Inorganic mercury kinetics. This diagram is complicated by the fact that inhaled vapor is oxidized to Hg++ so that both species are present. The inhaled vapor is highly mobile, readily crosses cell membranes, the blood-brain barrier, and the placenta. The Hg++ species is much less mobile, crossing the blood-brain barrier and placenta much more slowly than dissolved vapor. Source: Elinder et al. 1988. Reprinted with permission from Biological Monitoring of Toxic Metals; copyright 1988, Plenum Publishing Corporation.