Q(H I)/Q(He II) ratios attained by our most extreme models (typically those with α_m = α_u = −1.8 and M_u = 300 M_⊙) compared to those of the BPASS default model ( α_m = α_u = −2.35 and M_u = 300 M_⊙) and observational constraints from the literature. Left-hand panel: time evolution of the photon flux ratio at two metallicities, illustrating the brief lifetime of the hardest ratios. Right hand panel: both the hardest ratios attained at a given metallicity and the mean ratio over the first 20 Myr of stellar evolution are shown. In both panels, we compare to observational data as indicated in the labels and in Sect. 6. The filled region in the left hand panel (and error bars on the relevant point in the right hand panel) indicates the range of values measured for different regions in the resolved galaxy SBS 0335-052E (D = 54 Mpc, Kehrig et al. 2018).