# Encyclopedia

.The biological half-life or elimination half life of a substance is the time it takes for a substance (drug, radioactive nuclide, or other) to lose half of its pharmacologic, physiologic, or radiologic activity, as per the MeSH definition.^ Half-Life activity .
• Gordon Freeman - Wikia Gaming - Walkthroughs, games, guides, and more 2 February 2010 14:49 UTC gaming.wikia.com [Source type: General]

^ Radioactive half-life: the time required for a quantity of a radioisotope to decay by half.
• CDC Radiation Emergencies | Glossary of Radiological Terms 31 January 2010 12:55 UTC www.bt.cdc.gov [Source type: Academic]

^ The half-life of a substance does not equal half of its full duration of radioactivity.
• Half-life | Definition of Half-life at Dictionary.com: 2 February 2010 7:58 UTC dictionary.reference.com [Source type: FILTERED WITH BAYES]

.In a medical context, half-life may also describe the time it takes for the blood plasma concentration of a substance to halve ("plasma half-life") of its steady-state.^ Half Life is actually the amount of time a substance takes to decay half the original amount.
• Half-Life - Uncyclopedia, the content-free encyclopedia 31 January 2010 12:55 UTC uncyclopedia.wikia.com [Source type: General]

^ In a medical context, half-life may also describe the time it takes for the blood plasma concentration of a substance to halve ("plasma half-life") of its steady-state.
• Biological half-life - Psychology Wiki 2 February 2010 7:58 UTC psychology.wikia.com [Source type: Academic]
• Biological half-life - Psychology Wiki 31 January 2010 12:55 UTC psychology.wikia.com [Source type: Academic]

^ Half-life: the time any substance takes to decay by half of its original amount.
• CDC Radiation Emergencies | Glossary of Radiological Terms 31 January 2010 12:55 UTC www.bt.cdc.gov [Source type: Academic]

.The relationship between the biological and plasma half-lives of a substance can be complex, due to factors including accumulation in tissues, active metabolites, and receptor interactions.^ Different radioactive substances have different half-lives.

^ The relationship between the biological and plasma half-lives of a substance can be complex, due to factors including accumulation in tissues, active metabolites, and receptor interactions.
• Biological half-life - Psychology Wiki 2 February 2010 7:58 UTC psychology.wikia.com [Source type: Academic]
• Biological half-life - Psychology Wiki 31 January 2010 12:55 UTC psychology.wikia.com [Source type: Academic]

^ Examples of biological half-lives .
• Biological half-life - Psychology Wiki 2 February 2010 7:58 UTC psychology.wikia.com [Source type: Academic]
• Biological half-life - Psychology Wiki 31 January 2010 12:55 UTC psychology.wikia.com [Source type: Academic]

[1]
.Biological half-life is an important pharmacokinetic parameter and is usually denoted by the abbreviation t½.^ The original article was at Biological half-life .
• Biological half-life - Psychology Wiki 2 February 2010 7:58 UTC psychology.wikia.com [Source type: Academic]
• Biological half-life - Psychology Wiki 31 January 2010 12:55 UTC psychology.wikia.com [Source type: Academic]

^ Biological half-life .
• Biological half-life - Psychology Wiki 2 February 2010 7:58 UTC psychology.wikia.com [Source type: Academic]
• Biological half-life - Psychology Wiki 31 January 2010 12:55 UTC psychology.wikia.com [Source type: Academic]

^ Biological half-life is an important pharmacokinetic parameter and is usually denoted by the abbreviation t 1/2 .
• Biological half-life - Psychology Wiki 2 February 2010 7:58 UTC psychology.wikia.com [Source type: Academic]
• Biological half-life - Psychology Wiki 31 January 2010 12:55 UTC psychology.wikia.com [Source type: Academic]

[2]
.While a radioactive isotope decays perfectly according to first order kinetics where the rate constant is fixed, the elimination of a substance from a living organism follows more complex kinetics.^ The effective half-life of a given isotope is the time in which the quantity in the body will decrease to half as a result of both radioactive decay and biological elimination.
• half-life Facts, information, pictures | Encyclopedia.com articles about half-life 2 February 2010 7:58 UTC www.encyclopedia.com [Source type: FILTERED WITH BAYES]

^ The nucleus eventually reaches a stable number of protons and neutrons through one or more radioactive decays.
• CDC Radiation Emergencies | Glossary of Radiological Terms 31 January 2010 12:55 UTC www.bt.cdc.gov [Source type: Academic]

^ The decay rate is simply the number of radioactive atom decays occurring over a specified time.
• http://www.iem-inc.com/prhlfr.html 31 January 2010 12:55 UTC www.iem-inc.com [Source type: Reference]

See the article rate equation.

