Transport of organic environmental contaminants to animal products

A large number of chemical contaminants potentially may be present in agricultural environments, leading to exposure of animals and potential residues in animal products. The contamination may be either widespread, as a result of aerial transport of industrial emissions, or localized, as a result of accidental emissions and spills, improper waste disposal, contaminants in useful products, and areas of past use of products now banned. The halogenated hydrocarbons, including the polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), polychlorinated biphenyls (PCBs), and persistent organochlorine insecticides remaining from past use, are the contaminants of most concern. Depending on the degree and pattern of chlorine substitution, these compounds are resistant to degradation and tend to accumulate in the fat of animals and their products. Other classes of environmental contaminants as exemplified by the PAHs, phthalate esters, acid phenolics, and nitrosamines also may occur widely in the environment. These compounds are unlikely to be transported to animal products because the compounds are water-soluble or can be metabolized to water-soluble products, which are excreted in the urine and thus do not bioaccumulate in products such as milk and meat. The points of entry of environmental contaminants into agricultural environments usually are plants and soils. Lipophilic compounds such as the halogenated hydrocarbons are not taken up and translocated by plants. Contamination of plants is mainly a surface phenomenon resulting from aerial deposition of emissions or deposition of compounds volatilized from the surface of contaminated soil. Thus, fibrous roughages used primarily in feeding cattle and other ruminants will be the most important pathway of animal exposure and transport to human foods. The second pathway of animal exposure is by ingestion of contaminated soil while grazing or when confined to unpaved facilities. As in the case of feed sources, cattle is the species most vulnerable to exposure by the soil ingestion pathway under most commercial management systems, but poultry and swine are more vulnerable in those infrequent situations in which these species have access to contaminated soil.