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

MERCURY 231
2. HEALTH EFFECTS
containing mercury) were not reported. It was also unclear whether the results for the mercury exposure
group were inordinately influenced or skewed by the individual dentists with the highest exposures and/or
blood levels. These confounding factors precluded the use of the Ngim et al. (1992) study for the derivation
of an MRL, but the study does provide support both for the premise that low-dose chronic exposure to
metallic mercury can result in adverse health sequelae and for the chronic inhalation MRL that is based on
the Fawer et al. (1983) study of occupationally exposed individuals.
Other occupational studies further support the ability of metallic mercury to induce neurological deficits.
Several studies have reported significant effects on tremor or cognitive skills among groups exposed
occupationally to comparable or slightly higher (up to 0.076 mg/m 3 ) levels (Ehrenberg et al. 1991; Piikivi et
al. 1984; Roels et al. 1982). Difficulty with heel-to-toe gait was observed in thermometer plant workers
subjected to mean personal breathing zone air concentrations of 0.076 mg/m3 (range, 0.026–0.27 mg/m3 )
(Ehrenberg et al. 1991). Tremors have also been reported in occupationally exposed workers with urinary
mercury concentrations of 50–100 µg/g creatinine and blood levels of 10–20 µg/L (Roels et al. 1982). By
comparison, blood mercury levels in the Fawer et al. (1983) study averaged 41.3 and 16.6 µmol Hg/L for
the exposed and control groups, respectively. Urinary mercury levels for the exposed workers in the Fawer
et al. (1983) study averaged 11.3 µmol Hg/mol creatinine (about 20 µg/g creatinine), compared with
3.4 µmol/mol creatinine in the controls. Piikivi et al. (1984) found decreases in performance on tests that
measured intelligence (based upon a similarities test) and memory (evaluating digit span and visual
reproduction) in chloralkali workers exposed for an average of 16.9 years (range, 10–37 years) to low levels
of mercury, when compared to an age-matched control group. In this study, significant differences from
controls were observed on these tests among 16 workers with blood levels ranging from 75 nmol/L to
344 nmol/L and urine levels ranging from 280 nmol/L (about 56 µg/L) to 663 nmol/L. Abnormal nerve
conduction velocities have also been observed in chloralkali plant workers at a mean urine concentration of
450 µg/L (Levine et al. 1982). These workers also experienced weakness, paresthesias, and muscle cramps.
Prolongation of brainstem auditory evoked potentials was observed in workers with urinary mercury levels
of 325 µg/g creatinine (Discalzi et al. 1993). Prolonged somatosensory-evoked potentials were found in
28 subjects exposed to airborne mercury concentrations of 20–96 mg/m 3 (Langauer-Lewowicka and
Kazibutowska 1989). All of these studies substantiate the ability of chronic, low- to moderate-level
exposure to metallic mercury vapors to cause neurological deficiencies.