Traditional risk assessments of chemicals are failing to protect the environment

Regulatory decisions based on current risk approaches are flawed simply because the science underpinning the risk of chemicals is inappropriate in many cases. A fundamental problem is to use one methodology for all compounds, irrespective of their toxic mode of action in organisms. According to the theories of Druckrey and Küpfmüller, the character of a poison is primarily determined by the reversibility of critical receptor binding. Chemicals showing irreversible or slowly reversible binding to specific receptors will produce cumulative effects with time of exposure, and whenever the effects are also irreversible (e.g. death) they are reinforced over time; these chemicals have time-cumulative toxicity.This concept was validated by Druckrey and co-workers with genotoxic carcinogens, the action of which is described by what is now known as the Druckrey-Küpfmüller equation: c x t˄n = constant, where c = exposure concentration, t = median time to effect, and n is an exponent > 1, which reflects reinforcement of the effect over time. Using data generated by Sanchez-Bayo, Tennekes demonstrated that the Druckrey-Küpfmüller equation also described the toxicity of (non-genotoxic) neonicotinoid insecticides to arthropods. This discovery showed that the theories of Druckrey and Küpfmüller were generally applicable in toxicology and, perhaps even more importantly, that risk assessment procedures needed to be revised, because the risks of time-cumulative toxins had been seriously underestimated.

While most toxicants with a generic mode of action can be evaluated by the traditional concentration–effect approaches, a certain number of chemicals, including carcinogens, methylmercury, rodenticides, neonicotinoids and cartap insecticides have toxic effects that are reinforced with time of exposure (time-cumulative effects). Therefore, the traditional risk approach cannot predict the impacts of the latter chemicals in the environment. New assessment procedures are needed to evaluate the risk that the latter chemicals pose on humans and the environment.
Since imidacloprid and other neonicotinoid insecticides have time-cumulative effects on arthropods, the risk of foraging worker bees feeding on tiny levels of residues becomes an issue that cannot and should not be ignored Given that honey bee workers can live up to a few months in winter time, any residue concentration found in pollen will have a lethal effect provided there is sufficient time of exposure. The values of n are 2.2 and 5.8 for thiamethoxam and imidacloprid, respectively.