Relative importance of each heating mechanism in the neutral medium with respect to metallicity for the 37 galaxies of our sample. Top: photoelectric effect heating. Middle: photoionization heating. Bottom: cosmic ray heating. The green shaded area indicates the 90% confidence interval computed on the whole sample, with the median relation plotted in red. The black vertical line is the limit above which photoelectric heating dominates (≈22% Z_⊙). No PAHs are currently detected in the DGS below the orange vertical line.