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

MERCURY 229
C
2. HEALTH EFFECTS
No MRLs were derived for acute- or intermediate-duration inhalation exposure to metallic mercury vapors.
Available studies were either deficient in their reporting of details of experimental protocols and results,
used an insufficient number of experimental animals, or tested only one dose/concentration level.
An MRL of 0.0002 mg/m 3 has been derived for chronic-duration inhalation exposure (365 days
or longer) to metallic mercury vapor.
A significant increase in the average velocity of naturally occurring tremors compared to controls was
observed in a group of 26 mercury-exposed workers (from 3 industries) exposed to low levels of mercury
for an average of 15.3 years (range, 1–41 years) (Fawer et al. 1983). To estimate an equivalent continuous
exposure concentration, the average concentration assumed for the 8 hour/day exposures was multiplied by
8/24 and 5/7 (0.026 mg/m 3 x 8/24 hours/day x 5/7 days/week=0.0062 mg/m3 ). Uncertainty factors of 10 for
variability in sensitivity to mercury within the human population and 3 for use of a minimal-effect LOAEL
in MRL derivation were then applied to the calculated 0.0062 mg/m3 value, yielding a chronic inhalation
MRL of 0.2 µg/m3 . Although this MRL is based on experimental data from an adult working population,
there is no experimental or clinical evidence to suggest that it would not also be sufficiently protective of
neurodevelopmental effects in developing embryos/fetuses and children, the most sensitive subgroups for
metallic mercury toxicity.
Inhaled metallic mercury is quickly absorbed through the lungs into the blood, and 70–80% is retained. Its
biological half-life in humans is approximately 60 days. The half-life is different for different physiological
compartments (e.g., 21 days in the head versus 64 days in the kidneys) (Hursh et al. 1976). Since the
duration of exposure influences the level of mercury in the body, the exposure level reported in the Fawer et
al. (1983) occupational study was extrapolated from an 8-hour day, 40-hour workweek exposure to a level
equivalent to a continuous 24 hour/day, 7 day/week exposure, as might be encountered near a hazardous
waste site containing metallic mercury.
Gentry et al. (1998) used the neurobehavioral information on a control group and one exposure group from
the Fawer et al. (1983) study to derive an inhalation MRL for elemental mercury based upon a benchmark
dose (BMD) analysis. Dose-response analysis could be performed on four measures of hand tremor, with
tasks performed both at rest and with a load. The exposure level of the exposed group to metallic mercury
was assumed to be the mean TWA exposure of 0.026 mg/m 3 . A physiologically based pharmacokinetic
model for metallic mercury vapor was found to be linear through the region of concern from the LOAEL to