Certain chemical and physical characteristics are important for estimating the potential hazard involved for environmental toxicants. In addition to information regarding effects on differ-ent organisms, knowledge about the following properties is essential to predict the environmental impact: the degradability of the substance; its mobility through air, water, and soil; whether or not bioaccumulation occurs; and its transport and biomagnification through food chains. (See Box: BioaccumulationBiomagnification.) Chemicals that are poorly degraded (by abiotic or biotic pathways) exhibit environmental persistence and thus can accumulate. Typical examples of such chemicals include the persistent organic pollutants (POP) such as polychlorinated biphenyls and similar substances. Lipophilic substances such as the once-widespread organochlorine pesticides (eg, DDT) tend to bioaccumulate in body fat, resulting in tissue residues. Slowly released over time, these residues and their metabolites may have chronic adverse effects such as endocrine disruption. When the toxicant is incorporated into the food chain, biomagnification occurs as one species feeds on others and concentrates the chemi-cal. Humans stand at the apex of the food chain. They may be exposed to highly concentrated pollutant loads as bioaccumula-tion and biomagnification occur. The pollutants that have the widest environmental impact are poorly degradable; are relatively mobile in air, water, and soil; exhibit bioaccumulation; and also exhibit biomagnification.
Bioaccumulation & Biomagnification
If the intake of a long-lasting contaminant by an organism exceeds the latter’s ability to metabolize or excrete the substance, the chemical accumulates within the tissues of the organism. This is called bioaccumulation.Although the concentration of a contaminant may be virtually undetectable in water, it may be magnified hundreds or thou-sands of times as the contaminant passes up the food chain. This is called biomagnification.The biomagnification of polychlorinated biphenyls (PCBs) in the Great Lakes of North America is illustrated by the following residue values available from Environment Canada, a report pub-lished by the Canadian government, and other sources.Thus, the biomagnification for this substance in the food chain, beginning with phytoplankton and ending with the herringgull, is nearly 50,000-fold. Domestic animals and humans may eat fish from the Great Lakes, resulting in PCB residues in these spe-cies as well.