[C ii] versus CO, which is normalized using the total infrared continuum (TIR). Big green filled circles show ISO/LWS data of the inner S1, N1 regions of M 33, while open circles show data of the outer S2, N2 regions. The lowest [C ii]/CO ratio observed with ISO/LWS in M 33 is 1000 (lower blue dashed line), while the highest ratio is 41 200 (upper blue dashed line). Black solid and dotted lines indicate lines of constant density n and FUV field G₀, respectively, from the standard K99 PDR model with A_V = 10 mag and solar metallicity Z = 1.