Measures of pollutant concentration

Measures of pollutant concentration are used to determine risk assessment in public health. 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. Measures or defined limits include: no-observed-adverse-effect level (NOAEL), also called no-effect concentration (NEC), no-observed-effect concentration (NOEC) or similarly lowest-observed-adverse-effect level (LOAEL) acceptable operator exposure level (AOEL) ECx (in percentage). 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. There is not much debate on the existence of an NEC, but the assignment of a value is another matter. Current practice consists of the use of standard tests. 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, also called a no-observed-effect level, or NOEL). This NOEC has been severely criticized on statistical grounds by several authors and it was concluded that the NOEC should be abandoned. A proposed alternative is the use of so-called ECx – the concentration(s) showing x% effect (e.g. an EC50 in a survival experiment indicates the concentration where 50% 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 may give the impression that an effect is accepted, and this is in conflict with the aim of maximally protecting the environment. In addition ECx values do depend on the exposure time.