Hon. Norine E. Noonan
Assistant Administrator for Research
   and Development
U.S. Environmental Protection Agency
Ronald Reagan Building
1300 Pennsylvania Ave., NW -- Rm. 41209
Washington, DC 20004

Dear Ms. Noonan:

EPA's 1994 draft risk assessment for dioxin and related compounds was extensively reviewed and critiqued by the agency's Science Advisory Board in 1995; as a result the agency decided to prepare a revised draft assessment. In its review, the SAB noted inadequacies, large uncertainties, and gaps in the data on human exposure rates, sources, and time trends. For example, it noted that more recent data on the TEQ content of beef indicated a level 3 to 4 times lower than the value used in the draft assessment, and it noted that limited sediment core and human tissue data appeared to show that there had been substantial decreases in emissions and exposure since the 1970s.

When the SAB issued its report in 1995, it was anticipating that the agency would proceed expeditiously to finalize the risk assessment; therefore, it viewed the emissions and exposure data presented at that time as the best available, and appeared willing to accept the exposure portion of the draft assessment on that basis with only minor revisions. Since then, however, the picture has changed considerably. In the five years since the SAB review, the agency has commendably undertaken a "Dioxin Exposure Initiative" ("DEI") designed to reduce the uncertainties and fill the gaps noted by the SAB. The DEI has resulted in a plethora of new studies and published papers, which EPA has made publicly available on a special section of its website, www.epa.gov/ncea/dei.htm

These post-1995 DEI studies contain significant new data on human exposure that will have to be factored into a revised risk assessment. Among the findings that stand out are --

• The estimate of daily adult TEQ intake of dioxins and furans (PCDD/Fs), based on new data from food sampling and lake sediment cores, is roughly one-third of the estimate in the 1994 draft assessment.
  
  
• Modeling of time-trends in emissions based on the new data shows a striking increase -- in the range of 20-fold -- from roughly 1930-1940 to the late 1960s, then a sharp decrease to pre-1940 levels between roughly 1970 and 1980, and a further 75% reduction between 1987 and 1995. (And since 1995, stringent new regulations on municipal and medical waste combustors have gone into effect, which have undoubtedly resulted in further reductions, both currently and forward.) This time-trend exposure profile also appears to call into question the theories concerning dioxin emission sources contained in the 1994 draft.
  
  
• Empirical data show a roughly 50% reduction in total TEQ when foods such as beef, pork, and fish are cooked.