Cloudy predictions of the line intensities as a function of ionization parameter, U, and radiation field, G₀. Each connected track correspond to a given density (black = 10^1.0 cm^-3; light gray = 10^5.0 cm^-3, in steps of 0.5 dex). The observed ratios are indicated by the yellow (dwarf galaxies) and red hashed (normal galaxies) bands. The [O i]/[C ii] and [C ii]/L_TIR values corrected for non-PDR [C ii] emission. The stars/squares show the fiducial H ii region/PDR models for the dwarfs (green symbols) and the normal galaxies (red symbols). Left: results for the high-metallicity (2 Z_ISM) grid of models. Right: results for the low-metallicity (0.25 Z_ISM) grid of models. The fiducial model for normal galaxies is added to the low-metallicity panel for comparison (smaller red symbols).