Distribution of N_H(z) with redshift for blazars from this work. Blue objects belong to the Silver sample, while in cyan low-z data are shown. Error bars were computed within XSPEC at 90% confidence level (Δχ² = 2.71), as were the upper limits (denoted with arrows). All curves and coloured areas were obtained from Campana et al. (2015): the solid line, along with its corresponding 1- and 2-sigma envelopes in brown and green, respectively, is the median of the absorbed LOS distribution, representing the mean absorption contribution from both a diffuse WHIM and additional intervening over-densities. We take the 2-sigma lower envelope as the minimum contribution from a diffuse WHIM alone.