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

MERCURY 220
2.5 RELEVANCE TO PUBLIC HEALTH
OVERVIEW
2. HEALTH EFFECTS
The nature and severity of the toxicity that may result from mercury exposure are functions of the
magnitude and duration of exposure, the route of exposure, and the form of the mercury or mercury
compound to which exposure occurs. Since the ultimate toxic species for all mercury compounds is
thought to be the mercuric ion, the kinetics of the parent compound are the primary determinant of the
severity of parent compound toxicity. It is differences in the delivery to target sites that result in the
spectrum of effects. Thus, mercury, in both inorganic and organic forms, can be toxic to humans and other
animals.
Ingestion of methylmercuric chloride, for example, is more harmful than ingestion of an equal amount of
inorganic salts (e.g., mercuric chloride or mercuric acetate), since methylmercury is more readily absorbed
through the intestinal tract (about 95%) than are mercuric salts (about 10–30%). In turn, ingestion of
inorganic mercury salts is more harmful than ingestion of an equal amount of liquid metallic mercury,
because of negligible absorption of liquid metallic mercury (about 0.01%) from the gastrointestinal tract.
There is insufficient information to develop a complete matrix of effects for different mercury forms by
route of exposure. The information on inhalation exposure to mercury is limited primarily to metallic
mercury; only a few case studies are available for exposure to inorganic dusts or volatile organomercurials.
Inorganic salts of mercury do not readily cross the blood-brain barrier or the placenta. They are, therefore,
ultimately less toxic to the central nervous system and the developing fetus than either absorbed metallic
mercury or organic mercury compounds. Metallic mercury is more readily oxidized to mercuric mercury
than is methylmercury, so its transport across the placenta and into the brain may be more limited than that
of methylmercury. Once in the central nervous system, however, metallic mercury vapor is oxidized to the
mercuric ion (Hg ++ ), which is then trapped in the central nervous system due to the limited ability of the
mercuric ion to cross the blood-brain barrier. Mercurous salts are relatively unstable in the presence of
sulfhydryl groups and readily transform to metallic mercury and mercuric mercury. Thus, mercurous
forms of mercury will possess the toxic characteristics of both metallic and mercuric mercury. All
mercury compounds may ultimately be oxidized to divalent (or mercuric) mercury, which preferentially
deposits in the kidneys, and all mercury compounds may cause some degree of renal toxicity. While this is not