Ratio of the [O I] intensity, I([O I]), to the [C II] intensity arising from the neutral PDR region, I_n([C II]), as a functionof n(H₂) for a set of constant thermal pressure, P_th, appropriate to [C II] emitting regions from PDRs (not from H⁺ regions). The ratios for P_th = constant (solid blue lines) are labeled in units of K cm⁻³. We limit the range of kinetic temperatures, T_k, appropriate to the [C II] emitting regions of PDRs to 50–200 K. The solid black lines show the solutions for the ratio along T_k = 50, 75, 100, 150, and 200 K. The horizontal lines represent the measured ratio (solid line) or measured limits (dashed lines) for the four LOS and velocity ranges discussed in the text. The LOS labels are indicated in the figure.