The traditional approach to toxicity testing is to consider dose (concentration)-effect relationships at arbitrarily fixed exposure durations which are supposed to reflect ‘acute’ or ‘chronic’ time scales. This approach measures the proportion of all exposed individuals responding by the end of different exposure times. Toxicological databases established in this way are collections of endpoint values obtained at fixed times of exposure. As such these values cannot be linked to make predictions for the wide range of exposures encountered by humans or in the environment. Thus, current toxicological risk assessment can be compromised by this approach to toxicity testing, as will be demonstrated in this paper, leading to serious underestimates of actual risk. This includes neonicotinoid insecticides and certain metallic compounds, which may require entirely new approaches.
In order to overcome this handicap, an increasing number of researchers are using a variant of the traditional toxicity testing protocol which includes time to event (TTE) methods. This TTE approach measures the times to respond for all individuals, and provides information on the acquired doses as well as the exposure times needed for a toxic compound to produce any level of effect on the organisms tested. Consequently, extrapolations and predictions of toxic effects for any combination of concentration and time are now made possible. We will demonstrate that this approach is superior to current toxicological testing procedures, and has important implications for risk assessment of chemicals. There is no doubt that once toxicologists realise the full potential and advantages of time-to-event approaches, they will become the standard tool for analysis of toxicity from pulse exposures, and the latent toxic effects emerging after exposure has ceased, because both of these phenomena are time related.
An improper understanding of the mechanisms of toxicity with time explained in this paper can be found in the current regulatory framework for honey bees (Apis mellifera). The European Food Safety Authority (EFSA) has recommended the inclusion of chronic toxicity tests in pesticide risk assessment of honey bees, thereby expanding current oral and contact acute toxicity data for 24 and 48 h (EPPO guidelines 170 and OECD 213 and 214). Mortality is recorded daily during at least 48 h and compared with control values. If the mortality rate is increasing between 24 and 48 h whilst control mortality remains at an accepted level, i.e. ≤ 10%, the duration of the test is extended to a maximum of 96 h. The results are used to calculate the LD50 at 24 h and 48 h and, in case the study is prolonged, at 72 h and 96 h. EFSA proposes to use mortality data and a mathematic model based on Haber’s rule to detect repeated dose effects. This approach is bound to fail because it does not take into account that toxic effects may be reinforced by exposure time, as indicated for imidacloprid. Such data cannot be used for prediction of toxic effects for any combination of concentration and time because contain no more than 4 t50 values (one for each day).
What would be required is information on the exposure concentrations and exposure times needed to kill bees. Mortality should be determined under continuous exposure conditions to a range of concentrations at defined time intervals (say after 1, 2, 3, 7 and 14 days of exposure), specially in the case of oral exposure. In that way, not only LC50 values can be established for each of these time points but also the t50s can be estimated by regression analysis. Once this information is obtained, the risk assessment should consider the pesticides residues found in pollen, and the frequency of such residues in the environment, in order to estimate the time to 50% mortality (i.e. t50). Since imidacloprid and other neonicotinoid insecticides have time-dependent 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. In the example shown here, 50% of worker bees would die within 7-12 days if feeding on a field where 11% of plants have residues of imidacloprid in the specified range. By contrast, standard hazard quotients (HQ) for dietary NOEL of 20 μg L-1 are misleading because they suggest that imidacloprid poses no danger to honey bees. Given that honey bee workers can live up to a few months in winter time the NEC for imidacloprid is close to zero, which means that any residue concentration found in pollen will have a lethal effect provided there is sufficient time of exposure. Recommendations of this kind have been suggested before, but their implementation has not happened yet.
Tennekes HA, Sánchez-Bayo F (2011) Time-Dependent Toxicity of Neonicotinoids and Other Toxicants: Implications for a New Approach to Risk Assessment. J Environment Analytic Toxicol S4:001. doi:10.4172/2161-0525.S4-001