care workers are due to much more frequent injections (an average of one to two injections per day, according to some published surveys) and the practice of registering. Registering means that once a needle is inserted, the drug user will draw back the plunger of the syringe to examine for the presence of blood to ensure that the needle has been properly placed into a vein. Registering, then, involves contamination of both the needle itself and the hub, barrel, and plunger of the syringe. Although the syringe is typically rinsed before reuse, residual blood may adhere or remain, which may be released into the next person who uses the syringe by subsequent agitation (by drawing up and administrating the drug solution). Studies of the survivability of HIV in dried or aqueous states (Resnick et al., 1986) suggest that transmission may occur even if there is a delay of a day or more before the needle and syringe are reused by a different person.

A related practice of direct needle sharing has been termed booting (Inciardi, 1990; Ouellet et al., 1991), which involves additional steps in the basic injection pattern described above. Booting is the practice performed after registering and administering the drug solution. In this process, with the needle still in the vein, the injector draws back on the plunger of the syringe to fill the barrel with blood and then reinjects the blood, sometimes repeating this practice several times. More commonly reported with cocaine than with heroin injection, this practice allegedly enhances the euphoria associated with the drug's effects. Others, however, describe the motivation for engaging in this practice as economic, that is, to wash out all traces of the drug when administering it. The volume of blood that remains in the barrel of the syringe following booting is greater than that for the practice of registering and, at least theoretically, may be associated with a higher risk of transmission to anyone who subsequently uses a booted syringe. Empirical data on the risk of transmission for the practice of booting are sparse because few injection drug users can report reliably on whether previously used syringes were booted. Nevertheless, in one study, booting was associated with increased HIV seropositivity among injection drug users (Lamothe et al., 1993).

The setting in which drug injection takes place can also be related to direct sharing. A shooting gallery is a clandestine location where injection drug users go to rent needles and syringes. As used syringes are returned to a common container to be rented again, this process amounts to sequential anonymous sharing of needles and syringes (Friedland and Klein, 1987; Ouellet et al., 1991). Results of a study in which researchers tested used syringes collected from shooting galleries in Miami shed some light on the potential risk associated with injection drug use in the context of a shooting gallery. They showed that 20 percent of those syringes that had visible blood residue were positive for HIV, compared with 5.1 percent of those that had no visible blood residue (Chitwood et al., 1990). In a follow-up