revised final - Agency for Toxic Substances and Disease Registry ...

MERCURY 235
2. HEALTH EFFECTS
As a method of comparison to evaluate whether use of another method to derive an MRL might result in a
different intermediate oral MRL value for inorganic mercury, ATSDR used the same data (Dieter et al.
1992; NTP 1993) to calculate a benchmark dose for inorganic mercury. Using the most sensitive end point
identified in this study (relative kidney weight changes in rats), the experimental data were used to obtain a
modeled dose-response curve. Benchmark doses were then determined for the 10% response level
(0.38 mg/kg/day) and the 5% response level (0.20 mg/kg/day). After adjusting the 5-days/week exposures
in the study to 7-days/week equivalent doses, the 10 and 5% response-base benchmarks became 0.27 and
0.15 mg/kg/day, respectively. Application of 10-fold uncertainty factors for each inter- and intraspecies
variability resulted in estimated human benchmark doses of 0.003 mg/kg/day for the 10% response level
and 0.002 mg/kg/day for the 5% response level. These values strongly support the current existing
intermediate oral MRL of 0.002 mg/kg/day for inorganic mercury.
No MRL for chronic-duration oral exposure to inorganic mercury was derived, because the study results
showed decreased survival rate for male rats at all LOAELs.
Organic Mercury
Acute, Intermediate, or Chronic Inhalation MRLs: No inhalation MRLs were derived for organic mercury
compounds, due to the absence of data or to the lack of sufficient information regarding exposure levels
associated with the reported observed effects.
Acute and Intermediate Oral MRLs: No MRLs were derived for acute or intermediate oral exposure to
organic mercury compounds due to the absence of data or to the lack of sufficient information regarding
exposure levels associated with the reported observed effects.
Chronic Oral MRL for Methylmercury: Hair levels are typically used as an index of exposure to
methymercury. A number of studies report that hair mercury levels correlate with total intake levels and
with organ-specific levels of mercury. Suzuki et al. (1993) analyzed 46 human autopsies in Tokyo, Japan
and reported that hair mercury levels were highly significantly correlated with organ Hg levels in the
cerebrum, cerebellum, heart, spleen, liver, kidney cortex, and kidney medulla, when the total mercury or
methyl mercury value in the organ was compared with the hair total mercury or organic mercury,
respectively.