Dioxin has been described as the most poisonous substance known to man--an environmental contaminant that can justify closing down entire towns on a long-term basis. The discovery of dioxin in the late 1980s in the effluent of kraft pulp mills resulted in a worldwide revolution in bleaching technology, which is still going on. Most pulp mills in the U.S. have gone to ECF (elemental chlorine free) bleaching methods, which virtually eliminate the generation of dioxin.
Nevertheless, the dioxin controversy is alive and well. So little is known about this family of chemicals that environmentalists, industry, and the Environmental Protection Agency (EPA) cannot agree on most basic issues: the seriousness of the threat, the validity of animal studies for predicting the effect on humans, how dioxin affects humans, or where most of it comes from and how it spreads in the environment.
To sort out what is known from what is unknown and find a basis for making good risk assessments where human health was involved, in December 1994 the EPA asked its Science Advisory Board (SAB) to evaluate the dioxin reassessment that EPA had been working on for the last several years. Chapters 1-7 in that document review the literature on health effects, Chapter 8 is on modeling and Chapter 9 is on risk assessment.
The SAB's evaluation of the EPA dioxin reassessment was written after hearing comments from the public and EPA staff on May 15-16, 1995. It was published in September 1995 under the title "Re-Evaluating Dioxin: Science Advisory Board's Review of EPA's Reassessment of Dioxin and Dioxin-Like Compounds." The 98-page review is available from the Science Advisory Board, U.S. EPA, 401 M St. SW, Washington, DC 20460, 202/260-8414, fax 260-1889. EPA's final assessment document will appear later in 1996, after being revised to take into account the SAB's comments.
I wrote away for the SAB review after seeing two contradictory reports of what the SAB had told the EPA. One report was from the Wall Street Journal, the other originally from the Sunday Gazette-Mail of an unidentified city, posted on the Internet last October.
Even after the SAB review came, and I had studied it for a few hours, I still could not account for the way those reports described it. This is partly because I did not have a copy of the EPA document that is being commented on, and the SAB review refers to it frequently. It is also partly due to the fact that the subject matter is technical and complex, and the review has no index. It is hard for either scientists or the public to generalize about the subject matter because so little is known for certain about dioxin, and every statement has to be qualified. When the SAB refers to "major deficiencies" in EPA's coverage of a certain question, it is hard for the reader to tell how serious the criticism is, since hundreds of sections or statements are evaluated, some favorably and some unfavorably. In effect, the SAB review is a 98-page Rorschach test for commentators who have strong emotional involvement with the issue but little understanding of the substance.
My impression was that the Science Advisory Board's comments were practical, impartial, specific, and judicious--about as far from a "stunning indictment" as you can get. In general, I got the impression that if the SAB were grading the EPA's efforts, it would have given the first seven chapters, on health, an A; Chapter 8, on modeling, would have gotten a C or D; and Chapter 9, on risk assessment, a D. Rather than generalize further about the SAB's review, I will simply reprint excerpts from it, and then summarize those two newspaper articles, so that readers can draw their own conclusions. Note that the Wall Street Journal article does not describe a publication, but a public hearing, held by the SAB to learn the viewpoints of the EPA staff and the public.
The Dioxin Reassessment Review Committee (DRRC) met on May 15-16, 1995 to hear briefings by EPA staff and public comments on EPA's reassessment of dioxin and dioxin-like compounds. The DRRC addressed 43 health and exposure-related issues.
The Committee commended EPA for doing a very credible and thorough job assembling, integrating, and analyzing a very large body of data on dioxin emissions, environmental levels, exposures, and body burdens. The Committee's recommendations largely addressed refinements, not substantive revisions. The committee agrees that current levels of dioxin-like compounds in the environment derive primarily from anthropogenic sources and that the air/plant/animal pathway is probably the primary way in which humans are exposed, and supports the use of Toxic Equivalencies (TEQ) for exposure analysis.
The Health Assessment draft provides a comprehensive review of the scientific literature on the biology of dioxin. The committee is confident that final versions of Chapters One through Seven will not need further review by the SAB. The Committee agrees with the use of the TEFs as a basis for developing an overall index of public health risk, but their practical application depends on the reliability of the TEFs and the availability of representative and reliable exposure data.
Chapter Eight, on modeling, reflects a great deal of effort, but several members of the Committee found the exposition of important points to be unclear. Chapter Eight is also weakened by its reliance on the standard EPA default assumption of a linear non-threshold model for carcinogenic risk.
Almost all the members of the Committee concur with EPA's judgment that dioxin, under some conditions of exposure, is likely to increase human cancer incidence.
Chapter Nine, on risk assessment, displays both strengths and weaknesses, and needs to be revised considerably to deal with the weaknesses.
[The EPA had given the SAB a long list of "charges" or issues to address.]
