Half-life whole body

Endosulfan does not bioaccumulate to high concentrations in terrestrial or aquatic ecosystems. In aquatic ecosystems, residue levels in fish generally peak within 7 days to 2 weeks of continuous exposure to endosulfan. Maximum bioconcentration factors (BCFs) are usually less than 3,000, and residues are eliminated within 2 weeks of transfer to clean water (NRCC 1975). A maximum BCE of 600 was reported for a-endosulfan in mussel tissue (Ernst 1977). In a similar study, endosulfan, isomers not specified, had a measured BCE of 22.5 in mussel tissue (Roberts 1972). Tissue concentrations of a-endosulfan fell rapidly upon transfer of the organisms to fresh seawater for example, a depuration half-life of 34 hours (Ernst 1977). Higher BCFs were reported for whole-body and edible tissues of striped mullet (maximum BCF=2,755) after 28 days of exposure to endosulfan in seawater (Schimmel et al. 1977). However, tissue concentrations decreased to undetectable levels 48 hours after the organisms were transferred to uncontaminated seawater. Similarly, a BCE of 2,650 was obtained for zebra fish exposed to 0.3 pg/L of endosulfan for 21 days in a flow-through aquarium (Toledo and Jonsson 1992). It was noted that endosulfan depuration by fish was rapid, with approximately 81% total endosulfan eliminated within 120 hours when the fish were placed in a tank of water containing no endosulfan. 

Transfer of technetium from seawater to animals has been studied by laboratory experiments using 95mTc, The advantages of 95mTc over "Tc are 95mTc emits y-rays, thus whole-body counting techniques can be used, and it has a sufficiently high specific activity because of its relatively short half life (61 d). 

The elimination half-life of the substance gives important information about the duration of internal exposure following an episode of exposure. The half-hfe (half-time, Ty ) is the time taken for the concentration of a substance in the blood, tissue/organ, or whole body to decline to half of its original value. 

Alkali metals (K, Rb, Cs) behave similarly and sometimes one is accumulated preferentially when another is deficient. A similar case is made for Sr and Ca (Whicker and Schultz 1982a). The most important alkali metal isotope is Cs because of its long physical half-life (30 years) and its abnndance as a fission prodnct in fallout from nuclear weapons and in the inventory of a nuclear reactor or a fuel-reprocessing plant. Cesium behaves much like potassium. It is rapidly absorbed into the bloodstream and distribnted throughout the active tissues of the body, especially muscle. The P and y radiation from the decay of Cs and its daughter, Ba, result in essentially whole-body irradiation that harms bone marrow (Hobbs and McClellan 1986). 

After four weeks repeated administration of [ C]diethanolamine (7 mg/kg bw per day on five days per week) to rats, the urinary excretion of radiolabel (during the washout phase ) was followed for another four weeks. The log-linear response with time was a first-order process with a whole-body elimination half-life of about six days (Mathews et al., 1997). 

Chronic toxicity may be quantitated in a similar manner to acute toxicity, using the TD50 concept. Measurement of chronic toxicity in comparison with acute toxicity measurements may reveal that the compound is accumulating in vivo and may therefore give a rough approximation of the probable whole-body half-life of the compound. 

Human pharmacokinetic studies indicate that methylmercury has a half-life in blood and the whole body of about 50 days (CDC 2005). Hair grows at about 1 cm/month with a delay of around 20 days between current blood concentration and appearance of mercury in hair (Myers et al. 2003). Thus, postnatal maternal hair can be analyzed sequentially to evaluate timing of methylmercury exposure during pregnancy. However, the potential that this affords to document critical periods of prenatal methylmercury exposure has yet to be realized. 

Half-life Time required for an organ, tissue, or the whole body to... 

There have been relatively few studies on Mn bioavailability from various types of diets as well as from individual factors in the diet. However, to better understand the requirement of Mn in humans it is essential to obtain such information. While Mn deficiency in humans appears to be rare (see Chapter by Keen et al.), our knowledge about the signs of human Mn deficiency as well as our means to clinically assess Mn status is very limited. The physiological requirement of Mn, i.e., the amount that must be absorbed to balance the daily excretion and retention in growing subjects, is not known. The observed whole body turnover rate in human adults (a half-life of about 40 days) and available estimates of total body Mn content (20 mg) (26) speaks for a daily turnover of about 0.25 mg. With a low degree of absorption, the dietary requirement will be much higher. 

All chicks fed 2000 ppm Mn were then switched to 14 ppm Mn and serially killed on days 0, 3, 7, 10 and 14 following the diet switch. Tissue Mn concentrations are presented in Table I. Depletion of tissue Mn was curvilinear with time. Log-transformation of the data, however, revealed a linear (P<,01) reduction in Mn concentration in each of the tissues. Half-life (the number of days required to attain one-half of the initial tissue Mn concentration) of the tissue Mn determined by regression analysis on the log-transformed data was 6.0, 7.3 and 1.1 days in bone, pancreas and bile, respectively. Suso and Edwards (25) obtained a whole-body biological half-life of 5 days in chicks administered 5 Mn orally. From these investigations, it is clear that tissue... 

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