Alberto Gambino & Andrea Quirico
The metal exists in three fundamental forms two of which inorganic (Hg++ and Hg3+) and the third one organic methyled form(CH3Hg).
The principal engagement street by the humans is through alimentary cycle. There are saprophyte bacteria of the intestinal flora with turning capacity the inorganic Hg salts into organic compounds, so that the inhaleted mercury as steam can be absorbed through the intestinal mucosa.
The human emivita of the organic mercury is 70 days and the inorganic one is 40 days.
Mercury is also absorbed through skin and lungs, then it is given by the blood to all organs and the main way of excretion is urine. The mercury penetrates into cellular membranes by with its liposolubility and it interferes with the cellular metabolism, in particular Hg++ has big affinity with the sulphur of the sulphidrilic group SH giving rise to S-Hg which inhibits a few enzymic systems, blocking 2 important metabolic phases of the Krebs cycle , attaching Acetil-coA and the synthesis of the P450 cythocrome .
Also known as “mad hatter syndrome”, there are two types of poisoning:
- acute: very rare due to casual events (ingestion, epithelial lesions)
- chronic: micromercurialism with the following precocious symptoms: gingivitises and stomatitises, caused by Hg deposited in the salivary glands and poured in mouth with the saliva.
The potential mutagenicity of the mercury derives from his chemical tie capacity with RNA and then in the possible alteration of the mechanisms of replication of the genetic material; also low amounts show an increase in the frequency of the gpt gene mutation which takes to the appearance of mytotic figures and resulting aneuploidia and poliploidia of the emergent cells.
About the 1% of the organic mercury is directly expelled with the urine, but can be acetylated in the liver or combined with cysteine or glutathione. The n-acetil-omocystein-metilic mercury complex enters the circulation and is removed through the kidneys. The plasmatic mercury is expelled in minimum amount with the urine through the mechanism of the glomerulare filtering. To evaluate with an acceptable approximation the exposure level we can base on the analysis of the excretion profile of the porphyrines.
The nefrotoxicity of the mercury has been also evaluated through the measurement of a few cellular enzymes typical of the tubular epithelium as the b-glutamyltranspherase and b-galactosidase, b-glucuronidase and the n-acetil-b-glucosaminydase (NAG).
In nervous cell the immediate damage of the mercury is correlated to its perturbing action the electric function. The ion mercury is able to force the passage through the sodium and calcium channels causing depolarization and a accentuated release of neurotransmitters. After such action an irreversible block of the neurotransmitters emission follows and then a functional paralysis of the neuron itself. The existence of enzymatic inhibitions has been confirmed in particular of the ADP ribossilation which constitutes a fundamental process of the neuronal proteins metabolism. The enzyme inhibition of prevents formation of ribosilated actin and tubuline (structural proteins of the cytoskeleton) with consequent production of inactive intermediates. The final effect is the formation of neurofibril tangles like the ones discovered in the Alzheimer disease. Elimination from the SNC is slow due to his oxidation to ionic bivalent mercury that cannot anymore exceed the ematoencephalic barrier and this supports the store.
The mercury chloride and the metal mercury induce auto-immune phenomena. The pathogenic mechanism with which the pathology is set up consists in a policlonal b-lymphocytes activation through T cells with production of direct antibodies against the self which arrange themselves in a linear way along the basal membrane.
It causes the appearance of a membranous glomerulonefrite with subepithelial deposits of IgG which tends to evolve towards the nephrosic syndrome, preceded by increase of circulating IgE .
The appearance of autoimmune reactions has been proved also in the lung with similar demonstrations to the Goodpasture syndrome.
Various jobs suggest that the mercury not cause such damages:
lymphocyte levels in subjects with and without amalgam restorations
failure to detect effect of amalgam restoration on peripheral blood lymphocyte population
Only the 5% of the patients is clearly allergic to the mercury present in the amalgam. We have to remember that other researches have highlighted cutaneous ipersensibility reactions to the mercury given out patch test in 2 -35% of the subject bearers of obturations. The allergy symptoms are:
- urticaria eczema, eritematose reactions with interest of face, neck, arms and thorax, oral lichenoidi reactions.
MATERNAL EXPOSURE TO THE MERCURY AND PLACENTAR FETUS TRANSFER.
The mercury has been discovered in the maternal milk and many scientific publications document the passage through the placentar barrier.
Not all the chemical forms of the mercury have the same capacity to cross the placentar barrier, the two forms in absolute most subject to diffusion are the metal mercury and the metylmercury due to their liposolubility.
Some researches show the existence of alterations in the development of the nervous system of the fetus already to low mercury concentrations in the tissues.
In a recent study the concentration of the mercury has been evaluated in the amniotic liquid comparing it with the number and the occlusal extention of the obturations and a statistically significant relation exists between occlusal extention of the restorations and concentration of the metal. The ADA has performed a research on the results of the pregnancies by sending questionnaires to dentists and dental assistants concerning the mercury use; however the results have not pointed out any correlation between the levels of abortions or the incidence of congenital anomalies in the progenie.
ALLOWED MAXIMUM DOSE
The maximum levels of exposure for the airy polluting are said TLV (threshold limit value), among these the Hg steams are included . A mercury concentration in the air lower than 1 mg/m3 is shown as maximum dose.
|Hg vapour of inorganic and organic not alcalin Hg
|etyl and metyl Hg
TLV-TWA = Hg vapour concentration which a subject can tollerate 8h/die in a week
TLV-C = maxim airy Hg concentration during working time
TVL-STEL = maxim airy Hg concentration acceptable for at last 15 minuts in a working day
IDHL = danger of life