The background for this question is the observation in the late 1970s that dioxins are produced by the combustion of many common materials, including municipal solid waste. This led some scientists to suggest that dioxins had been with us since "the advent of fire" and that dioxins could be produced by natural combustion (for example, by forest fires). At that time, there were some suggestions that observed levels of dioxins were primarily the result of coal combustion or perhaps of wood burned in small stoves. This speculation was largely refuted by sediment core studies, both in the United States (primarily in the Great Lakes) and in Europe, which indicated that environmental dioxin levels increased significantly beginning about 1935-40 (see Volume II, pages 3-92 to 3-94). Since the advent of fire clearly predated this time, it can be concluded that dioxins were largely anthropogenic and associated with events taking place around 1935-40. What were these events? Coal combustion could be ruled out because the consumption of coal in the United States was essentially constant from the turn of the century until about 1970; this record did not agree with the sediment core data. The explanation is likely to be the introduction of chlorinated organic compounds (polyvinyl chloride and chlorinated pesticides are but two examples) in the 1935-40 time-frame....
...The Committee agrees that the available scientific evidence strongly indicates that current levels of dioxin-like compounds in the environment derive from anthropogenic sources and that the air-to-plant-to-animal pathway is most probably the primary way in which the food chain is impacted and humans are exposed. However, the environmental data are limited and EPA should not lose sight of other potentially important exposure pathways that may impact on some parts of the population, e.g., point source to water to fish, and cigarette smoking....
...The final chapter on risk assessment had, of necessity, the limitations imposed on it by its reliance on the contents of the first eight chapters. Also, having been prepared after the external peer reviews devoted to the earlier chapters, it was not as thoroughly reviewed as were the preceding chapters. It needs to be revised to reflect the changes being made in Chapters 1-8, and the areas of weakness discussed above in Sections 4.12.1 and 4.12.2. Chapter 9 would greatly benefit from an external peer review....
The EPA last fall issued a three-volume report on a four-year study, nine chapters of which dealt with dioxin. That report was reviewed by an independent panel of 39 scientists which met May 16-17 and issued "a stunning indictment of the lack of defensible science in setting EPA policy [in which they] told EPA that the agency had overstated the risks of dioxins, that its conclusions were not scientifically defensible and that it [the panel] could not endorse the report as currently drafted."
One of the panel members explains how EPA got so far off base, saying: "Regulatory agencies don't want to disprove anything; they just want to know if there is enough data to support a regulatory decision. If the data are ambiguous, a regulatory agency will simply pick and choose the data that support its position and ignore the rest."
The French Academy of Sciences has said publicly that it had looked at the same data and arrived at very different conclusions. An unnamed EPA source was quoted in Risk Policy Report, a newsletter, as saying that he didn't know who had done their evaluation, because there were no dioxin experts in France.
A bar graph, perhaps from the EPA, shows sources of dioxin in the U.S. environment. It says that the pulp and paper industry produces a tenth of one percent. Motor vehicles produce 5.5%; metals production 5.2%; incineration 0.15%; but "other and natural" sources are responsible for over 82%.
This newspaper article was put on the Internet October 18 by Dave Pettingill, under the subject "Re: Environmental risks are based on politics, not science." His introduction reads, in full: "The final final report issued by the Health and Exposures Panels of the Science Advisory Board regarding the dioxin reassessment is now available. Get your copy hot off the press by calling the SAB at 202-260-8414, or fax: 202-260-1889."
The article was based on "a summary and several key chapters of its report obtained last week" [probably in the first part of October]. A final version of the report was expected to be delivered to EPA Administrator Carol Browner within a week.
The Science Advisory Board, according to author Ken Ward, has given its general approval to the conclusions of a three-year EPA study of the toxic chemical dioxin. He lists four conclusions:
The second and third conclusions overstate the effects on humans. Compare Conclusion #2 to what the SAB wrote (p. 3-4, Executive Summary): "Vis-a-vis cancer, the Committee notes that all of the evidence available argues strongly that TCDD exerts its carcinogenic effect primarily through its effectiveness as a promoting agent stimulating cell replication in a reversible manner, and inhibiting apoptosis [programmed cell death], both mechanisms presumably mediated through the Ah receptor and associated transduction mechanisms. TCDD is thus not a complete carcinogen, and, to avoid confusion should not be designated as such in the EPA document. Almost all Members of the Committee do concur with EPA's judgment that 2,3,4,8-TCDD, under some conditions of exposure, is likely to increase human cancer incidence. The conclusion with respect to dioxin-like compounds is less firm...." The Committee would classify it as "probably carcinogenic to humans--with limited supporting information from human studies."
Conclusion #3 appears to raise an alarm that is unjustified on the basis of the SAB review, which states on p. 46, "...The Committee's consensus is that chloracne is the only lesion of note clearly established as being related to TCDD exposure.... Chloracne has also been found in people exposed to related compounds such as dibenzofurans and PCBs." It states on p. 59, concerning immunodeficiency, "Although the overall data suggest that dioxin and related compounds can produce immune effects, there are insufficient supporting data [from lab studies] to establish fully whether these effects can occur at or near two orders-of-magnitude above background levels.... Based upon the extensive experimental animal, and the very limited human, database the majority of the Committee agreed that sufficient data exist to indicate immune effects could occur in the human population from exposure to dioxin or dioxin-like agents at some dose levels.... There are not sufficient clinical data to assess human sensitivity."
In summary, I would say that the EPA's reassessment, and the SAB's review of it, are likely to be misinterpreted by other reporters as well, simply because both documents are technical, complex and hard to describe in a straightforward way, even by unbiased commentators. The reader who wants to understand what these documents really say should study them personally.
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