The problem with tallying 'dioxin.' (Environmental Protection Agency's 'Estimating Exposure to Dioxin-like Compounds' report released)

Dioxins and furans are chlorinated pollutants that can form during any of several combustion and industrial activities. Together with polychlorinated biphenyls (PCBs), these related families of chemicals possess a similar ability to bind with a protein in the body -- the ah receptor -- and then enter cells to turn genes on or off inappropriately.

However, not all family members exhibit this toxic activity. For convenience, authors of EPA's new three-volume tome Estimating Exposure to Dioxin-Like Compounds (SN: 9/17/94, p.181) have tended to broadly term as "dioxin" all those compounds that do: 7 of the 75 dioxins, 10 of 135 furans, and 11 of 209 PCBs. To account for their respective potencies, each was assigned a toxic equivalency (TEQ), which relates its potential for harm to 2,3,7,8-TCDD, the most toxic of the dioxins.

Though data are very limited, the report notes, what is known suggests that annual dioxin deposition rates of about 1 nanogram TEQ per square meter of land are typical for remote regions of the United States. Rates two to six times that are common in urban areas. High-concentration "hot spots" may occur near industrial emitters of these compounds.

Overall, EPA officials estimate that incineration generates about 90 percent of the dioxin in air. Among these sources, hospital-waste incinerators may be the largest polluters. The reason, EPA suspects, is that there are so many of them (more than 6,000 nationally), they use fairly unsophisticated pollution controls, and they burn chlorine-rich wastes.

Though municipal-waste incinerators are the next biggest culprit, EPA says that new, mandated controls "should substantially reduce [their] emissions in the near future." Other sources include incinerator ash, diesel vehicles, manufacturing of chlorinated chemicals, wood burning, and papermaking.

However, notes William Farland, director of EPA's office of health and environmental assessment in Washington, D.C., researchers can today account for the source of only 10 to 50 percent of all dioxinlike compounds. He says this suggests that other major, unidentified sources may exist -- or that there may be a major overcounting of dioxins if and when they become resuspended in air and fall out again as they recycle through the environment.

The Chlorine Chemistry Council in Washington, D.C., has hired a team of 18 experts (most from universities and state health departments) to investigate the soundness of the "source" numbers and the methodology EPA used to compute its risk estimates for dioxinlike compounds. John A. Moore, president of the D.C.-based Institute for Evaluating Health Risks, is cochairing the panel. Moore, a former head of EPA's pesticides programs, notes that some 90 percent of the TEQs that EPA estimates are in the environment come from chemicals other than TCDD. He expects his panel may challenge some TEQs for these chemicals and how this metric has been applied.

For instance, Moore observes, "there is a suspicion that [EPA] may not have used the receptor theory as it should...if you've got a [less potent dioxin] sitting in that receptor, it makes it not available to TCDD," the very potent one. So assigning risk, he maintains, "is not as simple as asking how much stuff is out there and assuming receptors will be available for all." Moreover, he notes, there are data to suggest that some chemicals may neutralize the activity of TCDD.

Lynn Goldman, EPA's assistant administrator for toxic substances, agrees that "there are significant data gaps that are critical to our understanding and effective management of dioxin." As such, she said last week, "we are calling on all parties to voluntarily submit any data that can help us better understand dioxin exposure" -- from information on releases into the environment to data on dioxin concentrations in air, water, soil, food, animal feed, and human tissues.