Tittabawassee River Watch Editorial
Tittabawassee River Watch EditorialBack to editorial page
Richard H. Reithz, Midland Daily News Editorial
Seeking illumination in a cloudy landscape
On April 13, Dr. Linda Birnbaum, a senior scientist at the Environmental Protection Agency, gave a seminar on the topic "Dioxin and Human Health" at Swan Valley High School. Dr. Birnbaum spoke for an hour, summarizing a large body of data on this family of chemicals (dioxin congeners, usually referred to as just "dioxin"), and then answered questions from the audience.
We learned from Dr. Birnbaum that everybody in the United States (and probably in the world) has low but still measurable levels of dioxin congeners in their body. The primary source of these materials appears to be our diet. The good news is that measurements show blood levels of dioxin congeners have been coming down for the past few decades. EPA expects them to drop still further since major emission sources for dioxin congeners have been controlled. The bad news is that Dr. Birnbaum feels the "margin of safety" between background levels of dioxin congeners found in human populations and levels thought to cause adverse effects in animals is not very large.
As a practicing toxicologist, I am aware that a variety of adverse effects are known to occur in animals given high doses of one specific dioxin: 2,3,7,8 tetrachloro dibenzo-p-dioxin (TCDD). Consequently I share her concern that high doses of this particular chemical could produce adverse effects in humans. However, I believe that the hazards predicted for exposure to low levels of "dioxin-like" mixtures are seriously overestimated because of EPA's adoption of the "Toxic Equivalent" (TEQ) approach. Because this is such an integral part of EPA's hazard estimations (and MDEQ's 90 ppt Direct Contact Criteria) I think it deserves further discussion. It's complicated so please, bear with me!
THE TEQ APPROACH: TCDD is never found alone in analyses of environmental samples or human and animal tissues. It is always part of a complex mixture of environmentally and biologically persistant chemicals. TCDD is usually a minor component in this mixture of chemicals (as it is in soil samples from the Tittabawassee flood plain where the majority of the TEQ comes from compounds other than TCDD).
Within this complex mixture several chemicals have structures resembling TCDD (multiple halogenations, planar aromatic rings). Dr. Birnbaum stated that although none of these other compounds have been nearly as well studied as TCDD, some members of this group seem to induce a spectrum of biological responses similar to those seen with TCDD. She said that faced with the task of regulating a "É Broad Spectrum of Chemicals with Unknown Toxicity but with Striking SAR (structure/activity relationships)É" EPA needed to develop a simple approach. Thus the Toxic Equivalent (TEQ) methodology was born.
In this process, each dioxin congener is assumed to act through a common mechanism. Contributions from each congener are assumed to be independent and additive. The total toxicity of the mixture, expressed in terms of TCDD equivalents (TEQ), is calculated by (a) multiplying the amount of each congener by its relative potency and (b) summing the products. Risk assessments (or, in the case of MDEQ, Direct Contact Criteria) are based on the toxicity of TCDD and the calculated TEQ of the mixture.
Determining the relative potency of the congeners and TCDD is not simple. Due to the diversity of responses considered (thousands of papers have been published on different effects produced by TCDD and/or its congeners), toxic equivalent factors (TEF) cannot be calculated directly. Instead a subjective process is used. A panel of scientists uses scientific judgment to reach "consensus" as to what the TEF values might be and then assigns a single numerical value to each congener. EPA notes that " ... consensus TEF values have been described as conservative estimates of the relative potency of a chemical in order to protect humans and wildlife."
TEQ methodology is certainly simple. Scientists disagree on whether it is reliable. Dr. Steven H. Safe reported in 1998 that many of the effects of dioxin congeners were not additive in his experimental systems. Toyoshiba et al., (2004) went further: "This study suggests that relative potency factors (RPFs) used to determine TEFs are not consistent with the WHO (World Health Organization) TEF values, that the observed responses fail to support dose additivity, and that the different congeners have significant differences in dose-response shape."
Data from Table 9-3 in EPA's Dioxin Reassessment Document (2003) gives some indication of the uncertainty and bias associated with the current process for assigning TEF values. EPA notes that 20 different endpoints from subchronic animal studies were considered in assigning a TEF to 2,3,4,7,8-PCDF (a pentachloro furan analog of TCDD). Relative potencies in these studies ranged from 0.018 to 0.600 - a 33 fold range . The TEF assigned to this chemical by the panel was 0.5, or about 83% of the highest value observed in any of the 20 studies.
Thus a high degree of conservativism is introduced by the TEQ process. This conservativism is multiplied by the conservativism already present in estimating the toxicity of the reference compound TCDD. For example, when evaluating the carcinogenicity of TCDD and its congeners, cancer incidences are taken from the most sensitive species and organ systems, the tumor count is the sum of benign tumors, malignant tumors, and "precancerous lesions", a 95% upper limit on the slope factor is substituted for the "best estimate" of the slope factor, and an interspecies conversion factor which assumes humans always develop more tumors than laboratory animals is applied. When you consider the compounding effect of all this conservatism, you may understand why I do not share Dr. Birnbaum's concern about the background levels of dioxin congeners which are present in our bodies.
Dr. Richard H. Reitz is a board-certified toxicologist with a PhD in biochemistry from Northwestern University. Dr. Reitz has authored or coauthored 73 publications in Toxicology and served on the National Academy of Science and EPA's science advisory boards, including time as an invited expert for one of EPA's reassessments of dioxin. Dr. Reitz lives in Midland and has run an independent consulting firm since his retirement from Dow Chemical in 1993.
©Midland Daily News 2005
David Linhardt response to MDN 04/17/05 article
The "access" to the public forum that the MDN gives to those that believe that "dioxins are our friends" while denying that same access to others that believe that dioxins are harmful to the health of Midland and riveside residents clearly demonstrates the bias that the MDN has shown in its reporting of these issues.
While dioxin "supporters" are allowed to express their very lengthy opinions in the printed issues, critics are generally relegated to e-mail responses which are not widely read by Midland subscribers.
Before we place too much reliance on the wisdom of the MDN in scientific matters, let's all remember that the MDN was a strong supporter of the failed nuclear power plant, as was a certain large employer.
The MDN "legacy" -- supporting nuclear power, afraid to challenge "we love dioxins" and de-facto censorship.
Formerly of Midland
Back to editorial page