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

(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.
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%.
References in periodicals archive ?
Terminal elimination half-lives of the brominated flame retardants TBBPA, HBCD, and lower brominated PBDEs in humans.
Exhibit 5 HALF-LIFE FRAMEWORK Target Half-lives (in months) Hi 14 18 22 Organizational Med 7 9 11 Complexity Low 1 3 5 Low Med Hi Technical Complexity Source: Art Schneiderman, Analog Devices, Inc
The authors suggested that exposure pathways and/or sources might be missing in the prediction or that the intrinsic elimination half-lives of PBDEs in humans are underestimated.
The time lapse between the peak in emissions and sample collection for biomonitoring is the most influential factor controlling the shape of concentration age trends for chemicals with human metabolic half-lives longer than 1 year.
By measuring the proportions of radioactive elements and their daughters, researchers can determine how many half-lives have passed since the formation of the radioactive elements in such objects as stars.
Most empirical estimates of human elimination kinetics for persistent chemicals reflect apparent elimination half-lives that represent the aggregated effects of intrinsic elimination, ongoing exposure, and changes in body weight.
The half-lives of the new isotopes range from seconds to tens of seconds, the researchers estimate.
BACKGROUND: Most empirical estimates of human elimination kinetics for persistent chemicals reflect apparent elimination half-lives that represent the aggregated effect of intrinsic elimination, ongoing exposure, and changes in body weight.
In both cases, the measured half-lives and energies associated with proton decay indicated that the protons must have tunneled through a barrier that could not be uniform in all directions.
For the participants included in our analyses, we found that years since residing in a water district was significantly associated with serum PFOA, which yielded half-lives of 2.9 and 8.5 years for water districts with higher and lower exposure levels, respectively.
Their half-lives range from 20 to 700 milliseconds.