  An intriguing fact about cosmic gas clouds is that they all appear to have
neutral (atomic) hydrogen column densities smaller than 10^{22} cm^{-2}.
Observations of damped Ly-alpha (DLA) absorption systems further indicate that
the maximum N(HI) decreases with increasing metallicity. It is generally
assumed that this trend is due to a dust-induced selection bias: DLA systems
with high N(HI) and high metallicity contain so much dust that the background
QSO becomes too dim to be included in optically selected surveys. Here, it is
argued that this explanation may not be viable. Instead, it is proposed that
conversion to molecular hydrogen determines the maximum HI column density.
Molecular hydrogen forms on the surface of dust grains and is destroyed by
photodissociation. Therefore, the molecular fraction correlates with both the
dust content and, because of self-shielding, the total hydrogen column density,
and anticorrelates with the intensity of the incident UV radiation. It is shown
that the first relation can account for the observed anticorrelation between
the maximum N(HI) and metallicity.
