title: Environmental persistence of chemicals.
abstract: BACKGROUND
  The hazard criterion of persistence as it applies to chemicals in the environment is reviewed and discussed. This quantity can not be measured directly in the environment, thus it must be estimated using models that synthesise information on chemical half-lives and partitioning properties, the nature of the environment and how the chemical is released into the environment.
  
  
  METHODS
  It is suggested that the preferred criterion is the average residence time of the chemical in the environment, i.e. conceptually the sum of the life-times of all molecules (attributable only to losses by degrading reactions) divided by the number of molecules. If all chemical fate processes are first order, this persistence is independent of the quantity of chemical introduced and whether introduction is steady- or unsteady-state in nature. It is shown that in a multimedia environment persistence is affected not only by degradation kinetics, but also by mode-of-entry and partitioning. For screening level purposes a Level II equilibrium model may be adequate but a Level III model is generally preferable for estimating the average persistence. If a distribution of persistences is required a dynamic Level IV model must be used.
  
  
  CONCLUSIONS
  The implications for regulating chemicals on the basis of persistence are discussed.
  
  
  CONCLUSIONS
  It is concluded that the preferred strategy is to use Level II, III, and IV models and that the use of only degradation kinetics or media-specific half-lives can be misleading and uneconomical.
authors.affiliation: Canadian Environmental Modelling Centre, Trent University, Peterborough, Ontario, K9J 7B8, Canada. dmackay@trentu.ca
authors.forename: Donald
authors.initials: D
authors.lastname: Mackay
authors.forename: Eva
authors.initials: E
authors.lastname: Webster
mesh_terms: D004785:Environmental Pollutants; D006207:Half-Life; D007700:Kinetics; D008962:Models, Theoretical; D013997:Time Factors
chemical_list: D004785:Environmental Pollutants