Half-lives methyl mercury

The half-life of mercury is long, but there are two phases, the first being around 2 days, then the terminal phase, which is around 20 days. However, the half-life will depend on the form of mercury. Thus, methyl mercury has a half-life of about 70 days, whereas for inorganic mercury, this is about 40 days. 

Methylmercury is a primary example of this type of model in humans. A human mercury one-compartment PK model has been developed on the basis of several exposure datasets in which blood or hair concentrations have been measured at various times after methyl mercury exposure (Stern 1997 Kershaw et al. 1980 Sherlock et al. 1984). The one-compartment model takes into account the elimination half-life of mercury from blood and methylmercury volume distribution (in blood) to back-calculate the... 

Self-Test 13.8A In 1972 grain treated with methyl mercury was released for human consumption in Iraq, resulting in 459 deaths. The half-life of methyl mercury in body tissues is 70. d. How many days are required for the amount of methyl mercury to drop to 10.% of the original value after ingestion ... 

The biological half-life in humans for methyl mercury is about 70 days because elimination is slow, irregular, and individualized, there is a considerable risk of an accumulation of mercury to toxic levels. A precise relationship between atmospheric levels of alkyl mercury and concentrations of mercury in blood or urine has not been shown. Clinical observations indicate that concentrations of 50-100pg mercury/lOOml of whole blood may be associated with symptoms of intoxication concentrations around 10-20pg mercury/ 100 ml are not associated with symptoms. In a study of 20 workers engaged in the manufacture of organic mercurials and exposed for 6 years to mercury concentrations in air between 0.01 and O.lmg/m, there was no evidence of physical impairment or clinical laboratory abnormalities. Low levels of methyl mercury in the blood do not seem to affect the results of behavioral performance tests. ... 

On the basis of chemical profile, Wood (38) predicted that arsenic, selenium, and tellurium will be methylated in the environment, and lead, cadmium, and zinc will not. Elemental concentration in the aquatic food chain has been reported for As (39), Hg (40), Cd (41), Pb (42), and Cu (43). The biological half-life of methylmercury in fish, for example, is one to two years (44). Pillay et al. (40) implicated heavy coal burning in the mercurial contamination of plankton and fish populations of Lake Erie. Other metals, notably cadmium, have been shown to be incorporated into the grazing grasses surrounding a coal burning source (27). Trace element contamination, therefore, can enter the food chain at various points. Disposal of solid wastes in the form of ash and slag is yet another environmental consideration (45). 

During long-term constant exposure (several months) to methyl mercury in food, there is a linear relationship between daily intake of methyl mercury and the concentration of mercury in blood. The mercury concentration in blood (pg/L) corresponds to the daily intake of methyl mercury (pg/ day) multiplied by 0.5-1. When exposure is continuous, the blood mercury concentration is proportional to the concentration in the brain, the critical organ for methyl mercury toxicity. Because of mercury s short half-life in the blood (2-4 days), evaluation of blood mercury is of limited clinical value if a substantial amount of time has passed since time of exposure [43]. 

The model simulations were in close agreement with the observed results from the distribution and metabolism studies. Physiological processes that were highlighted by the results and the discrepancies that did occur include the probable active transport into the brain (versus passive diffusion) of a methyl-mercury-cysteine complex, the bidirectional transport of methylmercury between the gut lumen and gut tissue as a more important determinant of methylmercury fecal excretion than biliary secretion, the importance for the determination of methylmercury half-life in rats of the recycling of mercury from ingested hair, and the need for better estimates of the rate constants for the demethylation of methylmercury in order to adapt the model to other species. 

A wide variation of half-time of methyl mercury in hair has been observed in Iraqi patients. The half-life varied from 35 to 180 days (Al-Shahristani and Shibab, 1974) with a deviant part of the population studied with longer half-life (see Fig. 13). The reason for this variation has not been clarified and may be of genetical origin. 

The clearance half life for new born infants is unknown. Children have a somewhat shorter half live than that in adults which is about 70 days. Lactating women have a relatively short half time of about 45 days. However, at toxic methyl mercury levels, the elimination rate, especially from the brain, may be considerably slower. In estimates of risks to human health, attempts to determine the lowest concentration associated with the onset of toxic signs and symptoms have been made. For methyl mercury these figures have been given and they are mentioned in the environmental health criteria document. For blood it is 200-500 ig per litre and in hair 50-125 mg/kg. The figure of 50 is in accordance with the figure mentioned by Professor Tsubaki. However, he showed also a figure of 10-12, of a boy with methyl mercury compatible symptoms. 

The debate about the relation between autism and mercury (as thimerosal) in vaccines continues, without useful conclusions [37 ]. In a population-based study of the pharmacokinetics of mercury after immunization of 72 premature infants weighing 2000-3000 g at birth, the mean maximal blood mercury concentration was 3.6 fig/l, and it occurred at 1 day after immunization the maximal mean stool mercury concentration was 35 ng/g, and it occurred on day 5 urine mercury was almost undetectable [38 ]. The blood mercury half-life was 6.3 (95% Cl = 3.9S.8) days, and mercury concentrations returned to prevaccination values by day 30. The blood half-life of intramuscular ethyl mercury from thimerosal in vaccines given to premature infants is substantially shorter than that of oral methyl mercury in adults. Because of the differing pharmacokinetics, exposure guidelines based on oral methyl mercury in adults may not be accurate for children who receive thimerosal-containing vaccines. 

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