Elimination Kinetics and Biological Half-Life

The elimination or depuration of chemicals from aquatic and terrestrial organisms often follows first order kinetics and can be described by Eq. (8)  

An important criterion in hazard assessment of organic chemicals is the biological half-life (ti/2). The half-life of a chemical is the time required to reduce the concentration of this chemical by one-half in tissue, organ, or in the whole organism. If the elimination rate k2 was determined the ti/2can be calculated by Eq. (11)  

However, if the eUmination phase takes a long time, as is the case for highly superhydrophobic persistent chemicals, the increase in body weight has to be considered [25a]. Compensation for so-caUed growth dilution can be made if the growth rate constant (kg) during the elimination phase is known by using 

In case that the kg is not known, this adjustment can be eliminated by multiplying the chemical concentration by the total weight of the organism. Estimation of ty2 based on body burden provides a better basis for comparisons of ti/2 of a chemical among studies with the same organism [25 a] (see also Sect. 8.2.3). However, recently it was shown that the half-Ufe of a chemical in different aquatic organisms is dependent on its lipid content [29 a, b, 40]. For persistent lipophilic chemicals ty2 increases with the lipid content of the organism (Fig. 1). 

Equations to Predict the Half-Life (t /2) or Elimination Rate Constant (k2) 

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