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toxic, toxin, toxicant

toxic:            a toxic substance, usually used in the plural:
                                        toxics flowed from the factory.

toxin:            a poisonous substance that is a specific product of the metabolic activities of a living organism.

toxicant:     a toxic agent, usually one produced by human-made activity, such as pesticide.

Although strictly speaking toxin should be used only for biologically produced substances and not as a synonym for toxic or toxicant, popular usage interchanges the terms. A toxic agent is anything that can produce an adverse biological effect.