" Social values, concerns

FIG. 11.8.2 Disciplines contributing to environmental decisions.

FIG. 11.8.2 Disciplines contributing to environmental decisions.

ical will combine with another substance. Because different structural forms of a chemical may exhibit different degrees of toxicity, the chemical structure of the substance being assessed is critical. For example, the free cyanide ion dissolved in water is highly toxic to many organisms (including humans); the same cyanide combined with iron is much less toxic (blue pigment). Cyanide combined with an organic molecule may have completely different toxic properties.

Solubility. Solubility is a substance's ability to blend uniformly with another. The degree of water and lipid (fat) solubility of a chemical is important in risk assessment. Solubility has significant implications for activities as diverse as cooking or chemical spill cleanup. To estimate the degree of potential water contamination from a chemical spill, it is necessary to know the chemical makeup of the material spilled to judge the extent that chemical contamination will be dispersed by dissolving in water. Likewise, the degree of lipid solubility has important implications, particularly in such processes as bioaccumulation.

Bioaccumulation. The process of chemical absorption and retention within organisms is called bioaccumulation. For example, a fat-soluble organic compound ingested by a microorganism is passed along the food chain when an organism eats the microorganism, then another predator eats the organism. The organic compound, because it is fat-soluble, will concentrate in the fat tissue of each animal in the food chain. The pesticide DDT is an example of a chemical that bioaccumulates in fish, and then in humans and birds eating those fish.

Transformation. Biotransformation and transformation caused by physical factors exemplify how chemical compounds are changed into other compounds. Biotransformation is the change of one compound to another by the metabolic action of a living organism. Sometimes such a transformation results in a less toxic substance, other times in a more toxic substance.

Chemical transformation is prompted by physical agents such as sunlight or water. A pesticide that is converted into a less toxic component by water in a few days following application (e.g., malathion) carries a different long-term risk than a pesticide that withstands natural degradation or is biotransformed into a toxic compound or a metabolite (e.g., DDT). The ability to withstand transformation by natural processes is called persistence.

Understanding basic chemical and physical properties helps to determine how toxic a chemical can be in drinking water or in the food chain, and whether the substance can be transported through the air and into the lungs. For example, when assessing the risk of polychlorinated biphenyls (PCBs), it must be recognized that they biode-grade very slowly and that they are strongly fat-soluble, so they readily bioaccumulate. When monitoring their presence, it must also be recognized that they are negligibly soluble in water: concentrations will always be much higher in the fat tissue of a fish, cow, or human than in the blood, which has a higher water content.

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