Although the column density of the cold obscuring material does not depend on the properties of the AGN (eg. its luminosity) it does depend on the properties of the host galaxy. Indeed, the absorbing NH strongly correlates with the presence of a stellar bar in the host galaxy (Maiolino et al. 1999). In particular, while non-barred Sy2s are characterized by an average log NH ~ 22 (cm-2), most of the strongly barred Sy2 galaxies are Compton thick. This finding indicates that stellar bars are effective in driving gas into the nuclear region to obscure the AGN. This result is in line with other studies, as reviewed by Sakamoto (1999), which indicate that bars are effective in driving gas into the nuclear region, though these other studies are not specifically focused on AGNs. We might speculate that, more generally, non-axisymmetric potentials (eg. distorted morphologies and galaxy interactions) drive gas into the nuclear region. Although there are some observational indications in this direction, a systemic study, similar to that on barred systems, has not been performed yet. This will be possible with the new Chandra and XMM data.