## Examples of biological half-lives

### Water

.The biological half-life of water in a human is about 7 to 14 days.^ The original article was at Biological half-life .
• Biological half-life - Psychology Wiki 2 February 2010 7:58 UTC psychology.wikia.com [Source type: Academic]
• Biological half-life - Psychology Wiki 31 January 2010 12:55 UTC psychology.wikia.com [Source type: Academic]

^ Everything about Half-Life 2 is remarkable.
• Half-Life 2 (pc) reviews at Metacritic.com 2 February 2010 7:58 UTC www.metacritic.com [Source type: General]

• Half-Life Faq 31 January 2010 12:55 UTC www.bluesnews.com [Source type: FILTERED WITH BAYES]

It can be altered by behavior. .Drinking large amounts of alcohol will reduce the biological half-life of water in the body[citation needed].^ The original article was at Biological half-life .
• Biological half-life - Psychology Wiki 2 February 2010 7:58 UTC psychology.wikia.com [Source type: Academic]
• Biological half-life - Psychology Wiki 31 January 2010 12:55 UTC psychology.wikia.com [Source type: Academic]

^ Biological half-life .
• Biological half-life - Psychology Wiki 2 February 2010 7:58 UTC psychology.wikia.com [Source type: Academic]
• Biological half-life - Psychology Wiki 31 January 2010 12:55 UTC psychology.wikia.com [Source type: Academic]

^ Drinking large amounts of alcohol will reduce the biological half-life of water in the body [ citation needed ] .
• Biological half-life - Psychology Wiki 2 February 2010 7:58 UTC psychology.wikia.com [Source type: Academic]
• Biological half-life - Psychology Wiki 31 January 2010 12:55 UTC psychology.wikia.com [Source type: Academic]

.This has been used to decontaminate humans who are internally contaminated with tritiated water (tritium).^ This has been used to decontaminate humans who are internally contaminated with tritiated water (tritium).
• Biological half-life - Psychology Wiki 2 February 2010 7:58 UTC psychology.wikia.com [Source type: Academic]
• Biological half-life - Psychology Wiki 31 January 2010 12:55 UTC psychology.wikia.com [Source type: Academic]

.Drinking the same amount of water would have a similar effect, but many would find it difficult to drink a large volume of water.^ Drinking the same amount of water would have a similar effect, but many would find it difficult to drink a large volume of water.
• Biological half-life - Psychology Wiki 2 February 2010 7:58 UTC psychology.wikia.com [Source type: Academic]
• Biological half-life - Psychology Wiki 31 January 2010 12:55 UTC psychology.wikia.com [Source type: Academic]

^ She even manages to flirt with Gordon a little; and while you'd think flirting with a completely mute character would be difficult, it doesn't seem to present her with too many problems.
• Half-Life 2: Episode One Review | PC Game Reviews - Yahoo! Video Games 2 February 2010 7:58 UTC videogames.yahoo.com [Source type: General]

^ If so, the ensemble effect would differ markedly between units, although the fundamental timbre would be the same.
• Ken Freeman & The Birth Of String Synthesis 2 February 2010 14:49 UTC www.soundonsound.com [Source type: General]

.The basis of this decontamination method (used at Harwell) is to increase the rate at which the water in the body is replaced with new water.^ The basis of this decontamination method is to increase the rate at which the water in the body is replaced with new water.
• Biological half-life - Psychology Wiki 2 February 2010 7:58 UTC psychology.wikia.com [Source type: Academic]
• Biological half-life - Psychology Wiki 31 January 2010 12:55 UTC psychology.wikia.com [Source type: Academic]

