Comparison of the LyC opacity (thick black line) with the line centre opacities of each transition (thin coloured lines) considered in this study as a function of H I column density. The gas opacity of each line is estimated according to Eq. (6) and assumes solar abundances (X/H) = (X/H)_⊙. Each panel corresponds to different sets of dust depletion δ_X, metallicity (X/H), and relative singly-ionised metal fraction to H I neutral fraction (f_X⁺/f_HI) values. δ_X, 0 is the dust depletion based on MW measurements (see text and Table 1). The ${\tau }_{\mathrm{X}}$   =  1 horizontal line marks the transition between optically thin and thick regimes. The dotted vertical line depicts the Lyman limit (i.e. N_HI = 1.6 × 10¹⁷ cm⁻²) below which ionising photons can efficiently escape the medium.