set of local galaxies belonging to the SHINING sample, and data collected in the literature (Herrera-Camus et al., in prep.; Zhao et al. 2016). High-redshift galaxies are taken from Carilli & Walter (2013) supplemented by data from Wang et al. (2013), De Breuck et al. (2014), Schaerer et al. (2015), Bañados et al. (2015), Yun et al. (2015), and Bothwell et al. (2017). The sample of ULIRGs at z~ 0.3 is from Magdis et al. (2014). Upper right panel: [CII]/FIR ratio as a function of CO(1–0)/FIR ratio for all galaxies included in the previous panel with available CO data. We also plot the PDR model grid for constant G₀ and n_H from Kaufman et al. (1999). Lower left panel: [CII]/FIR ratio as a function of L_FIR/M_H2 ratio for the galaxies with available CO(1–0) observations. Lower right panel: FIR luminosity as a function of CO luminosity. We also plot the relationships followed by local and high-redshift star-forming galaxies (gray dashed line) and by local ULIRGs and distant SMGs (red dashed line), from Genzel et al. (2010).