Editing Half-life (section)

==In biology and pharmacology==
{{See also|Biological half-life}}
A biological half-life or elimination half-life is the time it takes for a substance (drug, radioactive nuclide, or other) to lose one-half of its pharmacologic, physiologic, or radiological activity. In a medical context, the half-life may also describe the time that it takes for the concentration of a substance in [[blood plasma]] to reach one-half of its steady-state value (the "plasma half-life").

The relationship between the biological and plasma half-lives of a substance can be complex, due to factors including accumulation in [[tissue (biology)|tissues]], active [[metabolite]]s, and [[receptor (biochemistry)|receptor]] interactions.<ref name="SCM">{{cite book|title=Spinal cord medicine|author1=Lin VW|author2=Cardenas DD|publisher=Demos Medical Publishing, LLC|page=251|url=https://books.google.com/books?id=3anl3G4No_oC&pg=PA251|year=2003|isbn=978-1-888799-61-3}}</ref>

While a radioactive isotope decays almost perfectly according to first order kinetics, where the rate constant is a fixed number, the elimination of a substance from a living organism usually follows more complex chemical kinetics.

For example, the biological half-life of water in a human being is about 9 to 10 days,<ref>{{cite book|last1=Pang|first1=Xiao-Feng|title=Water: Molecular Structure and Properties|date=2014|publisher=World Scientific|location=New Jersey|isbn=9789814440424|page=451}}</ref> though this can be altered by behavior and other conditions. The biological half-life of [[caesium]] in human beings is between one and four months.

The concept of a half-life has also been utilized for [[pesticide]]s in [[plant]]s,<ref name=tebuau>{{cite web|last1=Australian Pesticides and Veterinary Medicines Authority|title=Tebufenozide in the product Mimic 700 WP Insecticide, Mimic 240 SC Insecticide|url=https://apvma.gov.au/node/14051|publisher=Australian Government|access-date=30 April 2018|language=en|date=31 March 2015}}</ref> and certain authors maintain that [[environmental impact of pesticides|pesticide risk and impact assessment models]] rely on and are sensitive to information describing dissipation from plants.<ref name=acs>{{cite journal|last1=Fantke|first1=Peter|last2=Gillespie|first2=Brenda W.|last3=Juraske|first3=Ronnie|last4=Jolliet|first4=Olivier|title=Estimating Half-Lives for Pesticide Dissipation from Plants|journal=Environmental Science & Technology|date=11 July 2014|volume=48|issue=15|pages=8588–8602|doi=10.1021/es500434p|pmid=24968074|bibcode=2014EnST...48.8588F|doi-access=free|hdl=20.500.11850/91972|hdl-access=free}}</ref>

In [[Basic reproduction number|epidemiology]], the concept of half-life can refer to the length of time for the number of incident cases in a disease outbreak to drop by half, particularly if the dynamics of the outbreak can be modeled [[Exponential decay#Applications and examples|exponentially]].<ref name = "Balkew">{{cite thesis |last=Balkew |first=Teshome Mogessie |date=December 2010 |title=The SIR Model When S(t) is a Multi-Exponential Function |publisher=East Tennessee State University |url=https://dc.etsu.edu/etd/1747 }}</ref><ref name = "Ireland">{{cite book |editor-first=MW|editor-last=Ireland |date=1928 |title=The Medical Department of the United States Army in the World War, vol. IX: Communicable and Other Diseases |location=Washington: U.S. |publisher=U.S. Government Printing Office |pages=116–7}}</ref>