Bioaccumulants are Substances that increase in Concentration in living organisms as they take in contaminated air, water, or food because the substances are very slowly metabolized or excreted. (See: biological magnification.)

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Other /More definition:
bioaccumulants are "Substances that increase in concentration in living organisms as they take in contaminated air, water, or food because the substances are very slowly metabolized or excreted" (EPA 1992, 4).