Luminosity line ratios predicted in the context of PDR and XDR models, as a function of column density, gas density, and strength of the incident radiation field (see text for details). The contours show the ratios measured in each source in our sample, color coded as shown in the legend. XDR models can reproduce the observed high [C II]/[C I] ratios only at low column densities, N_H ≲ 10²² cm⁻², whereas PDR models are compatible with the observed constraints over a wider range of parameters. The relatively low [C II]/CO(7–6) ratio observed in the targeted quasars is suggestive of typically large column densities, N_H ≳ 10²³ cm⁻² for both PDR and XDR environments. Finally, the low observed [C I]/CO(7–6) ratios point to a high density n_H ≳ 10³ (10⁵) cm⁻³ in PDRs (XDRs). Overall, a dense (n_H > 10³ cm⁻³) gas cloud with column densities of N_H ∼ 10²³ cm⁻² impinged by UV radiation from newborn stars appears to best describe the observed line ratios. However, we stress that no single model accurately reproduces simultaneously all of the observed line ratios, thus suggesting that a single cloud is too simplistic a description of the observed phenomenology.