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October 21, 2002
Volume 80, Number 42
CENEAR 80 42 p. 40
ISSN 0009-2347


METAL MATTERS
As EPA looks at regulation of metals, it faces issues not found with organic chemicals

CHERYL HOGUE

Most of the chemicals that the Environmental Protection Agency regulates are organic compounds, ranging from pesticides to dioxins. But increasingly, EPA is faced with deciding whether and how to regulate metals and metal-containing compounds.

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FORGING AHEAD EPA is grappling with the complexities of toxic metals in the environment.
Assessing the dangers from metals and metal compounds to human health and the environment involves concerns that are different from those associated with organic chemicals. For instance, organic chemicals generally are broken down over time in the environment; metals, because they are elements, are not. Also, metals can switch from one oxidation state to another depending on environmental conditions. Hexavalent chromium, for example, classified as a known human carcinogen, when released to the air can reduce to trivalent chromium, which is not carcinogenic, while Cr3+ can transform into Cr6+ in soils.

Each form of a metal--various inorganic compounds, different species of the same element, or organometallic compounds--has its own set of environmental characteristics such as toxicity and bioavailability. And some metals, such as copper, chromium, and zinc, are essential nutrients at low levels but are toxic at higher concentrations.

Because of these issues, EPA is developing a series of scientific documents to guide its offices as they assess metals in the environment for possible regulation. The documents will steer the agency as it sets national standards for air quality and water quality, maximum contaminant levels for metals in drinking water, and soil cleanup levels for Superfund sites. The agency also will use them as it reviews pesticide registrations.

The guidance documents also will help EPA as it examines large groups of metals and metallic compounds to rank them for such actions as listing on the Toxics Release Inventory. The agency currently requires TRI reports for any compound containing a metal if the metal itself "can cause, or reasonably be anticipated to cause, a toxic effect." EPA has delisted specific metal compounds from TRI when data show the compound is less toxic than the metal it contains and that the metal is not released in its elemental form in the environment.

EPA Deputy Administrator Linda J. Fisher says assessment of hazards from metals applies to each of the environmental media that EPA regulates--water, air, and soil. This issue is "probably going to tie the agency up in knots" in the future if EPA does not soon address it, she says.

The first of the three anticipated EPA documents is an action plan. EPA released a draft action plan in June and plans to issue a final version in coming months. The draft plan proposes five broad issues that EPA should consider when it assesses metals: chemical speciation, bioavailability, bioaccumulation, persistence, and toxicity.

EPA's Science Advisory Board (SAB) recently reviewed the draft action plan and agreed that four of those issues--chemical speciation, bioavailabilty, bioaccumulation, and toxicity--are key to assessing the hazards posed by metals. Understanding chemical speciation of the most toxic forms of metals "is clearly important," but so is understanding the environmental chemistry that regulates the rate of metal species formation, their stability, and their prevalence, according to a report from SAB.

William P. Wood, the scientist who chairs EPA's internal technical panel on the metals assessment action plan, says the concept of toxicity "has limited meaning" for metals because these chemicals' toxicity depends on their bioavailability. This characteristic refers to the extent to which a substance is or can be taken up by an organism through a particular type of exposure, such as through the skin or by ingestion.

The SAB report says: "Bioavailability exerts a major influence on bioaccumulation and toxicity. In aqueous systems, factors such as salinity, complexation with humics, and methylation by bacteria are examples of processes that will influence uptake/toxicity by organisms."

SAB rejects one of the top five concerns listed in EPA's draft action plan: environmental persistence of a metal.

"Persistence should not be considered a key issue" in EPA's efforts on metals, SAB writes. While the concept of persistence is helpful when the environmental fate of organic compounds is involved, it has "little meaning" for metals, which do not degrade. "Persistence is inherently part of the environmental chemistry of metals," it explains. Instead, the report continues, persistence is better addressed through environmental chemistry factors such as stability of a chemical species.

SAB also recommends that EPA's strategy on metals include the combined effects of different metals, including nutrition and toxicological considerations. The agency should consider possible interactions--additive, synergistic, or antagonistic effects--from mixtures of metals, the report says. It notes that Superfund sites frequently contain mixtures of metallic elements.

AFTER EPA COMPLETES its action plan, the agency will prepare guidance to its offices on assessing the hazards and risks of metals. Vanessa T. Vu, SAB's staff director, says this second framework document will lay out the scientific principles the agency is to consider when it scrutinizes metals but will not include instructions for assessments. According to Wood, this framework will be a blueprint for future EPA guidance documents specific for its program offices on water, air, waste, or toxic compounds and pesticides. EPA's goal is to have the framework finished by the end of 2003.

Finally, the agency will put together specific assessment guidelines that differentiate between different metals and metal compounds for each program office. The target date for completion of this project is spring 2004.

The draft action plan is available at http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=51736.



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