No effect concentration

No effect concentration (NEC) is a risk assessment parameter that represents the concentration of a pollutant that will not harm the species involved, with respect to the effect that is studied. It is often the starting point for environmental policy [Bruijn et al, 1997, Chen & Selleck 1969] .

Industry is continually synthesizing new chemicals, the regulation of which requires evaluation of the potential danger for human health and the environment. Risk assessment is nowadays considered essential for making these decisions on a scientifically sound basis.

There is not much debate on the existence of an NEC [Van Straalen 1997, Crane and Newman 2000] but the assignment of a value is another matter. Current practice consists of the use of standard tests, which were developed in the 1970s. In the standard tests groups of animals are exposed to different concentrations of chemicals and different effects such as survival, growth or reproduction are monitored. These toxicity tests typically result in a No Observed Effect Concentration (NOEC). This NOEC has been severely criticized on statistical grounds by several authors [Suter 1996, Laskowski 1995, [http://www.bio.vu.nl/thb/research/bib/Kooy96.html Kooijman 1996] , Van der Hoeven 1997] and it was concluded that the NOEC should be abandoned [ [http://www.oecd.org/document/30/0,2340,en_2649_34365_1916638_1_1_1_1,00.html OECD Document No 54 of "Series on Testing Assessment", 2006] ] . A proposed alternative is the use of so called ECx –concentrations, or the concentrating showing x % Effect (e.g. an EC5 in a survival experiment indicates the concentration where 5 % of the test animals would die in that experiment). ECx concentrations also have their problems in applying them to risk assessment. Any other value for x other than zero means an effect is accepted, this is in conflict with the aim of protecting the environment [Bruijn et. al 1997] . The aim of environmental legislation is to achieve no effect of pollutants to the environment. In addition ECx values do depend on the exposure time [Kooijman 1981, [http://www.bio.vu.nl/thb/research/bib/JageHeug2006.html Jager et al 2006] ] , it is essential to realize that ECx values decrease for increasing exposure time, until equilibrium has been established. This is because effects depend on internal concentrations [Kooijman 1981, Péry et al 2001a] , and that it takes time for the compound to penetrate the body of test organisms. The standard is to start with organisms that were not previously exposed to the compound. The exposure period during which the decrease is substantial depends on the properties of the test compound and of the organism and the type of effect. This may lead to the situation that an EC50 might be lower than an EC10 concentration depending on the test protocol used. It is clear that for the protection of the environment this is a non workable situation.

Therefore a biology based approach must be chosen [ [http://www.oecd.org/document/30/0,2340,en_2649_34365_1916638_1_1_1_1,00.html OECD Document No 54 of "Series on Testing Assessment", 2006] ] . Biology-based methods not only aim to describe observed effects, but also to understand them in terms of underlying processes such as toxico-kinetics, mortality, feeding, growth and reproduction (Kooijman 1997). This type of approach starts with the description of the uptake and elimination of a compound by an organism is, as an effect can only be expected if the compound is inside the organism, and where the No Effect Concentration is one of the modeling parameters. As the approach is biologically based it is also possible by using Dynamic Energy Budget theory [ [http://www.bio.vu.nl/thb/research/bib/Kooy2000.html Kooijman, 2000] ] to incorporate multiple stressors (e.g. effects of food restriction, temperature [Heugens, 2001, 2003] ), and quite a few processes that are active under field conditions (e.g. adaptation, population dynamics, species interactions, life cycle phenomena [Sibly and Calow (1989)] ). The effects of these multiple stressors are excluded in the standard test procedures by keeping the local environment in the test constant. It is also possible to use these parameter values to predict effects at longer exposure times, or effects when the concentration in the medium is not constant. If the observed effects include those on survival and reproduction of individuals, these parameters can also be used to predict effects on growing populations in the field [Kooijman 1997, Hallam et al 1989] .

References

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Bibliography

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