(VCCEP) Tier 1 Pilot Submission for BENZENE - Tera

intracellular pathogen, Listeria monocytogenes. Benzene exposure was stopped for half of
each group, and the other half was exposed for an additional 7 days after infection, for a total of
12 days. Bacterial proliferation was measured at 4 and 7 days after infection for all mice. No
effect on bacteria counts in the spleen was observed one day after infection. Pre-exposure to
benzene at 300 ppm only resulted in a 7-fold increase in bacteria counts at day 4 after infection.
With continued benzene exposure after infection, dose-dependent increases in spleen bacteria
were observed at 30 ppm and above at day 4. However, by day 7, spleen bacterial counts had
returned to normal for all treatment groups. The authors suggest that benzene exposure
caused a delay in cell-mediated immune response, because there was a temporary increase in
spleen bacterial cell counts. Both T- and B-lymphocytes were depressed at benzene exposures
≥30 ppm, and counts did not return to control levels even after cessation of exposure. The
immune system LOAEL was determined to be 30 ppm, and a NOAEL of 10ppm was identified in
this study. Reduced tumor resistance mediated via T-cells was observed in 90% of male
C57Bl/6J mice exposed to 100 ppm benzene for 100 days (20 weeks), 6 hours/day, 5
days/week, and challenged with 10.00 polyoma virus-induced tumor cells/mouse. Lethal tumor
incidence in control mice, and those exposed to 10 or 30 ppm, was 30% of the group or less
(NOAEL = 30 ppm). Stoner et al. (1981) reported that exposure of female BNL mice to
200 ppm (640 mg/m 3 ) benzene for 10 to 20 days, or to 400 ppm for 5, 12, or 22 days,
suppressed T-cell-dependent primary antibody response to tetanus toxin on day 21 after
immunization. Exposure to 200 ppm for 5 days showed no effect. The NOEL was determined
to be 50 ppm (160 mg/m 3 ) after 5, 10, or 20 days of exposure.
Effects on the immune system appear to be operative at doses that also produce
hematotoxicity. The studies described above provide a profile of benzene-induced suppression
and alteration of the immune system by the oral and inhalation routes, addressing effects on
mitogen-stimulated lymphocyte proliferation, T- and B-lymphocyte responses, primary antibody
response to sheep RBC and spleen cell proliferation (Heish et al., 1988b; White et al., 1984;
Rozen et al., 1984), natural killer cell and interleukin-2 (Fan, 1992), and resistance to pathogens
(Rosenthal and Snyder, 1985, 1987). Immunotoxic LOAELs ranged from 8 to 27 mg/kg/day for
benzene in drinking water, and 10.2 to 30 ppm by inhalation in mice. In the only rat study,
Robinson et al. (1997) reported a NOEL of 200 ppm for humoral response. Only Rosenthal and
Snyder (1985) established a NOAEL of 10 ppm (32 mg/m 3 ). Using the inhalation NOAEL of
10 ppm, an estimated European Air Quality Standard (AQS) average daily dose (ADD) in mice
was calculated to be approximately 12.8 mg/kg/day, an estimated NOAEL similar to the animal
LOAEL for hematopoietic effects. Immune effects in animals seem to be initiated at higher
exposures than other hematologic effects. Benzene immunotoxic effects are probably
reflections of bone marrow toxicity (BU 2000). The equivalent human inhalation ADD was
estimated to be 35 ppm (112 mg/m 3 ) (CONCAWE, 1996).
6.2.8 Metabolism
It is now generally believed that benzene-linked toxicity/carcinogenicity is caused by metabolites
of benzene interacting with target organs (Ross, 2000; Valentine et al., 1996). Benzene is
metabolized in liver, and metabolites are released from the liver into the general circulation, and
from there, to the target organs and to the kidneys for excretion. A large amount of research
has focused on the metabolism of benzene in liver, via a variety of in vivo and in vitro
techniques, and metabolic pathways leading to the final products of excretion have been
developed using measurements of dose vs. absorption by different routes of administration.
The proposed catabolic pathway models have been developed for evaluating the
pharmacokinetics of benzene in experimental animals and man. Little work has been carried
out on metabolism of benzene in organs other than liver or of the further metabolism of liver-
Benzene VCCEP Submission
March 2006
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