Consensus Report

Health Risks from Dioxin and Related Compounds: Evaluation of the EPA Reassessment (2006)


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The U.S. Environmental Protection Agency (EPA) is responsible for assessing health risks from dioxin and related compounds, which are chemicals that are unintentionally released into the environment from many industrial processes. Public concern over dioxin began when it was found in Agent Orange, an herbicide widely used during the Vietnam War. Dioxin and related compounds persist in the environment and build up in the food chain. People are exposed primarily when they eat beef, pork, fish, and dairy products, although occupational or accidental exposure are typically much higher. Efforts to reduce dioxin and related compounds in the environment in recent years have resulted in an almost 90% decrease in emissions. This report evaluates EPA's latest draft "Reassessment" of the health risks from dioxin and related compounds. The report finds that EPA comprehensively reviewed the available scientific literature, but it did not adequately quantify the uncertainties associated with the risks, nor did it adequately justify the assumptions used to estimate them. Using different assumptions may result in a lower estimated cancer risk for humans exposed to low doses of dioxin and related compounds. The report also encourages EPA to further address health risks other than cancer by establishing a "reference dose" below which it anticipates no adverse effects--information valuable for establishing risks faced by populations, including workers, who may be exposed to more than the reference dose. This study was sponsored by EPA, the U.S. Department of Health and Human Services, and the U.S. Department of Agriculture.

Key Messages

  • EPA did not adequately quantify the uncertainty associated with responses at the estimated value of the POD. The estimated value of the response at a particular effective dose (like the ED01) is typically uncertain for a variety of reasons related to the challenge of conducting an epidemiological study or an animal study.
  • EPA's decision to rely solely on a default linear model lacked adequate scientific support.
  • Although EPA addressed many sources of variability and uncertainty qualitatively, the committee noted that the Reassessment would be substantially improved if its risk characterization included more quantitative approaches. Failure to characterize variability and uncertainty thoroughly can convey a false sense of precision in the conclusions of the risk assessment.
  • Even with the inherent uncertainties, the committee concludes that the toxic equivalency factor methodology provides a reasonable, scientifically justifiable, and widely accepted method to estimate the relative potency of DLCs. However, the committee noted that the Reassessment should acknowledge the need for better uncertainty analysis of the toxicity values and should provide at least some initial uncertainty analysis of overall toxicity of environmental samples.
  • The committee finds that the recent dose-response data released by the National Toxicology Program after submission of the Reassessment represent good examples of new and compelling information that warrants consideration in a revised risk assessment.
  • The committee identified three areas that require substantial improvement in describing the scientific basis for EPA's dioxin risk assessment to support a scientifically robust risk characterization:(1) Justification of approaches to dose-response modeling for cancer and noncancer end points; (2) Transparency and clarity in selection of key data sets for analysis; (3) Transparency, thoroughness, and clarity in quantitative uncertainty analysis
  • To assess the total magnitude of emissions of TCDD, other dioxins, and DLCs, EPA used a bottom-up approach that attempted to identify all emission-source categories (such as combustion, metal processing, and chemical manufacturing and processing) and then estimated the magnitude of emissions for each category. The committee concludes that a top-down approach would also provide useful information and could give rise to significantly different estimates of the historical levels of emissions of TCDD, other dioxins, and DLCs. A top-down approach would account for measured levels in humans and the environment and consider the emission sources required to account for these levels.