Distribution data obtained from humans exposed to benzene vapors 3
may serve as an
illustration of the pathways depicted in Figure 5-2. The amount of benzene taken up through the
lungs into the blood and retained is approximately 30 percent of the inhaled dose. The rest is
exhaled, 50 percent immediately and another 20 percent excreted unchanged via the lungs. Studies
in rodents indicate that benzene is also readily (>90 percent) taken up by ingestion. Finally,
although much less effective, dermal exposure may also give rise to some absorption (about 0.2
percent of the applied dose). Once taken up, over half of the benzene absorbed is rapidly
translocated to organs with a rich blood supply, such as the liver and the kidneys, as well as to
tissues rich in fat such as adipose tissue, brain, and bone marrow. Benzene may also be found in
the placenta and fetus following exposure of pregnant women to benzene. Of importance is the
fact that benzene is only slowly released from fatty tissues. Coincidentally, complex
transformation processes convert the nonpolar, fatsoluble benzene into a host of polar water-
soluble metabolites that are excreted in the urine. For several organic compounds there is a
substantial database relative to metabolism, but for most substances our knowledge is incomplete.