See Fig. 4 for a general description of the plot. Left panel: additional GRB data from Arcodia et al. (2016) are plotted. Right panel: the distribution of N_H (z) with redshift for blazars from this work (blue stars) and for quasars from the literature (black symbols are related to Page et al. 2005; magenta to Nanni et al. 2017; grey to Eitan & Behar 2013; dark green and orange to Shemmer et al. 2006 and Shemmer et al. 2008, respectively; red to Saez et al. 2011; cyan to Yuan et al. 2006; purple to Grupe et al. 2006; lime to Shemmer et al. 2005; dark red to Campana et al. 2015; brown to Ricci et al. 2017; dark orange to Corral et al. 2011). Squares stand for generic non-blazars AGN, while stars for blazars. Error bars were computed within XSPEC at 90% confidence level (Δχ² = 2.71), as were the upper limits (denoted with arrows below the related symbol).