Posted on October 10, 2011 by Michael Rodburg
Dioxins, a class of chemicals whose most notorious denizen is 2,3,7,8-terachlorodibenzodioxin, a/k/a TCDD, have been of public concern since the 1970’s, but their pathway to regulatory consensus has been a series of twists and turns, potholes and dead ends ever since. Once branded the most potent animal carcinogen ever tested, its human carcinogenicity remains controversial today. On August 29, 2011, following swiftly on the heels of a Science Advisory Board (SAB) review critical of several aspects of USEPA’s May, 2010 reanalysis of key issues related to dioxin toxicity, USEPA announced that it would delay the cancer risk portion of its final Integrated Risk Information System (IRIS) assessment and move only to a final non-cancer assessment by the end of January, 2012. The USEPA reanalysis was in response to a 2006 critique by the National Academy of Sciences (NAS).
TCDD gained notoriety in the 1970s as a contaminant in Agent Orange, the defoliant of choice used during the Vietnam War between 1962 and 1971. It is a chemical that is not commercially produced; rather it is the inadvertent by-product of numerous processes, including the manufacture of some chemicals, pulp and paper, and most combustion processes, including the burning of household waste. Because of the ubiquity of the sources from which dioxins are produced, the public may be exposed through eating beef, dairy products, pork or fish, or by living near municipal waste incineration.
USEPA’s first risk assessment of dioxins was issued in 1984; seven years later it began a reassessment in a process that is ongoing. USEPA’s 1994 draft reassessment went through SAB review in 1995, which resulted in a revised reassessment in 2000, a second SAB review in 2000-2001, a second revised draft reassessment in 2003, a NAS review in 2006, a USEPA response to NAS’ comments in 2010, and the August 26, 2011 SAB review of USEPA’s response to the NAS report. The beat goes on.
Dioxin levels in the environment, mostly in soil, sediments and biota, have been declining regularly since the early seventies as pollution control efforts have ratcheted down inadvertent production and emissions. USEPA’s reassessment impacts mostly whether and to what extent a site requires clean-up. A significantly lowered USEPA cleanup target for dioxin in soils raises the specter of reopening hundreds of sites that were remediated under current guidance to a 1 part per billion target for residential soils and a 5-20 ppb target for non-residential soils. USEPA estimates that 104 CERCLA sites may need to be re-evaluated if it adopts a substantially lowered target. Even without a cancer risk assessment, USEPA’s announcement that it would move forward with its non-cancer risk is likely to result in final guidance that sets a cleanup target for dioxin in residential soil at 72 parts per trillion, a 92.8% reduction from the current target, and a commensurate lowering for non-residential soils to .95 ppb.
USEPA’s decision to split the cancer and non-cancer assessments likely pleased no one, including USEPA Administrator Lisa Jackson, who stated in 2009 that the Agency would complete the assessment by December 2010. Environmentalists have pushed hard on USEPA for years and are likely not pleased that the cancer analysis has been again derailed by scientific critique. Many in industry have resisted lowered clean up levels for years, echoing many of the criticisms of USEPA’s cancer risk analysis by the NAS and SAB. SAB’s 84 page report issued on August 26, 2011 generally lauded USEPA’s efforts in its May, 2010 report responding to the 2006 NAS Report.
Nonetheless, SAB provided additional recommendations “to further enhance the transparency, clarity, and scientific integrity” of the Report. Two critical elements of TCDD assessment were singled out as deficiencies by SAB: “(1) nonlinear dose-response for TCDD carcinogenicity, and (2) uncertainty analysis of TCDD toxicity.” With everything else going on within and outside USEPA in the legislative, political and regulatory arena, it will be interesting to see if USEPA can or will meet its self-imposed deadline of end of January 2012 for the non-cancer risk assessment; surely the cancer assessment is not now likely to proceed with much haste.
For more information, please contact the author, Michael Rodburg.