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

MERCURY 353
2. HEALTH EFFECTS
unknown. This information is needed to design better treatment drugs and protocols. Mechanistic studies
are needed on how mercury (in its various forms) is excreted and how such activities can be enhanced.
An important priority research and data need is a study of the effects of dietary selenium on the absorption
and toxicity of methylmercury. Primates would be the most appropriate species for such a study. Oral
dosage levels (in food) should cover an sufficient dose range to provide useful information for high fish
consuming populations. Mercury excretion should also be measured and compared with controls at least
weekly, with the entire study length being not less than 6 months, and preferably one to two years in
duration. Concurrent neurobehavioral testing should be included, if possible, and be conducted at fixed
intervals depending upon the duration of the study.
Comparative Toxicokinetics. There is only limited data available on species differences in
absorption rates following oral exposures to all forms of mercury, and the results are negative (i.e., no
differences) (Clarkson 1971, 1972a; Friberg and Nordberg 1973; Nielsen and Andersen 1990; Rice
1989b). There are data concerning inhalation absorption of metallic and inorganic mercury (Berlin et al.
1969; Cherian et al. 1978; Clarkson 1989; Hursh et al. 1976); however, the data are insufficient to allow
for interspecies comparisons (Ostlund 1969). Studies comparing the inhalation absorption of all forms of
mercury in humans and animals are needed to improve the utility of animal data in assessing human risk.
The limited information available on dermal exposure suggests that dermal absorption of both inorganic
and organic mercury compounds occurs in humans and animals, although no comparison of the rate or
extent of absorption can be made between species (Gotelli et al. 1985; Hursh et al. 1989; Laug and Kunze
1949; Schamberg et al. 1918). As with inhalation exposure, studies comparing the dermal absorption of all
forms of mercury in humans and animals are needed to improve the utility of animal data for assessing
human risk.
The distribution of mercury in humans and animals appears to be similar. The lipophilic nature of metallic
mercury results in its distribution throughout the body in humans (Takahata et al. 1970) and in animals
(Berlin and Johansson 1964; Berlin et al. 1966). Distribution of inorganic mercury compounds resembles
that of metallic mercury; however, human distribution is preferentially to the kidneys, liver, and intestines.
Also, levels in the brain are substantially lower, as these compounds have a lower lipophilicity.
Distribution of organic mercury compounds is also similar to that of metallic mercury. The ability of
methylmercuric compounds to cross the blood-brain and placental barriers enables ready distribution to all
tissues, although, again, the highest levels are found in the kidneys. Phenylmercuric compounds are