^ The puzzles in HL2 increase in difficulty so that the player is constantly having to use skills they have learned with new skills they are being taught.
• Half-Life 2 - Mark Sivak | ETC-Press (Beta) 14 January 2010 7:20 UTC www.etc.cmu.edu [Source type: FILTERED WITH BAYES]

^ Accordingly, the methods are particularly useful in programs for screening and developing new therapeutic binding proteins, such as various antibody constructs directed against therapeutic targets.
• In vitro estimation of in vivo half-life binding proteins - Patent application 2 February 2010 7:58 UTC www.faqs.org [Source type: Academic]

### Alcohol

The removal of ethanol (alcohol) through oxidation by alcohol dehydrogenase in the liver from the human body is limited. .Hence the removal of a large concentration of alcohol from blood may follow zero-order kinetics.^ For example, ethanol may be consumed in sufficient quantity to saturate the metabolic enzymes in the liver, and so is eliminated from the body at an approximately constant rate (zero-order elimination).
• Elimination half-life - Psychology Wiki 31 January 2010 12:55 UTC psychology.wikia.com [Source type: Academic]

^ The following exercises illustrate how the half-life depends upon the concentration of reactant for zero-, first-, and second-order reactions.
• Chemical Kinetics: Half-Life 2 February 2010 7:58 UTC www.chm.davidson.edu [Source type: Academic]
• Half Life - ChemWiki 31 January 2010 12:55 UTC chemwiki.ucdavis.edu [Source type: FILTERED WITH BAYES]

^ The graph at the left shows how the concentration of reactant A varies with time for a zero-order reaction.
• Chemical Kinetics: Half-Life 2 February 2010 7:58 UTC www.chm.davidson.edu [Source type: Academic]
• Half Life - ChemWiki 31 January 2010 12:55 UTC chemwiki.ucdavis.edu [Source type: FILTERED WITH BAYES]

.Also the rate-limiting steps for one substance may be in common with other substances.^ Grab the other one, and step on that one.
• Half-Life 2 Walkthrough - IGN FAQs 2 February 2010 7:58 UTC faqs.ign.com [Source type: General]

^ Also the rate-limiting steps for one substance may be in common with other substances.
• Biological half-life - Psychology Wiki 2 February 2010 7:58 UTC psychology.wikia.com [Source type: Academic]
• Biological half-life - Psychology Wiki 31 January 2010 12:55 UTC psychology.wikia.com [Source type: Academic]

^ Rate the films you've seen to find other films you may like.
• San Diego Asian Film Festival 2008 : Half-Life 31 January 2010 12:55 UTC sdaff.bside.com [Source type: General]

For instance, the blood alcohol concentration can be used to modify the biochemistry of methanol and ethylene glycol. .In this way the oxidation of methanol to the toxic formaldehyde and formic acid in the (human body) can be prevented by giving an appropriate amount of ethanol to a person who has ingested methanol.^ Vortigaunt : [seeing the body of a person who has committed suicide lying in a chair] A poignant scene.
• Half-Life - Wikiquote 2 February 2010 14:49 UTC en.wikiquote.org [Source type: Original source]
• Half-Life - Wikiquote 2 February 2010 7:58 UTC en.wikiquote.org [Source type: Original source]
• Half-Life - Wikiquote 14 January 2010 7:20 UTC en.wikiquote.org [Source type: Original source]

^ When a person inhales or ingests a radionuclide, the body will absorb different amounts of that radionuclide in different organs, so each organ will receive a different organ dose .
• CDC Radiation Emergencies | Glossary of Radiological Terms 31 January 2010 12:55 UTC www.bt.cdc.gov [Source type: Academic]

^ A person who has ingested a radioactive material receives an internal dose to several different organs.
• CDC Radiation Emergencies | Glossary of Radiological Terms 31 January 2010 12:55 UTC www.bt.cdc.gov [Source type: Academic]

.Note that methanol is very toxic and causes blindness and death.^ Note that methanol is very toxic and causes blindness and death.
• Biological half-life - Psychology Wiki 2 February 2010 7:58 UTC psychology.wikia.com [Source type: Academic]
• Biological half-life - Psychology Wiki 31 January 2010 12:55 UTC psychology.wikia.com [Source type: Academic]

