  We calculate theoretical population ratios of the ground fine-structure
levels of some atoms/ions which typically exhibit UV lines in the spectra of
QSO absorbers redward the Ly-alpha forest: C0, C+, O0, Si+ and Fe+. The most
reliable atomic data available is employed and a variety of excitation
mechanisms considered: collisions with several particles in the medium, direct
excitation by photons from the cosmic microwave background radiation (CMBR) and
fluorescence induced by a UV field present.
  The theoretical population ratios are confronted with the corresponding
column density ratios of C I and C II lines observed in damped Ly-alpha (DLA)
and Lyman Limit (LL) systems collected in the recent literature to infer their
physical conditions.
  The volumetric density of neutral hydrogen in DLA systems is constrained to
be lower than tens of cm^-3 (or a few cm^-3 in the best cases) and the UV
radiation field intensity must be lower than two orders of magnitude the
radiation field of the Galaxy (one order of magnitude in the best cases). Their
characteristic sizes are higher than a few pc (tens of pc in the best cases)
and lower limits for their total masses vary from 10^0 to 10^5 solar masses.
  For the only LL system in our sample, the electronic density is constrained
to be n_e<0.15 cm^-3. We suggest that the fine-structure lines may be used to
discriminate between the current accepted picture of the UV extragalatic
background as the source of ionization in these systems against a local origin
for the ionizing radiation as supported by some authors.
  We also investigate the validity of the temperature-redshift relation of the
CMBR predicted by the standard model and study the case for alternative models.
