Herbicides Weeds and other nonfood plants can reduce yield by ca. 10%. Herbicides, or chemical weed killers, have largely replaced mechanical methods of weed control, especially where intensive and highly mechanized agriculture is practiced. Many herbicides can be classified as selective, killing weeds without harming the crop, whereas others are nonselective, killing all vegetation. Like insecticides, trace residues of herbicides can be found in final food products.
Fungicides Fungicides are chemicals used to kill or halt the development of fungi. Although many of these compounds are used to control fungal diseases on plants or seeds of plants, they are rather nontoxic. The notable exception is mercury-containing fungicides. A host of organic mercury compounds that were developed several decades ago has been replaced by other organic fungicides.
Industrial and environmental Contaminants More than 40,000 chemical substances are logged by the Environmental Protection Agency (EPA) in its inventory of chemicals subject to the Toxic Substances Control Act. Most do not pose a threat to human health, but some certain incidents have indicated that they can be a potential hazard.
Polychlorinated Biphenyls PCBs (trade name Aroclor®) were first synthesized in 1881 and generically refer to several chlorinated isomers of biphenyl. This family of ca. 200 chemical compounds exhibits physical characteristics ranging from light oily substances to greasy and waxy substances. The molecular structure of PCBs has considerable resistance to acids, bases, high temperature, and electrical current, and is nonflammable. PCBs were used in food packaging material made from recycled paper. It is estimated that about two-thirds was degraded or destroyed, but because of their high degree of stability, ca. 400 million pounds of discarded PCBs may be present in the environment. In 1978, rice oil was contaminated with 3000 ppm of PCBs because of discharges from PCB heattransfer agents into water used to irrigate rice fields. Over 1000 individuals had symptoms of chloracne, pigmentation of the skin and nails, edema, and weakness, and experienced vomiting, diarrhea, and weight loss (Yusho disease). Young children experienced growth retardation and 35.5% of the deaths in a 11-year period were traced to cancer.
Dioxins Trace amount of dioxins, such as chlorinated dibenzop -dioxins, occur in commercial chemical formulations, such as herbicides. Some of these compounds, such as tetrachlorodibenzop -dioxin (TCDD), are among the most potent toxicants. Documented toxic effects in humans include chloracne, fatigue, disturbances in the peripheral nervous system, and liver toxicity.