By its nature, aquatic toxicology may be the major part of environmental toxicology. Seventy percent of the earth's surface and more than ninety percent of the biosphere is water. This environment is perceived to be at risk from chemical contaminants. Thousands of synthetic chemical compounds are currently registered for use in industry and agriculture, and thousands of tons of these are produced annually. Regardless of the source or original intended use, portions of these chemicals are released either deliberately or unintentionally into the environment. Thousands of tons more of unintended byproducts accompany the synthetic chemicals. If not placed deliberately into the aquatic environment, hydrologic and atmospheric processes distribute these chemicals, eventually depositing them in aquatic systems. These anthropogenic chemicals join a multitude of natural products, chemicals that are synthesized by plants and animals or, like hydrocarbons, are formed through geochemical or pyrocatalytic transformations. These anthropogenic and natural toxic compounds are referred to as xenobiotics; how they interact with aquatic life is the concern of aquatic toxicology.