TOXICOLOGICAL
CHEMISTRYAND  
BIOCHEMISTRY
THIRD EDITION

In order to understand toxicological chemistry, it is necessary to have some understanding of the environmental context in which toxicological chemical phenomena occur. This in turn requiresan understanding of the broader picture of environmental science and environmental chemistry,which are addressed in this chapter. Also needed is an understanding of how environmentalchemicals interact with organisms and their ecosystems, as addressed by the topic of ecotoxicology.

Environmental science can be defined as the study of the earth, air, water, and living environments, and the effects of technology thereon.1 To a significant degree, environmental science hasevolved from investigations of the ways by which, and places in which, living organisms carry outtheir life cycles. This is the discipline of natural history, which in recent times has evolved intoecology, the study of environmental factors that affect organisms and how organisms interact withthese factors and with each other.

Environmental science can be defined as the study of the earth, air, water, and living environments, and the effects of technology thereon.1 To a significant degree, environmental science has evolved from investigations of the ways by which, and places in which, living organisms carry outtheir life cycles. This is the discipline of natural history, which in recent times has evolved intoecology, the study of environmental factors that affect organisms and how organisms interact withthese factors and with each other.

The Environment

            Traditionally, environmental science has been divided among the study of the atmosphere, the hydrosphere, the geosphere, and the biosphere. To an increasing extent during their brief time on earth, humans have used their ingenuity and technology to cause enormous perturbations in the natural environment. This has occurred to such a degree that it is now necessary to recognize a fifth sphere of the environment that is constructed and operated by humans, the anthrosphere.

            The atmosphere is the thin layer of gases that cover Earth’s surface. In addition to its role as a reservoir of gases, the atmosphere moderates Earth’s temperature, absorbs energy and damaging ultraviolet radiation from the sun, transports energy away from equatorial regions, and serves as a pathway for vapor-phase movement of water in the hydrologic cycle. The hydrosphere contains Earth’s water. Over 97% of Earth’s water is in oceans, and most of the remaining fresh water is in the form of ice. Therefore, only a relatively small percentage of the total water on Earth is actually involved with terrestrial, atmospheric, and biological processes. Exclusive of seawater, the water that circulates through environmental processes and cycles occurs in the atmosphere, underground as ground water, and as surface water in streams, rivers, lakes, ponds, and reservoirs. The geosphere consists of the solid earth, including soil, which supports most plant life. The part of the geosphere that is directly involved with environmental processes through contact with the atmosphere, the hydrosphere, and living things is the solid lithosphere. The lithosphere varies from 50 to 100 km in thickness. The most important part of it insofar as interactions with the other spheres of the environment are concerned is its thin outer skin, composed largely of lighter, silicate-based minerals. and called the crust. All living entities on Earth compose the biosphere. Living organisms and theaspects of the environment pertaining directly to them are called biotic, and other portions of the environment are abiotic. The anthrosphere consists of all the structures and devices made and operated by humans. The anthrosphere is composed of buildings, highways, parking lots, railroads, vehicles, aircraft, and other things that people make and do in Earth’s environment. It obviously has a major influence on all environmental phenomena, and any realistic treatment of environmental science must consider the anthrosphere along with the other four environmental spheres.

To a large extent, the strong interactions among living organisms and the various spheres of the abiotic environment are best described by cycles of matter that involve biological, chemical, and geological processes and phenomena. Such cycles are called biogeochemical cycles. Organisms participate in biogeochemical cycles, which describe the circulation of matter, particularly plant and animal nutrients, through ecosystems. As part of the carbon cycle, atmospheric carbon in CO₂ is fixed asbiomass; as part of the nitrogen cycle, atmospheric N₂ is fixed in organic matter. The reverse of these kinds of processes is mineralization, in which biologically bound elements are returned to inorganic states.