Half-times, biological

Half-time, Biological—Time required for an organ, tissue, or the whole body to eliminate one-half of any absorbed substance by regular processes of elimination. This is the same for both stable and radioactive isotopes of a particular element, and is sometimes referred to as half-time, symbolized as tbiol or Tb. 

Effective Half-time - Biological half-time x Radioactive half-life... 

Exposure. Americium may be detected in samples of urine, blood, feces, or body tissues. Due to the relatively long biological half-time of americium, short-term exposures cannot be readily distinguished from longer-term ones. No new biomarkers of exposure are needed at this time. 

Biological Half-time—The time required for a biological system, such as that of a human, to eliminate by natural process half of the amount of a substance (such as a chemical substance, either stable or radioactive) that has entered it. 

Half-time, Effective—Time required for a radioactive element in an organ, tissue, or the whole body to be diminished 50% as a result of the combined action of radioactive decay and biological elimination, symbolized as Te or Teff. 

The Leggett Model simulates the age-dependence of lead kinetics on such factors as bone turnover rates, partitioning between soft tissues and excreta, removal half-times in liver, kidneys, and red blood cells, and the deposition fraction in brain. The model structure represents a compromise between biological realism and practical considerations regarding the quantity and quality of information available to determine parameter values (Leggett 1993). 

An interesting study on accumulation and retention of 95mTc by the edible winkle (Littorina littorea L.) was carried out by Swift [54] who determined a concentration factor of 45 in the whole body and a biological half-time of 115... 

Radiocerium absorbed into the systemic circulation will be transported by blood proteins and be deposited predominantly in liver and bone. Deposition fractions will be about 0.45 for liver, 0.35 for bone, and 0.1 for other soft tissues with the remainder excreted in urine and feces. The retention times in liver and bone are long compared to the radioactive half-lives of the cerium isotopes. Therefore, their effective biological half-times in these organs will be approximately equal to their physical half-lives. Experimental data on internal organ distri-... 

Retention of cerium deposited in these tissues is generally considered to be very long, with a biological half-time in excess of a thousand days. Therefore, the effective half-time of retention in these internal organs will be approximately equal to the physical half-life. 

TABLE 24. Bioaccumulation (pgPbg 1 dry weight), concentration factor and biological half-time (/ 2- day) for lead uptake in three different trophic levels of organisms (modified from Reference 134)... 

Biological half times of cadmium in humans is lengthy. Based on body burden and excretion data, cadmium may remain in the human body for 13 to 47 years. Although cadmium is excreted primarily in urine and feces, it tends to increase in concentration with the age of the organism and eventually acts as a cumulative poison (Hammons et al. 1978). These phenomena have not been documented adequately in wildlife species. 

It is emphasized that Cr+3, probably because of its very low solubility in seawater, appears to have a much lower bioavailability to most groups of marine animals than Ct 6, which is more water soluble (Carr et al. 1982). The clam Rangia cuneata appears to be an exception it accumulated up to 19 mg Cr/kg in soft parts, on a dry weight basis, during exposure for 16 days to chromium-contaminated muds, and retained most of it for an extended period the estimated biological half-time was 11 days (Carr et al. 1982). In general, benthic invertebrates rarely accumulate chromium from contaminated sediments (82 to 188 mg Cr+Vkg) only a few examples have been recorded (Neff etal. 1978). 

Yamaguchi, S., K. Sano, and N. Shimojo. 1983. On the biological half-time of hexavalent chromium in rats. Industr. Health 21 25-34. 

Diazinon (phosphorothioic acid 0,0-diethyl 0-(6-mcthyl-2-(l-mcthylcthyl)-4-pyrimidinyl) ester) is an organophosphorus compound with an anticholinesterase mode of action. It is used extensively to control hies, lice, insect pests of ornamental plants and food crops, as well as nematodes and soil insects in lawns and croplands. Diazinon degrades rapidly in the environment, with half-time persistence usually less than 14 days. But under conditions of low temperature, low moisture, high alkalinity, and lack of suitable microbial degraders, diazinon may remain biologically active in soils for 6 months or longer. 

It has been shown that all doses of arsenic trioxide are characterized by different toxicokinetics parameters. Arsenic compounds have long half-times and the tendency to accumulate in the body. The excretion rate decreased with decreasing blood concentration. The present study confirms the ability of toxicokinetic models to improve the study of various toxic substances and to estimate the Biological Threshold Limit Values. 

---