Exposure and environmental impact

Mixing substances can result in interactive effects

In nature pesticides seldom occur alone, but in a mixture of different substances. These substances can affect each other so as to produce an additive or synergistic effect. An additive effect is when the observed effect corresponds to the sum of the toxicities of the individual substances. Substances can also interact so that the toxic effect is greater than the sum of effects of the individual substances, i.e. they have a synergistic effect.

Examples of interactive effects

Some laboratory studies have been carried out on the effects of low concentrations of pesticides on aquatic organisms. These clearly show that there is a risk of biological effects from pesticide use, even though it can be difficult to detect these effects. In a trial with mosquito larvae, researchers were able to show an interaction between atrazin and chlorpyrifos, two completely different pesticides. Atrazin is a herbicide that is not toxic to mosquito larvae even at high concentrations. Atrazin, which is a component of e.g. Totex strö, was banned from sale in Sweden in 1989, but is still present in aquatic environments. Chlorpyrifos is an approved insecticide that affects e.g. the swimming behaviour of mosquito larvae even at low concentrations. When mosquito larvae are exposed to atrazin and chlorpyrifos together, the atrazin affects the breakdown of chlorpyrifos in the larvae. This leads to the formation of breakdown products that are actually more toxic than the parent substance, which results in chlorpyrifos being more toxic at lower concentrations than if had acted alone. Similar effects can also be obtained with other insecticides belonging to the same group as chlorpyrifos (e.g. methylparathion, diazinon, malathion and trichlorfon). This type of interaction is difficult to predict in view of the fact that the pesticides are directed at completely different target organisms. The risk of interactive effects can increase with the number of pesticides used within an area. There is also a risk of interactive effects between a pesticide and another environmental toxin, e.g. metals or persistent organic compounds such as PCB and brominated flame retardants.