Electron density (n_e) as a function of H II region diameter (continuation of Fig. 16 of Kim et al. 2017). Black symbols indicate values determined in previous radio continuum surveys and green circles shows the distribution found for a sample of mm-RRLs by Kim et al. (2017), for which the electron densities are derived from mm-RRL emission. The data from the current work are represented by red stars. The solid black lines represent the best-fit line determined by least-square fit to all data with a Spearman’s rank coefficient of r = −0.92 and p-value ≪0.0013. The dashed red line represents HC H II regions parameter space with diameter ≤0.03 pc and n_e ≥ 10⁵ cm³ and the dashed black line represents UC H II regions parameter space with diameter ≤0.1 pc and n_e ≥ 10⁴ cm⁻³ (Kurtz et al. 2000).