Left panel: distribution of absorption column densities for the 35 X-ray detected quasars, derived by HR or spectral analysis. Right panel: A_V magnitudes vs. X-ray absorption column densities derived by spectral analysis. The black square represents values of N_H= 5 × 10²¹ cm^-2 and A_V = 0 for 8 sources (see Table 2). The blue solid line represents the relation obtained by assuming a Galactic dust-to-gas ratio, i.e., A_V = 10^{(log N_H−21.278)} mag (Maiolino et al. 2001).