Correlations between G₀ and density n₀ found by Young Owl et al. (2002) and Oberst et al. (2011). Reflection nebula points from Young Owl et al. (2002), Galactic PDRs as stated in Table 9. IRAS 23133+6050 and S 106 fit into the trend between the larger H ii regions with hotter exciting stars and the RNe PDRs with B type exciting stars. Dashed and solid black lines: theoretical prediction for the relationship between radiation field and density for a pressure bound H ii region and the best fit to the Young Owl et al. (2002) data. Green dot-dashed line: best fit to the Carina nebula data of Oberst et al. (2011). For a selection of sources we have also included the points representing the dense PDR component (denoted “dc”) as opposed to the default values representing the interclump medium (denoted “ic”).