Dioxin risks in perspective: past, present, and future

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Abstract

The United States Environmental Protection Agency (USEPA) and other U.S. and international agencies have focused extensive efforts on the evaluation of the potential health risks of exposures to chlorinated dioxins (PCDDs), furans (PCDFs), and related dioxin-like polychlorinated biphenyls (PCBs). Extensive regulatory efforts over the past 20 years have also been made to control emissions of these compounds and thus to reduce exposures in the general population. This paper reviews the available information on temporal trends in emissions, environmental levels, intake levels through foods, and human body burdens of dioxins. This paper also provides an overview and comparison of recent hazard assessments for dioxins from U.S. and international agencies. Available data on emissions, environmental and food levels, and human body burdens of dioxins in the general population indicate a several-fold reduction in exposures and body burdens in the general population over the three decades from 1970 to 2000. U.S. and international hazard assessments concur on certain aspects, but disagree on fundamental issues including the likelihood of a threshold for carcinogenic dose–response and the degree of safety factors needed in deriving a protective exposure limit. These disagreements have significant consequences for interpreting the potential health risks of current background dioxin exposure levels. However, whatever the degree of health risk that may be associated with current background exposures, the general population is experiencing several-fold lower exposures, and, therefore, lower health risks, currently compared to 30 years ago. In light of the dramatic declines in exposure already observed, further efforts to reduce exposures through attempts to control emissions or food levels should be carefully evaluated to understand the likely efficacy of the efforts and the relative costs and benefits.

Introduction

The toxicology and human health effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds (i.e., the polychlorinated dibenzodioxins and dibenzofurans, or PCDD/Fs) have been the focus of an extraordinary amount of research over the past 30 years. Toxicologically, TCDD presents a range of interesting features, including its high degree of potency for several endpoints, its status as a confirmed animal carcinogen and teratogen, large inter-species and inter-strain variability in responses, and its receptor-based mechanism of toxicity.

However, while toxicological and epidemiological research has yielded a robust database on the toxicity of these compounds in animals and humans, significant differences persist among scientists and risk management agencies over how to assess current and future risks attributable to exposures to these compounds. For example, several authoritative agencies and scientific organizations have concluded that a daily TCDD toxic equivalency (TEQ) dose of 1–4 pg/kg-day is likely to be without adverse health effects (ECSCF, 2001; JECFA, 2001). Some scientists have even suggested that low doses of TCDD might be associated with a “hormetic” (i.e., health protective) response (Kayajanian, 2002). Conversely, the United States Environmental Protection Agency (USEPA) has suggested that TCDD doses in the range of 1 pg/kg-day, and even far lower, may pose a significant health risk (USEPA, 2000). Whether or not adverse, non-cancer effects have been observed in cohorts who have been exposed to elevated levels of dioxins will likely remain a controversial subject for many more years. Similarly, while there is compelling evidence that TCDD is a potent animal carcinogen, there is still considerable debate as to whether even very high TCDD exposures cause cancer in humans.

One issue not in dispute is the fact that numerous environmental controls have been implemented over the past 20 years in an effort to reduce TCDD releases to the environment and thus reduce exposures in the general population. Although the precise nature of the historical and current sources has not been fully elucidated, USEPA reported that TCDD emissions from quantified sources decreased, from more than 14,000 g TEQ/yr to about 3300 g TEQ/yr between 1987 and 1995 (USEPA, 2000). The relatively long elimination half-life of TCDD and many of the other PCDD/F compounds in humans (more than 7 years) is such that body burdens do not change rapidly in response to changes in intake exposure levels. However, given that two decades have passed since scientists began measuring TCDD levels in industrial sources, the environment, and human tissues (albeit not in a highly consistent and systematic manner), it might be possible to observe concurrent temporal trends that relate TCDD sources to actual human body burdens. Indeed, numerous authors have noted decreases in PCDD/F body burdens (Jackson and Michalek, 2001; Papke, 1998; Wittsiepe et al., 2000), and recent pharmacokinetic analyses have concluded that these drops in body burden levels in the general population are direct evidence that TCDD intake levels have decreased dramatically over the past 30 years (Aylward and Hays, 2002; Lorber, 2002).

In order to understand whether and how future policy decisions will influence dioxin-related health risks, it is critical to identify and assess the nature of any historical temporal trends that relate sources to exposures in the general population. Such information should lead to more informed decisions regarding the gain in health benefits associated with the cost of current and additional regulatory controls. This paper provides a review of temporal trends in quantified dioxin emissions from industrial and non-industrial sources, dioxin levels in the environment and foods, and dioxin body burdens of the general population. This review also summarizes regulatory agencies’ assessments of tolerable exposures to dioxin and provides a critical comparison of recent U.S. and international assessments of dioxin hazards. We integrate the information on exposure trends and international hazard assessments to provide perspective on past, current, and future risks of general population exposure to dioxin.

Section snippets

Dioxin exposure: temporal trends

Dioxins and furans (PCDD/Fs) are formed during incomplete combustion of organic material where chlorine is available in the feedstock or in the air supply during the combustion process, and they are also produced as trace contaminants in various industrial processes (USEPA, 2000). Focus on dioxins as contaminants began in the 1940s and 1950s in industrial settings, where chloracne was observed in worker populations involved in the manufacture of chlorinated phenoxy herbicides and exposed during

Dioxin hazard assessments

The USEPA has focused extensive resources toward assessing the potential health hazards from dioxins, beginning in the 1970s with reviews, suspensions, and cancellations of the registrations of pesticides containing TCDD as a trace contaminant. In 1985, USEPA published its first Health Assessment Document for TCDD. In 1991, USEPA commenced the Dioxin Reassessment, which was intended to incorporate the rapidly growing body of knowledge regarding TCDD into a scientifically sophisticated

Conclusions: historical, current, and future risks

Declines in body burdens of dioxins in people in the U.S. and Western Europe have been nearly as steep as is possible, given the slow elimination of these compounds. The dramatic declines in body burdens over the past 30 years are indicative of substantial reductions in exposures (perhaps on the order of 90–95% reduction in intake since the late 1960s or early 1970s). Estimates of declines in quantified emission sources (between 1987 and 1995) can account for only a portion of this dramatic

Acknowledgements

We are grateful to Brent Finley for his assistance with the manuscript. This research was funded in part by the Chlorine Chemistry Council.

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