  We investigate the main physical properties of low-metallicity Asymptotic
Giant Branch stars, with the aim of quantifying the uncertainties that
presently affect the predicted chemical yields of these stars, associated to
mass loss and description of molecular opacities. We find that above a
threshold mass, M ~ 3.5Msun for Z=0.001, the results are little dependent on
the opacity treatment, as long as hot-bottom burning prevents the surface C/O
ratio from exceeding unity; the yields of these massive AGB stars are expected
to be mostly determined by the efficiency of convection, with a relatively mild
dependence on the mass-loss description. A much higher degree of uncertainty is
associated to the yields of less massive models, which critically depend on the
adopted molecular opacities. An interval of masses exists, say 2.0-3.0Msun,
(the exact range depends on mass loss), in which HBB may be even extinguished
following the cooling produced by the opacity of C-bearing molecules. The
yields of these stars are the most uncertain, the variation range being the
largest (up to ~ 2dex) for the nitrogen and sodium yields. For very low-mass
models, not experiencing hot-bottom burning (M< 1.5Msun),the description of
mass loss and the treatment of the convective boundaries are crucial for the
occurrence of the third dredge-up, with sizable consequences on the CNO yields.
