INTRODUCTION
Toxicity is the property or properties of a material that produces a harmful effect upon a biological system, and the agent that produces such a biological effect is termed as a toxicant. The majority of the toxic chemicals discussed in this volume are of man-made or anthropogenic origin. It is true that some of the materials that are produced naturally by biological systems are extremely potent, e.g., the fungal aflatoxins and venom; however, these materials are usually produced only in small amounts. In contrast, anthropogenically produced materials can amount to millions of kilograms per year. Toxicants enter the environment by a variety of routes from many different sources.
Toxicants introduced into the environment may come from two basic sources: point sources and nonpoint sources. Discharges from point sources include sewage discharges, waste streams from industrial sources, hazardous waste disposal sites, and accidental spills. Point discharges are generally easy to identify in terms of the types, rates of release, and total amounts of materials released. In contrast, nonpoint discharges are much more difficult to characterize. These include materials released from atmospheric emissions, agricultural runoff, contaminated soils and aquatic sediments, and urban runoff from such sources as parking lots and residential areas. In most situations, discharges from nonpoint sources are composed of mixtures of complex materials. Therefore, the amounts of toxicants released are difficult to assess, and the rates and timing of discharges are difficult to predict.
Many classes of chemicals can exhibit toxicity. One of the most commonly studied and discussed is the pesticide. Pesticide can refer to any substance that exhibits toxicity to an undesirable organism, but its toxicity often constitutes a broad spectrum. Industrial chemicals also are a major concern because of the large amounts transported and utilized. Metals, such as cadmium from mining and...