NI, OI, AlII, SiII and FeII, which are dominant ionization stages in the HI gas in our own Galaxy. Detailed ionization models for intergalactig radiation field at high redshift explored by Lu et al (1995) and Fan and Tytler (1994) show that ionization corrections are indeed minimal for systems with $\log$ N(HI)$>$ 20.

Another effect which could affect the abundance determinations is the presence of HII regions within the DLA intercepted by the line of sight (Steigman et al 1975). These regions of warm ($T$ $\simeq$ 10$^4$ K) ionized gas would contribute to the column densities of singly ionized species, such as FeII, SiII and AlII, but not to the HI column density. In the interstellar gas of our Galaxy the column density contribution of HII regions is generally negligible when the HI column density is as high as it is in DLA systems (N(HI) $\ga$ 10$^{20}$ cm$^{-2}$). If HII regions were more important in DLA than in our Galaxy they would produce different line profiles between neutral and ionized lines, when observed at high resolution. This effect is not observed in the DLA towards QSO 0000-2169 where the $b$ values, velocities and profiles of singly ionized species are the 