Distribution of log(R_PAH^*) vs. oxygen abundance color-coded by log([SIII]/[SII]) (tracer of the ionization parameter) for all H II regions in the sample. The red circles show the average values in different metallicity bins (reported in Table 2). The vertical gray dashed line marks 12 + log(O/H) = 8.2 as a threshold between the low- and high-metallicity regimes. The horizontal dashed line corresponds to the average R_PAH for diffuse ISM measured by Sutter et al. (2024). The black squares correspond to measurements for WLM and NGC6822 (from low- to high-metallicity) made by (Chown et al. 2025a) using JWST F770W and F2100W images. The PAH fraction drops significantly in the low-metallicity regime.