When the exposure of two different toxic chemicals produces a more severe effect than either one alone is called?

The answer is; synergism

An example of synergism in toxicology is the use of pyrethrin (an insecticide) and piperonyl butoxide(which in itself is not an insecticide). When used in combination, the pyrethrin becomes 10 times more toxic to target insects than the use of pyrethrin alone.

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