Toxicology is the study of physical or chemical agents that produce adverse effects on biological systems. This definition is a bit overly broad for our purpose, since, for example, it could include nontoxic oxygen-demanding substances that can kill fish by depleting oxygen in a stream. In this chapter and the several that follow we only examine agents that affect individual organisms directly. Indirect effects, such as eutrophication or deoxygenation, are described in Chapter 15.
Furthermore, we are mainly concerned here with environmental pollutants. The field of toxicology also deals with other toxins, such as pharmaceuticals, food additives, and those that occur naturally. Of particular interest are xenobiotics. Various forms of radiation, if capable of depositing enough energy to break chemical bonds, can also produce toxic effects. Radiation with sufficient energy is called ionizing radiation or just high-energy radiation. Examples include ultraviolet, x-ray, and gamma radiation from the electromagnetic spectrum, and high-energy particles such as alpha or beta radiation (helium nuclei and electrons, respectively) from radioactive decay.
This chapter focuses on the general principles of toxicological effects at and below the organism level (e.g., biochemical, cellular, organ systems). In subsequent chapters we detail higher-level effects such as ecosystem-wide changes, or organism effects that are specific to particular groups of organisms, such as aquatic or mammalian.
Toxicology is an interesting combination of the qualitative and the quantitative. A major activity in toxicology is examining exposed organisms to determine the ''how'' and ''what'' of a toxin's effect: ultimately, it is hoped, to the molecular level of understanding. Another large area of activity is the measurement of toxic responses, in either laboratory experiments or in field measurements. These responses are usually studied in probabilistic terms using the tools of statistics. Toxicity tests involve exposing organisms