.A person who has ingested ethylene glycol can be treated in the same way.^ A person who has ingested a radioactive material receives an internal dose to several different organs.
• CDC Radiation Emergencies | Glossary of Radiological Terms 31 January 2010 12:55 UTC www.bt.cdc.gov [Source type: Academic]

### Prescription medications

Substance Half-life Notes
Amiodarone 25 days
Cisplatin 30 to 100 h
Chlorambucil 1.53 h
Digoxin 24 to 36 h
Fluoxetine 1 to 6 days
The active metabolite of fluoxetine is lipophilic and migrates slowly from the brain to the blood. .The metabolite has a biological half-life of 4 to 16 days.^ Half-Life 2 took three days.
• Twenty Sided » half-life 31 January 2010 12:55 UTC www.shamusyoung.com [Source type: FILTERED WITH BAYES]

^ Also called biological half-life .
• Half-life | Definition of Half-life at Dictionary.com: 2 February 2010 7:58 UTC dictionary.reference.com [Source type: FILTERED WITH BAYES]

^ It's Official--Vivendi Universal to Release Half-Life 2 November 16 After all the turmoil, the highly praised shooter to get a release this holiday season.
• Half-Life 2 News, Half-Life 2 PC Video Game News 2 February 2010 7:58 UTC www.gamepro.com [Source type: General]

Methadone 15 to 60 h, in rare cases up to 190 h.[3]
Oxaliplatin 14 min[4]
Salbutamol 1.6 h

### Metals

.The biological half-life of caesium in humans is between one and four months.^ As for Half Life 3, there better be one!

^ MSG series and Half-Life for one.
• Ain't It Cool News: The best in movie, TV, DVD, and comic book news. 31 January 2010 12:55 UTC www.aintitcool.com [Source type: FILTERED WITH BAYES]

^ Attach objects to one another with the new Physics Gun in this Half-Life 2 mod.
• Half-Life 2 downloads on CNET 31 January 2010 12:55 UTC www.cnet.com [Source type: General]

This can be shortened by feeding the person prussian blue. The prussian blue in the digestive system acts as a solid ion exchanger which absorbs the caesium while releasing potassium ions.
.For some substances, it is important to think of the human or animal body as being made up of several parts, each with their own affinity for the substance, and each part with a different biological half-life (physiologically-based pharmacokinetic modelling).^ Half Life 3 Fallout is a part of this site.
• Half-Life - Wikia Gaming - Walkthroughs, games, guides, and more 31 January 2010 12:55 UTC gaming.wikia.com [Source type: FILTERED WITH BAYES]

^ Half-Life Story page is up!
• Half-Life Portal - an extensive Half-Life fansite - News Archive #2 14 January 2010 7:20 UTC www.halflifeportal.com [Source type: General]

^ Each radionuclide has its own characteristic half-life.
• http://www.iem-inc.com/prhlfr.html 31 January 2010 12:55 UTC www.iem-inc.com [Source type: Reference]

.Attempts to remove a substance from the whole organism may have the effect of increasing the burden present in one part of the organism.^ Attempts to remove a substance from the whole organism may have the effect of increasing the burden present in one part of the organism.
• Biological half-life - Psychology Wiki 2 February 2010 7:58 UTC psychology.wikia.com [Source type: Academic]
• Biological half-life - Psychology Wiki 31 January 2010 12:55 UTC psychology.wikia.com [Source type: Academic]

^ And when you see the water effects that are ever-present throughout HL2 , you may want to hold your hand under your chin to keep your jaw from dropping.

^ You understand that the Wii Console specifications and the Wii Network Service are constantly evolving and that we may update or change your Wii Console or the Wii Network Service in whole or in part, without notice to you.
• Amazon.com: Half Life 2: Xbox: Video Games 2 February 2010 7:58 UTC www.amazon.com [Source type: General]

.For instance, if a person who is contaminated with lead is given EDTA in a chelation therapy, then while the rate at which lead is lost from the body will be increased, the lead within the body tends to relocate into the brain where it can do the most harm.^ Vortigaunt : [seeing the body of a person who has committed suicide lying in a chair] A poignant scene.
• Half-Life - Wikiquote 2 February 2010 14:49 UTC en.wikiquote.org [Source type: Original source]
• Half-Life - Wikiquote 2 February 2010 7:58 UTC en.wikiquote.org [Source type: Original source]
• Half-Life - Wikiquote 14 January 2010 7:20 UTC en.wikiquote.org [Source type: Original source]

