Pesticide mixtures are more toxic than predicted.
Laetz, CA, DH Baldwin, TK Collier, V Hebert, JD Stark and NL Scholz. 2009. The synergistic toxicity of pesticide mixtures: implications for risk assessment and the conservation of endangered Pacific salmon. Environmental Health Perspectives doi: 10.1289/ehp.0800096.
Certain pesticide mixtures are more toxic to young fish than would be predicted based on the potencies of the individual compounds, according to study appearing in the journal Environmental Health Perspectives.
Since pesticides occur in the environment as mixtures, it is often difficult to understand and predict their risks. Synergy is the term used to describe a situation where the toxicity of mixtures is greater than the additive toxicities of the compounds that make up that mixture. Synergism is of concern because chemical safety in the US is based on toxicity evaluations of single chemicals.
In this study, researchers exposed juvenile Coho salmon to mixtures of two common types of pesticides: organophosphates and carbamates. They examined combinations of the five pesticides diazinon, malathion, chlorpyrifos, carbaryl and carbofuran. The concentrations used were higher than what is typically found in the environment, but comparable concentrations of the compounds are sometimes measured in nature.
The authors report the mixtures at the higher concentrations tested created more synergy linked effects. In some cases, mixtures killed the fish when exposure to only one of the chemicals did not.
Specifically, the authors looked at neurotoxicity by examining the effect the chemicals have on the brain. To do this, they examined the activity of the enzyme acetylcholinesterase in fish brain tissue.
The enzyme clears the neurotransmitter acetylcholine from the space between adjacent nerve cells (synapses). If acetylcholine builds up in this space, the resulting abnormal nervous system activity can lead to disorientation and death.
Coho salmon (Oncorhyncus kisutch) are threatened or extinct from much of their historic range. The authors note that pesticides could play a role in their declines. Habitat loss, dams and other factors may also contribute to their demise.
Other animal studies have found that mixtures pose more toxic threats than single compounds alone. However, a previous laboratory study by some of the same authors found different results. In that study, synergy did not result when Chinook salmon brains (rather the whole animal) were processed and exposed to similar pesticide mixtures.
The discrepancy between whole animal research findings and laboratory studies raises an important issue that is central to an ongoing debate about adequate health and safety testing without using whole animals and without testing mixtures.
The authors conclude that "single-chemical risk assessments are likely to underestimate the impacts of these insecticides on salmon in river systems where mixtures occur" and "may pose a more important challenge for species recovery than previously anticipated."
