Expanding HII region in a primodial mixture of hydrogen and helium environment with n_H = 10⁻³ cm⁻³ and an initial temperature of 100 K. The hydrogen ion fractions (HI solid and HII dashed lines) after t = 1500 Myr is shown in the left panel. The right panel shows the helium ion fractions (HeI solid, HeII dashed, and HeIII dotted lines) after t = 1500 Myr. The thin dashed line represents an ion fraction of 50%.