  We present HI line profiles for various models of circumstellar shells around
red giants. In the calculations we take into account the effect of the
background at 21 cm, and show that in some circumstances it may have an
important effect on the shape and intensity of the observed line profiles. We
show that self-absorption should also be considered depending on the mass loss
rate and the temperature reached by circumstellar gas.
  HI emission from circumstellar shells has been mostly reported from stars
with mass loss rates around 10$^{-7}$ solar masses per year. We discuss the
possible reasons for the non detection of many sources with larger mass loss
rates that are hallmarks of the end of the AGB phase. Although radiative
transfer effects may weaken the line emission, they cannot alone account for
this effect. Therefore, it seems likely that molecular hydrogen, rather than
atomic hydrogen, dominates the composition of matter expelled by stars at the
end of their evolution on the Asymptotic Giant Branch. However sensitive HI
observations can still yield important information on the kinematics and
physical properties of the circumstellar material at large distances from
central stars with heavy mass loss, despite the low abundance of atomic
hydrogen.
