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Approaches in ecological risk assessment are usually based on estimating the probability that organisms will be exposed to environmental concentrations of chemicals greater than some toxicological criterion (LC50, NOEC). Following this approach, quality standards for persistent chemicals in soil can be derived. For readily degradable chemicals emitted at intervals, the ecological risk can be related to the time taken for the chemical concentration to fall to a level causing no effect on most, e.g. 95%, of the species. A formula is derived for the maximum acceptable half‐life, assuming a log–logistic distribution of species susceptibilities and a simple exponential decay of the chemical. It is also assumed that the pesticide is distributed homogeneously through the top 20 cm of the soil. A numerical example is elaborated for carbofuran, demonstrating that for a recommended dose applied to soil, the maximum half‐life should be 47 days, if the ecotoxicological protection level is to be reached within one year after a single application. This estimate may vary between 35 and 61 days, depending on the margins of uncertainty attached to ecotoxicological data. Complete recovery not only includes the disappearance of the chemical but also the recolonization of treated soils. The half‐life condition derived in this paper should therefore be considered as a minimum requirement.

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