Bioaccumulation is the gradual accumulation of a certain chemical into the living tissue of an organism from its environment. This accumulation may result from direct absorption from the environment or from ingestion of food particles. 
The concentration of bioacumulation substances usually increases in with the age of an organism. Bioaccumulation is one of the factors used to asses the environmental hazard of a chemical. Chemicals with a higher tendency towards bioaccumulation form a greater hazard. When a substance bioaccumulates at each step of the food chain it will biomagnify 
Like tocicity and persistancy, bioaccumulation is an important criterion to determine its environmental impact. According to OSPAR criteria, a substance poses a risk for bioaccumulation when fishes accumulate by direct adsorption, more than 500 times the concentration of the surrounding water.
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- OSPAR CONVENTION 2005, Cut-Off Values for the Selection Criteria of the OSPAR Dynamic Selection and Prioritisation Mechanism for Hazardous Substances