Dioxins persist for a long time in the environment because they do not dissolve in water and are relatively immobile in soil and sediment. When animals consume plants, feed, and water contaminated with dioxins, they accumulate in the animals’ fatty tissue. Similarly, when humans consume these animals, the dioxins then accumulate in human fatty tissue. According to EPA, because dioxins also persist in the body for years, recent significant reductions in dioxin emissions into the air are unlikely to reduce human health risks in the near term. While EPA estimates that most exposure to dioxins occurs from eating commonly consumed foods, the draft reassessment report also estimates that limited exposure to dioxins results from breathing air containing trace amounts of dioxins; inadvertently ingesting soil containing dioxins; and absorbing through the skin minute levels of dioxins present in the soil. Some people may experience higher exposure levels than the general population as a result of food contamination incidents; workplace exposures; industrial accidents; or consuming unusually high levels of fish, meat, or dairy products. When calculating human exposures, dioxins are measured in picograms—that is, trillionths (0.000000000001) of a gram. Highly sophisticated measurement techniques and technologies are required to test foods for the presence of the 29 dioxins identified as having toxic effects. The several hundred known dioxin compounds can be placed in one of three closely related families: polychlorinated dibenzo-p-dioxins (CDD), polychlorinated dibenzofurans (CDF), and polychlorinated biphenyls (PCBs). CDDs and CDFs are byproducts of combustion and some industrial processes. According to EPA, U.S. emissions of CDDs and CDFs into the environment declined by 75 percent between 1987 and 1995 primarily as a result of reductions in emissions from municipal and medical waste incinerators. Some PCBs share certain characteristics with CDDs and CDFs and therefore are identified as “dioxin-like.” PCBs were at one time manufactured for use in products such as lubricants and industrial transformers but have not been made in the United States since 1977. However, because dioxins break down so slowly, past emissions remain in the environment for years—even decades—before they diminish. Consequently, a large part of humans’ current exposure to dioxins is due to releases of dioxins that were stored in soil and sediment, and to a lesser extent in vegetation and the atmosphere. These sources are called “reservoir sources.” EPA believes that with the reduction in current emissions from combustion and incineration, these reservoir sources have taken on more significance. According to EPA, dioxins always occur in the environment and in humans as complex mixtures of individual compounds. However, the complex nature of the dioxin mixtures to which people are exposed (through foods or other sources) complicates evaluation of the health risks such mixtures might pose. Scientists therefore developed the concept of toxic equivalency factors (TEFs) to facilitate risk assessment of exposure to these mixtures. Because TCDD is the best-understood dioxin, it is used as a frame of reference for estimating the toxicity of the other dioxins, and its TEF is set at 1.0. Only 1 of the other 28 dioxins included in EPA’s reassessment has a TEF of 1.0; most of the others have TEFs of 0.1 or less, meaning that they are considered less toxic to humans than TCDD. International experts review and periodically update the TEFs based on new data. For its reassessment of dioxins, EPA used the latest revisions that were made at an expert meeting organized by the World Health Organization in 1997. Since 1991, EPA has been updating its initial 1985 report assessing the health risks of dioxins. The October 2001 draft reassessment report exceeds 3,000 pages. Part I of the draft report provides information on exposure to dioxins, including chapters on dietary intake; part II addresses health assessment methodologies and specific health effects; and part III, the Integrated Summary highlights information in parts I and II on exposure and health effects and provides a risk characterization—a statement summarizing EPA’s assessment of the health risks associated with dioxins. In the reassessment, EPA studied the risks of cancer as well as noncancer health effects, such as neurological and reproductive impairments. Founded in 1948, the World Health Organization (WHO) is a specialized agency of the United Nations, with 191 member states. WHO’s functions include giving worldwide guidance in the field of health and setting global standards for health. WHO carries out these functions through a variety of offices and programs that often collaborate with each other and with other public health entities of WHO’s member states and nongovernmental organizations. The principal contributors to the WHO reassessments of dioxin risks that are discussed in this report have been (1) the International Agency for Research on Cancer, which coordinates and conducts both epidemiological and laboratory research into the causes of cancer; (2) the WHO European Centre for Environment and Health, which coordinates comprehensive