Expanding HII region in a pure hydrogen 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 = 100 (1500) Myr is shown in the left (middle) panel. Red lines show the results using the ten-band PWPL, blue lines using the three-band PWC approximation, and the black lines represent the CLOUDY equilibrium solution. The thin dashed line represents an ion fraction of 50%. The right panel shows the temperature profile of the HII region after t = 100 and 1500 Myr as dashed and solid lines, respectively.