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 : C ^ { 0 } , C ^ { + } , O ^ { 0 } , 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 .