Single or multiple exposures over a longer period of time represent chronic exposure. In the occupational setting, both acute (eg, accidental discharge) and chronic (eg, repetitive handling of a chemical) exposures occur. Exposures to chemicals found in the environment such as air and water pollutants are often chronic, resulting in chronic disease, as in the Minamata Bay, Japan, methyl mercury disaster. Sudden large chemical releases may result in acute massive population exposure with serious or lethal consequences. The tragedy in Bhopal, India, was such an event, in which methyl isocyanate was released into a crowded population area, resulting in almost 4000 deaths and more than half a million injuries. The release of dioxin in Seveso, Italy, contaminated a populated area with a persistent organic chemical having both acute and long-term chronic effects. More recently, the massive oil spill caused by the explosion of BP’s Deepwater Horizon drilling rig in the Gulf of Mexico highlighted the potential for long-term ecotoxic impacts involving widespread geographic areas.
Certain chemical and physical characteristics are important for the estimation of the potential hazard of environmental toxicants. Data on toxic effects of different organisms, along with knowledge about degradability, bioaccumulation, and transport and biomagnification through food chains, help in this estimation. (See Box: Bioaccumulation & Biomagnification for a classic example involving the Great Lakes.) Poorly degraded chemicals (by abiotic or biotic pathways) exhibit environmental persistence and can accumulate. Such chemicals include the persistent organic pollutants (POPs), polychlorinated biphenyls, dioxins and furans, and similar substances. Lipophilic substances such as the largely banned or abandoned organochlorine pesticides tend to bioaccumulate in body fat. This results in tissue residues that are 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. This concentrates the chemical in organisms higher on the food chain. Humans stand at the apex of the food chain. They may be exposed to highly concentrated pollutant loads as bioaccumulation 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.
If the intake of a long-lasting contaminant by an organism exceeds the latter's ability to metabolize or excrete the sub-stance, the chemical accumulates within the tissues of the organ-ism. This is called bioaccumulation. Although the concentration of a contaminant may be virtu-ally undetectable in water, it may be magnified hundreds or thousands 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 follow-ing residue values available from a classic Environment Canada report published by the Canadian government, and elsewhere. The biomagnification for this substance in the food chain, begin-ning with phytoplankton and ending with the herring gull, is nearly 50,000-fold. Domestic animals and humans may eat fish from the Great Lakes, resulting in PCB residues in these species as well. Source PCB Concentration (ppm)1 Concentration Relative to Phytoplankton Phytoplankton 0.0025 1 Zooplankton 0.123 49.2 Rainbow smelt 1.04 416 Lake trout 4.83 1,932 Herring gull 124 49,600 1Sources: Environment Canada, The State of Canada's Environment, 1991, Government of Canada, Ottawa; and other publications.