Lyα and metal line opacities as a function of H I neutral fraction and singly-ionised fraction respectively. Opacities are computed from Eq. (B.6) in Appendix B assuming f_CGM = 0.25, primordial hydrogen abundance and solar neighbourhood ratios (see Table 1). The different coloured curves correspond to z = 0, 2 and 6. Dashed curves show the longer wavelength resonant transition in case of doublets or multiplets.