There are also cases where potentially toxic substances may counter each other's effects. When the effect of an exposure to two or more chemicals is less than would be expected (if the known effects of the individual chemicals were added together), the effect produced is known as antagonistic, that is, less than additive toxicity (2 + 2 = 3). For example, selenium reduces the toxic effects of mercury. Other antagonists that have been identified are methionine and vinyl chloride, arsenic and selenium, and zinc and cadmium. Less-than-additive toxicity has been reported for the combination of atrazine with meth-oxychlor pesticides.
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