^ He decrypted the file, and you learned that Project: Locke was the codename for a project where DNA is put into a Template body to effectively clone a person.
• Gordon Stacy - City of Heroes Wiki - City of Heroes, City of Villains, Going Rogue, and more 2 February 2010 14:49 UTC cityofheroes.wikia.com [Source type: FILTERED WITH BAYES]
• Gordon Stacy - Paragon Wiki 2 February 2010 14:49 UTC wiki.cohtitan.com [Source type: FILTERED WITH BAYES]

^ I am naturally a man who hates deception, and wishes above all things to lead a life with its windows open and shades up, but I have been forced into the very reverse.
• "Doc." Gordon 2 February 2010 14:49 UTC home.comcast.net [Source type: Original source]

• Polonium in the body has a biological half-life of about 30 to 50 days.
• Caesium in the body has a biological half-life of about one to four months.
• Lead in bone has a biological half-life of about ten years.
• Cadmium in bone has a biological half-life of about 30 years.
• Plutonium in bone has a biological half-life of about 100 years.
• Plutonium in the liver has a biological half-life of about 40 years.

## Rate equations

### First-order elimination

.There are circumstances where the half-life varies with the concentration of the drug.^ There are circumstances where the half-life varies with the concentration of the drug.
• Biological half-life - Psychology Wiki 2 February 2010 7:58 UTC psychology.wikia.com [Source type: Academic]
• Biological half-life - Psychology Wiki 31 January 2010 12:55 UTC psychology.wikia.com [Source type: Academic]

^ There are games, and then there is the Half-Life legacy.
• 2404 - PC Gaming - Half-Life 2 Review 2 February 2010 7:58 UTC www.2404.org [Source type: General]

^ Is there a version of Half-Life for Linux?
• Alt.Games.Half-Life FAQ 2 February 2010 7:58 UTC www.faqs.org [Source type: General]

Thus the half-life, under these circumstances, is proportional to the initial concentration of the drug A0 and inversely proportional to the zero-order rate constant k0 where:
$t_{1/2} = \frac{0.5 A_{0}}{k_{0}} \,$
.This process is usually a logarithmic process - that is, a constant proportion of the agent is eliminated per unit time.^ You can use the following relationship to go between an element's decay constant , λ, and it's half-life : T ½ = ln 2 / λ The decay constant , λ, is the average number of decays per unit time.
• Half-life@Everything2.com 2 February 2010 7:58 UTC everything2.com [Source type: General]

^ 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.
• Half-life | Definition of Half-life at Dictionary.com: 2 February 2010 7:58 UTC dictionary.reference.com [Source type: FILTERED WITH BAYES]

^ Decay constant: the fraction of a number of atoms of a radioactive nuclide that disintegrates in a unit of time.
• CDC Radiation Emergencies | Glossary of Radiological Terms 31 January 2010 12:55 UTC www.bt.cdc.gov [Source type: Academic]

[5] Thus the fall in plasma concentration after the administration of a single dose is described by the following equation:
$C_{t} = C_{0} e^{-kt} \,$
• Ct is concentration after time t
• C0 is the initial concentration (t=0)
• k is the elimination rate constant
The relationship between the elimination rate constant and half-life is given by the following equation:
$k = \frac{\ln 2}{t_{1/2}} \,$
Half-life is determined by clearance (CL) and volume of distribution (VD) and the relationship is described by the following equation:
$t_{1/2} = \frac{{\ln 2}\cdot{V_D}}{CL} \,$
.In clinical practice, this means that it takes 4 to 5 times the half-life for a drug's serum concentration to reach steady state after regular dosing is started, stopped, or the dose changed.^ Playing Half-Life 2 will change a person.
• Microsoft | Half-life 2 14 January 2010 7:20 UTC www.projectcoe.com [Source type: General]

^ HALF-LIFE The half-life of a radioisotope is the amount of time it takes for half of the radioisotope to decay.

