[C II]/[O I](63 μm) ratio plotted against the ([C II]+[O I](63 μm)/TIR ratio for M 33. [C II], [O I], and TIR intensities are from positions with intensities above the local 3σ value. Colors correspond to the four different regions of the inner and outer galaxy, and above and below a TIR threshold, as described in Sect. 3.1. Left: observed ratios with typical 1σ errors. Right: ratios with corrected [O I] intensities, as described in Sect. 3.2.4. In addition, median and standard deviations are shown for the four regions (errorbars in color) (cf. Table 5).