Scaling relations between I₂₁ and I_Hα for H II regions. Left: I₂₁ as a function of I_Hα for the H II regions (grey points) in our sample. The white dots represent the median relation, while the blue dashed line is the best-fit power law obtained by fitting the data considering the errors associated with the quantities on both axes. The solid black line shows a linear relation normalised to match the data at high surface brightness. The slope and intercept of the best-fit power law together with the scatter with respect to the best model are reported in the top left corner. The meaning of the symbols and lines is the same across the three panels, except that the solid black line the right panel represents a one-to-one relation. Middle: I₂₁ as a function of I_Hα, corr. Right: as a function of I_Hα, corr.