^ At the time of the events of Half-Life 2.
• The Citizen – Half-Life 2 | PlanetPhillip 31 January 2010 12:55 UTC www.planetphillip.com [Source type: FILTERED WITH BAYES]

.So, for example, digoxin has a half-life (or t½) of 24–36 h; this means that a change in the dose will take the best part of a week to take full effect.^ Video: half-life 2 soundtrack part 3 .
• Half-Life 2 - Playfire 14 January 2010 7:20 UTC www.playfire.com [Source type: FILTERED WITH BAYES]

^ Half-life was the best game EVER created.
• FARK.com: (4002322) LHC tries its hardest to emulate Black Mesa. Gordon Freeman unavailable for comment 2 February 2010 14:49 UTC www.fark.com [Source type: FILTERED WITH BAYES]

^ Playing Half-Life 2 will change a person.
• Microsoft | Half-life 2 14 January 2010 7:20 UTC www.projectcoe.com [Source type: General]

.For this reason, drugs with a long half-life (e.g.^ There are circumstances where the half-life varies with the concentration of the drug.
• Biological half-life - Psychology Wiki 2 February 2010 7:58 UTC psychology.wikia.com [Source type: Academic]
• Biological half-life - Psychology Wiki 31 January 2010 12:55 UTC psychology.wikia.com [Source type: Academic]

^ Not too long ago, a creepy fan video appeared online depicting an interview with one of Half-Life 2’s Combine soldiers, which was based on the creepy Scientology interviews with Tom Cruise.

^ Some games when things became too hard I’d give up but not half life for some reason, it was hard but it was never unfair.
• On The Enemies of Half-Life | The Reticule 31 January 2010 12:55 UTC thereticule.com [Source type: General]

.amiodarone, elimination t½ of about 58 days) are usually started with a loading dose to achieve their desired clinical effect more quickly.^ The effect is more severe with a higher dose.
• CDC Radiation Emergencies | Glossary of Radiological Terms 31 January 2010 12:55 UTC www.bt.cdc.gov [Source type: Academic]

## Sample values and equations

 Variable Abbreviation(s) Example value Formula Dose (loading dose, or steady state/maintenance) D (LD or MD) 1000 mg =VdC0 Volume of distribution Vd 25 L =D/C0 Concentration (initial or steady-state) C0 or Css 40.0 mg/L =D/Vd Biological half-life T½ 14 h =0.7/Ke Elimination rate constant Ke 0.05/h =0.7/(T½) =Cl/Vd Elimination rate, or rate of infusion to balance elimination Kin 50 mg/h =Css×Cl Clearance Cl 1.25 L/h =Vd×Ke Bioavailability F 1 $= \frac{[AUC]_{A} imes dose_{B}}{[AUC]_{B} imes dose_{A}}$
.Note that the "0.7" constant is a commonly used log approximation, but not the actual value.^ A.) Actually, I just wanted to note that the flying technique used to descend featured in this map is called "sailing" and is listed under the HL2DQ Tricks text.
• Internet Archive: Free Download: Half-Life 2 (PC) - 1:36:57 14 January 2010 7:20 UTC www.archive.org [Source type: General]

Another commonly used approximation is 0.693 for -ln(0.5) = 0.69315...

## References

1. ^ Lin VW; Cardenas DD (2003). Spinal cord medicine. Demos Medical Publishing, LLC. p. 251. ISBN 1888799617.
2. ^ International Union of Pure and Applied Chemistry. "biological half life". Compendium of Chemical Terminology Internet edition.
3. ^ Manfredonia, John (March 2005). "Prescribing Methadone for Pain Management in End-of-Life Care". JAOA—The Journal of the American Osteopathic Association. Retrieved 2007-01-29.
4. ^ Ehrsson, Hans et al. (Winter 2002). "Pharmacokinetics of oxaliplatin in humans". Medical Oncology. Retrieved 2007-03-28.
5. ^ Birkett DJ (2002). For example, ethanol may be consumed in sufficient quantity to saturate the metabolic enzymes in the liver, and so is eliminated from the body at an approximately constant rate (zero-order eliminationPharmacokinetics Made Easy (Revised Edition). Sydney: McGraw-Hill Australia. ISBN 0-07-471072-9.

# Citable sentences

Up to date as of December 30, 2010

Here are sentences from other pages on Half-Life 2, which are similar to those in the above article.