Industrialized and large developing countries, especially China, India, and Brazil, produce staggering quantities of agricultural and industrial chemicals. Some of these chemicals are either toxic to start with or become toxic after their use in the industrial operations. Many of these chemicals are intentionally or unintentionally discharged into the environment, thus contaminating water, soil, and sediments. Incidents range from industrial chemical waste contamination at Love Canal, New York, to halogenated hydrocarbons and pesticide in groundwater to oil spills in Prince William Sound, Alaska. In addition, shipping of huge quantities of organic chemicals to different parts of the world has the potential of causing a worldwide environmental problem. While some degradation of organic chemicals may be due to abiotic mechanisms, for example, photochemical reactions in aquatic environments, most of the degradation of organic chemicals is by indigenous microbial populations.
In natural environment, a chemical may be present at a level, which on its own would cause no harm, but upon interaction with other chemicals may become much more toxic. For example, in production of petrochemical smog, ultraviolet light from the Sun, in the presence of oxygen, hydrocarbons, and nitrogen oxide interact to form peroxyacyl nitrates that are much more toxic than hydrocarbons or nitrogen oxide alone; this is known as synergism. On the other hand, there are also cases when potentially toxic substances may interact to counter each other's effect known as antagonism.
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