Time-dependent toxicity of neonicotinoids and other toxicants: Implications for a new approach to risk assessment

A fundamental goal of toxicology is to determine safe levels of exposure to potentially poisonous substances for humans and the environment. Traditionally, safe levels of a chemical have been derived from the non-observable effect level (NOEL) estimated in laboratory toxicity bioassays with organisms which are representative of certain taxa. There are fundamental problems with the validity of this approach, both conceptual and statistical in nature. Firstly, the outdated NOEL concept is being replaced by the no-effect concentration (NEC) level, which assumes that toxic chemicals do not have any effect on a population of organisms at very low concentrations. Recent developments in ecotoxicology, however, suggest that some toxicants can produce effects at any concentration level provided their exposure time is sufficiently long. Consequently, risk assessment of these chemicals, which includes neonicotinoid insecticides, some carcinogenic substances and certain metallic compounds, may require entirely new approaches. Secondly, the traditional approach to toxicity testing is to consider dose or concentration-effect relationships at arbitrarily fixed exposure durations which are supposed to reflect âacuteâ or âchronicâ time scales. This approach measures the proportion of all exposed individuals responding by the end of those fixed exposure times. However, the endpoint values obtained this way cannot be linked to make predictions for the wide range of exposures encountered by humans or in the environment, thus leading to serious underestimates of actual risk. 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. Examples are shown to demonstrate that this approach is superior to current toxicological testing procedures, and has important implications for risk assessment of chemicals, particularly when the chemical has delayed toxic effects in a time-dependent manner

Sources and toxicity of pollutants

Modern living standards depend largely on the production and usage of thousands of chemicals, many of which are toxic and synthetically produced. These substances are discharged into the air, soil, water bodies and the sea through a variety of ways, becoming pollutants of our environment. The investigation of their fate and impacts they have on ecosystems is called ecotoxicology, a multidisciplinary science which intends to evaluate the nature of the discharge, the transformation and distribution of toxicants in the environment, exposure, lethality and sublethal effects on organisms, population responses, and changes in community structure and ecosystem function. The sources and mode of action of some of the most common groups of toxicants are described in this chapter, leaving their fate and effects in organisms and ecosystems for the subsequent chapters

More realistic concentrations of agrochemicals for environmental risk assessments

Given the strict protocols for the registration of agro-chemicals in developed countries there is a need to re-consider whether the use of PECs for the assessment of risks and/or hazards in aquatic environments is a valid approach. As the exposure of aquatic organisms in natural systems is reduced by several processes, more realistic values of effective concentrations should be estimated. A correcting factor for such effective exposure is proposed here based on the partitioning and dissipation features of organic pesticides. The validation of this model is limited by the data available at the moment, but the evidence so far is quite promising

A new technique to measure bird's dietary exposure to pesticides

Current methods to analyse pesticide residues in wild animals are essentially forensic examination procedures. This paper describes a new approach to obtain samples from live birds for pesticide residue analysis without harm to the animal involved. The technique measures the dietary exposure and has been tested successfully in birds exposed to farm chemicals in Australia. Stomach flushing with warm water was used to obtain 'flushing' samples (stomach and droppings combined) from 11 species of birds at four locations near or away from cotton farms. Water samples from all sites were also taken. Immunoassays were used to analyse these samples for residues of DDE, DDT, diuron, endosulfan and parathion-methyl. ELISA is suited to environmental residue analysis because of its high sensitivity, small sample volume requirements, low cost and speed. Prior to the residue analysis, optimization of the assays and elimination of matrix effects are essential. Positive residues were found in 90% of the birds, their amounts varying up to 4 orders of magnitude amongst individuals and species, with predators and insectivorous having higher levels than granivorous and nectivores. The technique can be applied to the same animal over a period of time, thus providing a useful tool for monitoring programs in environmental studies. Its application to ecological risk assessment and exposure are discussed

Ecological Relative Risk (EcoRR): Another approach for risk assessment of pesticides in agriculture

A site-specific methodology was developed to assess and compare the ecotoxicological risk that agricultural pesticides pose to ecosystems. The Ecological Relative Risk (EcoRR) is a composite scoring index for comparing relative risks between different plant protection products, and is used to assess the potential ecological impact their residues have after being applied to agricultural systems. The EcoRR model is based on standard frameworks for risk assessment (e.g. PEC/toxicity), but takes account of factors such as persistence of residues and biodiversity of ecosystems. The exposure module considers the environmental concentrations of a substance, its persistence, bioaccumulation and probability of exposure in several environmental compartments (water, sediment, soil, vegetation, air). The toxicity module takes into account the biodiversity of the ecosystems affected, whereby the endpoints used are weighted by the proportional contribution of each taxon in a given environmental compartment. EcoRR scores are calculated independently for each compartment and affected areas, thus enabling pinpointing of where risks will occur. The procedure to calculate EcoRR scores is explained using an example, and a sensitivity analysis of the model is included. A simulated risk assessment of 37 pesticides intended for use in a cotton development is also given as a case study. Exposure data were obtained using fugacity model II in areas previously defined by spray drift models. Toxicity data to vertebrate taxa and crustaceans were obtained from several databases, and biodiversity data from local sources. EcoRR scores were calculated for each compartment both on-farm and off-farm, during a normal growing season and during a flood, and a comparative relative assessment for all pesticides is discussed. EcoRR scores were also compared to traditional assessments using quotients for some taxa in the aquatic and terrestrial environments, revealing a good correlation between both model

Concluding remarks

The new millennium started with a legacy of unprecedented contamination of the world ecosystems left in the wake of the various activities of humankind. Chemical pollutants have become so diverse (see Chapter 1) and widespread that there is hardly any region of the world that is not currently affected by their impacts. With the exception, perhaps, of the desert wilderness areas (for which information on pollution is still lacking), every other ecosystem on earth, from the polar regions to the tropics, whether on land or in the oceans, has been shown to contain residues or traces of organic and inorganic pollutants of anthropogenic origin

Comparison of acute toxicity of two neonicotinoid insecticides, imidacloprid and clothianidin, to five cladoceran species

The acute toxicity (48-hr) of old (imidacloprid) and new (clothianidin) neonicotinoid insecticides to five cladoceran species and species sensitivity distribution (SSD) for cladocerans and other aquatic organisms to these insecticides are compared here