Mercury

Requirements
Absorption and Metabolism
Sources of Hg
Hg Toxicity

I.  Requirements

1. Based on present evidence, Hg is not considered an essential element

II.  Absorption and Metabolism

1. When taken orally, tissue concentrations of Hg are much higher when it is consumed in organic (methyl) rather than in inorganic form
   1. Absorption of inorganic Hg is less than 2%
   2. Absorption of organic forms may be as high as 95% in some species
2. Hg poisoning from absorption through the skin has been reported
3. 50% of inhaled inorganic Hg was retained by rats
4. Absorption of Hg from amalgam tooth fillings has been reported (FASEB J. 3:2641, 1989)
5. The body excretes inorganic Hg much more readily than organic Hg
6. Secretion of either inorganic or organic Hg into milk is limited
7. The placenta is a barrier against inorganic but not organic Hg
8. Apparently, the body has no homeostatic mechanisms regulating Hg concentration in tissues

III.  Sources of Hg

1. Industrial contamination
   1. Microorganisms in pond and stream sediments can methylate inorganic Hg compounds (Nature 223:753, 1969) so pollution of streams with inorganic Hg is of great concern
   2. Methylation of inorganic Hg in the rumen of cattle appears to be negligible (J. Anim. Sci. 36:415, 1973)
2. Fish
3. Hg treated seed grain used for food

IV.  Hg Toxicity

1. Chronic low level toxicity
   1. 1.          Dysfunction of the central nervous system
   2. 2.          Gingivitis
   3. 3.          Digestive upsets, loss of appetite and weight
2. Acute Hg poisoning
   1. 1.          Nausea, vomiting, abdominal pain
   2. 2.          Shock
   3. 3.          Cardiac arrhythmias
   4. 4.          Excess salivation, foul breath, loose teeth, soft spongy gums
   5. 5.          Death due to uremia
3. Maximum tolerable limit for Hg is 2 ppm for both the organic and inorganic forms

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