[OIII]88μm/[N⁺]122μm line ratio as a function of the ionization parameter for three gas densities n_H = 10 (circle), 100 (square), and 1000 (triangle) cm⁻³, and for different values of the metallicity Z = 0.0004, 0.004, 0.02, and 0.05 (from purple to yellow) for f_cov = 0 (dotted line), 0.25 (dash-dotted line) and 0.5 (dashed line). The shaded region corresponds to the values spanned by this ratio in our samples.