[C ii] versus CO, which are normalized with the FIR continuum. 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. Small green circles show PACS observations of the BCLMP 302 H ii  region in M 33 (M11). In addition we show data from Milky Way GMCs, from star-forming regions, and from other galaxies, which are compiled from Stacey et al. (1991, S91), Hailey-Dunsheath et al. (2010, HD10), Graciá-Carpio et al. (2011, GC11), Israel et al. (1996, I96), Israel & Maloney (2011, I11), Madden et al. (1997, M97), Malhotra et al. (2001, M01), and Luhman et al. (2003, L03). 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. The dashed-dotted green line shows a K99 PDR model result for A_V = 1 mag, Z = 1, and n = 10³ cm^-3. The knee of this curve is for G₀ = 10^0.5 (cf. Fig. 18 in K99).