Distribution of R_PAH vs. log([SIII]/[SII]) for the resolved H II regions (gray points; as in Fig. 5c) and isolated SNRs (red points). Panels (a) and (b) correspond to R_PAH measurements from the observed and the local background subtracted fluxes. The black dashed line corresponds to the relation defined by Eq. (4). The cyan contours show the probability density distribution for H II regions; the levels correspond to 65, 80, 95, and 99 percentile intervals.