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

Modeling the HI 21-cm line profile from circumstellar shells around red giants