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Assessing Dioxin Risks: An Update

Issue July 2010

Dioxin is one of the most controversial and thoroughly studied environmental contaminants in history. Scores of scientists from government, industry and academia have studied the health effects of dioxin and tracked its fate through the environment.  Despite years of research and deliberation during which levels in the environment have steadily declined, the final chapter in the U.S. Environmental Protection Agency’s (EPA’s) voluminous risk assessment of dioxin (referred to as the dioxin reassessment) has yet to be finalized.  Current EPA Administrator Lisa P. Jackson, however, hopes to have the EPA issue a final reassessment by the end of the year.

Dioxin is not one chemical, but a complex family of compounds of similar composition.  Dioxin usually occurs as a mixture of its various family members, each of which varies greatly in levels of toxicity.  In fact, some members of the dioxin family are 10,000 times less toxic than the most toxic ones.  For the sake of simplicity, this article uses a common definition of dioxin that includes a family of 17 similar chemical compounds of concern to scientists.

Although no one manufactures dioxin intentionally, it is a trace byproduct of many manufacturing processes, especially those involving combustion.  In addition to dioxin being a by-product of manufacturing and combustion, there are also natural sources of dioxin, including forest fires and volcanoes.  Residential trash burning – or “backyard burning,” fireplaces, wood stoves and even cigarette smoking can emit dioxin.  

Human Exposure to Dioxin:  Trending in the Right Direction
Because there are natural sources of dioxin, it is likely that we have always been exposed to this substance and hence have carried trace levels of it in our blood.  Dioxin exposures and human blood levels rose with industrialization, probably peaked in the 1970s, and have declined dramatically since then, thanks to voluntary industry initiatives and government regulations.  Efforts to curtail dioxin emissions have been so successful that EPA estimates the largest single source of dioxin to the environment today is actually backyard trash burning.  In the future, dioxin blood levels are predicted to decline even further as they equilibrate with the current extremely low environmental exposure levels.

How are we as the general population currently exposed to dioxin?  Scientists tell us that 95 percent of our exposure to dioxin comes through the foods we eat, and that dioxin is most concentrated in the fat of animals.  The exposure path begins with air emissions to soils and waterways where dioxin is taken up in the food chain, via plant and aquatic life.  We become exposed to dioxin as we consume fatty meats and full-fat dairy products.

Our bodies eliminate dietary dioxin slowly, and we know from studies by the Centers for Disease Control and Prevention (CDC) that as a result, many of us have trace levels of dioxin in our blood.  Some of these levels are so low that until recently they could not be detected by the most advanced quantitative analytical methods.  The CDC is careful to point out that the presence of an environmental chemical in human blood does not mean that it will cause effects or disease.  Nevertheless, if we as consumers wish to reduce our personal dioxin levels, the Federal Interagency Working Group on Dioxin recommends that: “For most people, following Federal Dietary Guidelines will reduce fat consumption and, consequently, reduce dioxin exposure.”

Obstacles to Completing the Reassessment
Against a backdrop of reduced emissions, exposures and average blood levels of dioxin, there remains significant disagreement over pivotal scientific issues associated with assessing dioxin health risks.  EPA’s reassessment examines both cancer and non-cancer health effects.  The current draft version of the reassessment was issued in 2003, and submitted by EPA to the National Academy of Sciences (NAS) for peer review in 2004.  NAS published its review of the report in the summer of 2006, identifying critical areas of concern for EPA to address.  This past May, EPA released its draft response to the NAS review – a nearly 2000 page document – and that response has become the subject of public comment and review by a scientific panel of EPA’s Science Advisory Board (SAB).  The SAB is an independent peer-review mechanism that was established by Congress in 1978 with a broad mandate to advise EPA on technical and scientific matters.

At the heart of one controversy is the question of whether there is any level of chronic exposure to dioxin that does not lead to cancer.  EPA has taken an extremely cautious approach in this matter, maintaining that there is no safe level of dioxin intake.  The NAS, along with numerous researchers, cites evidence, however, that there is a threshold level below which no cancer effects occur.  Other nations and world-respected public health agencies, such as the World Health Organization, have adopted a threshold approach for dioxin cancer risk assessments and base safe exposure guidelines on sensitive non-cancer effects, such as behavioral and learning adverse effects.

Another point of disagreement is EPA’s methodology in setting a “reference dose,” a daily oral dose for a given duration that is likely to be without an appreciable risk of adverse effects over a lifetime.  EPA established this dose in response to the NAS recommendation to quantify non-cancer effects associated with dioxin exposure.  Although EPA opted to use human data over animal data, EPA’s quantitative approach to measuring an adverse effect level appears not to reflect the “best available science.”

For more information on EPA’s dioxin reassessment, please see:   http://cfpub.epa.gov/ncea/CFM/nceaQFind.cfm?keyword=Dioxin
 

Data for 1985 and 1995 are modeled average dioxin-TEQ for ages 20 to 70 (Lorber, 2002. A pharmacokinetic model for estimating exposure of Americans to dioxin-like compounds in the past, present, and future, Science of the Total Environment. 288. 81-95) Data for 2003-2004 are from U.S. CDC, 2005, Third National Report on Human Exposure to Environmental Chemicals, Atlanta, GA.

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