efforts, in collaboration with the International Programme on Chemical Safety, to evaluate the possible health risks of dioxins as well as methods of prevention and control of environmental exposure of the general population to these chemicals; and (3) the Joint Expert Committee on Food Additives of the United Nations’ Food and Agriculture Organization and WHO, which provides scientific evaluations as a basis for the development of food standards by the Codex Alimentarius (food code) Commission. To estimate dietary exposure to dioxins, EPA obtained and reviewed information on (1) the dioxins present in 10 types of foods with high fat content, (2) the toxicity of individual dioxins contained in these food types, and (3) the quantities of these foods that people in the United States typically eat. EPA has incorporated new studies following improvements in analytical capabilities to detect dioxins in food during the 1990s. However, in its draft reassessment report, EPA identified a number of limitations with the food data used to estimate dietary exposure that add uncertainty to the agency’s overall estimate of current average daily dietary exposure to dioxins. For example, in some cases, the studies available on the presence of dioxins in foods were not designed to estimate national averages. Further, while EPA used the accepted method for estimating the toxicity of the dioxins found in the 10 food types, EPA and others acknowledge that the method has limitations. Finally, EPA estimated the quantities of these foods consumed using U.S. Department of Agriculture (USDA) data on U.S. adults’ food consumption based on surveys made between 1989 and 1991; however, EPA believes the dietary habits of Americans have changed very little over the course of the past decade. A body of scientific research on foods in Europe, North America, and other locations indicates that the primary source of human exposure to dioxins is the dietary intake of foods, especially those containing animal fat. According to EPA’s October 2001 draft reassessment report, the average adult in the United States receives about 95 percent of his or her exposure to dioxins by eating commonly consumed foods, such as beef, pork, and poultry; fish; and dairy products. (EPA estimated small exposures to dioxins from the air and soil as well.) The 10 types of foods EPA analyzed for its reassessment are beef; pork; poultry; other meats, such as lamb and baloney; eggs; milk; dairy products, such as cheese and yogurt; freshwater fish and shellfish; marine fish and shellfish; and vegetable fat, such as corn and olive oils and margarine. These foods, only one of which is not of animal origin, are believed to be the major contributors to dietary exposure to dioxins. Even though vegetable fat products are estimated to contain low levels of dioxins, EPA included these foods in its analysis because they are high in fat and common in the American diet. EPA excluded fruits and vegetables from its analysis because data on dioxins in U.S. fruit and vegetable products, which generally contain little or no fat, are extremely limited. The existing data indicate that typically these products contain low levels of dioxins, which generally stem from residues—deposits on outer layers with little penetration to inner portions. Until recently, chemical analyses of dioxins in foods have focused primarily on two of the families of dioxins, the CDDs and CDFs, with less attention on identifying and measuring specific PCBs. The draft reassessment report includes an evaluation of PCB levels in the 10 food types. The draft report identifies estimated exposures to CDDs and CDFs together and identifies the estimated exposure to PCBs separately. This approach provides information that can inform potential regulatory approaches, among other things, because CDDs and CDFs result primarily from combustion and industrial processes, whereas PCBs, which persist in the environment from the 1970s and earlier, are no longer being manufactured. As shown in table 1, EPA estimated that the average adult in the United States is exposed daily to about 63 picograms of dioxins through dietary intake, with the highest exposure coming from beef and freshwater fish and shellfish. According to EPA, this exposure level is close to the level that has caused adverse noncancer effects in animals, such as effects on the development of reproductive systems. It is important to note that EPA’s dietary exposure estimates are averages, and they do not apply to adults with additional or unusual exposure to dioxins—for example, from diets unusually high in fat content or diets of foods high in dioxin content. To estimate any population’s dietary intake of dioxins, the specific dioxins present in the various foods must be identified and measured through chemical analyses of the foods. However, reliable estimates of the average concentrations of dioxins in specific foods nationwide have only recently begun to be available. In the past, data were available only from studies of dioxin concentrations in a specific food product or products in a specific location or a few locations, and these data were not sufficient to reliably estimate average national exposure.