half-life

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half-life

 [haf´līf″]
the time required for the decay of half of a sample of particles of a radionuclide or elementary particle; see also radioactivity. Symbol t½ or T½.

half-life

(haf'līf),
The period in which the radioactivity or number of atoms of a radioactive substance decreases by half; similarly applied to any substance, such as a drug in serum, whose quantity decreases exponentially with time. Compare: half-time.

half-life

(haf´līf) the time required for the decay of half of a sample of particles of a radionuclide or elementary particles; symbol t 1/2 or T 1/2.
antibody half-life  a measure of the mean survival time of antibody molecules following their formation, usually expressed as the time required to eliminate 50 per cent of a known quantity of immunoglobulin from the animal body. Half-life varies from one immunoglobulin class to another.
biological half-life  the time required for a living tissue, organ, or organism to eliminate one-half of a radioactive substance which has been introduced into it.

half-life

(hăf′līf′, häf′-)
n.
1. Physics The time required for half the nuclei in a sample of a specific isotopic species to undergo radioactive decay.
2. Biology
a. The time required for half the quantity of a drug or other substance deposited in a living organism to be metabolized or eliminated by normal biological processes. Also called biological half-life.
b. The time required for the radioactivity of material taken in by a living organism to be reduced to half its initial value by a combination of biological elimination processes and radioactive decay.

half-life (t-½)

Etymology: AS, haelf + lif
1 also called radioactive half-life. the time required for a radioactive substance to lose 50% of its activity through decay. Each radionuclide has a unique half-life.
2 the amount of time required to reduce a drug level to half of its initial value. Usually the term refers to time necessary to reduce the plasma value to half of its initial value. After five half-lives, 97% of a single drug dose will be eliminated. See also biological half-life, effective half-life.
The amount of time required for a substance to be reduced to one-half of its previous level by degradation and/or decay (radioactive half-life), by catabolism (biological half-life), or by elimination from a system (e.g., serum half-life)
Haematology The time that cells stay in the circulation—e.g., red blood cells, 120 days, which increases after splenectomy; platelets, 4–6 days; eosinophils, 3–7 hours; neutrophils, 7 hours
Immunology The time an immunoglobulin stays in the circulation: 20–25 days for IgG, 6 days for IgA, 5 days for IgM, 2–8 days for IgD, 1–5 days for IgE
Nuclear medicine The length of time required for a radioisotope to decay to one-half of the original amount having the same radioactivity; a radioisotope’s effective T1/2 is either the time of decay—physical T1/2—or the time to elimination from a biological system. See Biological half-life
Physiology The time that it takes for half of a molecule’s activity to decay
Research See Cited half-life, Citing half-life
Therapeutics The amount of time it takes for the serum concentration of a drug to fall 50%, which reflects its rate of metabolism and elimination of parent drug and metabolites in the urine and stool

half-life

T1/2 The amount of time required for a substance to be reduced to one-half of its previous level by degradation and/or decay–radioactive half-life, by catabolism–biological half-life, or by elimination from a system–eg, half-life in serum Hematology The time that cells stay in the circulation–eg, RBCs 120 days–which ↑ after splenectomy, platelets–4-6 days, eosinophils–3-7 hrs, PMNs–7 hrs Immunology The time an Ig stays in the circulation: 20-25 days for IgG, 6 days for IgA, 5 days for IgM, 2-8 days for IgD, 1-5 days for IgE Therapeutics The time that a therapeutic agent remains in the circulation, which reflects its rate of metabolism and elimination of parent drug and metabolites in the urine and stool. See Effective half-life.
Half life in hours
Drug  Adult  Children
Digoxin  6–51  11–50
Gentamycin  2-3
Lithium 8–35
Phenobarbital  50–150  40–70
Phenytoin 18–30  12–22
Procainamide  2–4
Quinidine  4–7
Theophylline  3–8  1–8
Tobramycin  2–3
Valproic acid  8–15
Advance/Lab Feb 1995, p19  

half-life

(haf'līf)
1. The period in which the radioactivity or number of atoms of a radioactive substance decreases by half; similarly applied to any substance whose quantity decreases exponentially with time.
Compare: half-time
2. Time required for the serum concentration of a drug to decline by 50%.
Half-lifeclick for a larger image
Fig. 188 Half-life . X = half-life. Note that the time taken to reach zero amount is not 2 x X.

half-life

the time required for half of the mass of a radioactive substance to disintegrate. For example, the half-life of 14C is 5,700 years.

Half-life

The time required for half of the atoms in a radioactive substance to disintegrate.

half-life

(haf'līf)
1. The period in which the radioactivity or number of atoms of a radioactive substance decreases by half; similarly applied to any substance whose quantity decreases exponentially with time.
Compare: half-time
2. Time required for the serum concentration of a drug to decline by 50%.

half-life

the time in which the radioactivity usually associated with a particular isotope is reduced by half through radioactive decay.
References in periodicals archive ?
Estimating intrinsic elimination half-lives of persistent chemicals therefore requires correcting for effects other than intrinsic elimination (i.
A few studies have exploited the information in the cross-sectional age-concentration relationship to estimate intrinsic human elimination half-lives for PCBs, dioxins, and furans.
Here we use two sets of age--concentration CSD and pursue three main goals: first, to provide estimates of intrinsic elimination half-lives from the human body at background exposure levels for nine PCB congeners; second, to compare half-life estimates with literature data to discuss plausible ranges for intrinsic elimination half-lives compared with apparent half-lives; and third, to evaluate the possibility to access information about historic exposure contained in the temporal evolution of the age-concentration relationship in cross-sectional population biomonitoring data.
This equation, Equation 9, predicts body burdens of chemicals with relatively short elimination half-lives to be less dependent on age.
Our analysis provides a transparent interpretation of the half-lives derived from exponential fitting of CSTD.
Under any exposure conditions, CSTD-derived half-lives are specific to the characteristics of the sampled population and the exposure trend, and not individual humans.
With respect to BMI, heavier individuals (who inherently have more adult measurements) had significantly higher mean half-lives than leaner individuals in paired comparisons above and below BMIs of 20 and 25 (Table 1).
Subgroups selected according to chloracne response showed significantly shorter mean half-lives with increasing severity of chloracne compared with the subgroup with no chloracne (Table 1).
Table 1 shows similar average half-lives for persons < 12 and < 18 years of age, and a significantly longer half-life for persons > 18 years of age.
Data from these samplings were not included in the modeling of half-lives but are reported separately.
The half-lives of the higher BDEs increased with decreasing number of bromine substituents (Table 3).
In two rubber mixers with a rich data set, a traditional elimination model yielded similar results for BDE-209, with half-lives of 14 and 16 days, respectively, calculated between days 10 and 34, to ensure that the uptake phase was over (Figure 2).