Repeated exposures to a chemical may result in the progressive increase of its concentration in an organism, organ or tissue; illness or other effects of the chemical's toxicity may increase with successive doses.
Factors involved in accumulation include selective binding of the chemical to tissue molecules, concentration of fat soluble chemicals in body fat, absent or slow metabolism of the chemical, and slow excretion of the chemical. Accumulation is a mass balance effect where input exceeds output.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.