Metallic Refinement

The EnvironmentalProtection Agency has decades of experience conducting riskassessments on contaminants in the environment. These evaluations,which factor in exposure and hazards, such as toxicity, guideEPA in deciding whether and how to regulate chemicals releasedto air, water, and land. Most of the pollutants that the agencyassesses are organic compounds. But EPA realizes that its riskassessment isn't a one-size-fits-all process and that itneeds adjustment for some substances.

Metals compose a major category of chemicals that need risk assessmentstailored to their unique properties. For instance, while most organicpollutants break down over time, metals naturally persist. Somemetals are essential nutrients at low levels. And the oxidationstate of a metal, which can change depending on environmental conditions,can determine its toxicity. Because of these and other concerns,EPA is in the midst of a multiyear effort that ultimately willprovide better guidance to regulators as they determine whetherto restrict releases of metals to the environment.

"We'repleased that EPA has recognized how different metals are,"said Jane C. Luxton, a partner with the Washington, D.C., officeof the law firm King & Spalding. "It's a large topicto grapple with and get right," said Luxton, who representsthe North American Metals Council. And EPA's work is groundbreaking."This really hasn't been done before," she said.

THIS EPA EFFORT is a legacy of former deputy administratorLinda J. Fisher. Fisher was second-in-command at the agency from2001 until she left in July 2003 shortly after her boss, ChristineTodd Whitman, resigned from the helm of EPA.

Fisher wanted EPA to have guidance on metals that would work forrisk assessments done in every agency program-for water, air,and soil contamination as well as pesticide registration. She saidissues around the unique properties of metals likely would "tiethe agency up in knots" if not addressed (C&EN,Oct. 21, 2002, page 40).

Though Fisher is gone from the agency, the work she fostered continues.In July, EPA released a draft document that zeroes in on the specialattributes and behaviors of metals and metallic compounds thataffect risk assessments.

The draft document doesn't set out a prescriptive method forassessing human health and ecological risks from metals, explainedRandall S. Wentsel, chief of the ecological and health processesbranch in EPA's Officeof Water. Instead, it provides "tools" so the variousprograms throughout the agency can apply a consistent set of scientificprinciples for assessing the hazards and risks of metals. EPA isparticularly interested in 19 metals suspected of having the potentialto harm human health and the environment.

There are three major kinds of assessments that can be used formetals, according to the draft. One type ranks or categorizes chemicalsbased on their potential to cause risk. This prioritization guidesEPA in focusing first on the contaminants posing the greatest risk.A second type of assessment would be done when EPA is setting anational environmental standard, such as a discharge limit fora water pollutant. A third category of EPA assessments addressesrisks associated with specific sites, such as a hazardous wastecleanup.

The document applies to metals and inorganic metal compounds, thoughit does discuss the transformation in the environment of metalsinto organometallic compounds, such as the methylation of mercury,Wentsel said. The document does not cover synthetic organometallicsubstances, he added.

When finalized, the document would affect air and water qualitystandards, the allowable level of contaminants in drinking water,cleanup of contaminated land, and registration of pesticides.

On July 27 and 28, EPA held what was billed as a "peer consultation"workshop for technical experts to examine and suggest changes tothe draft document. The reviewers included academics and scientistsfrom state, U.S., and Canadian regulatory agencies, as well asexperts from environmental consulting firms.

THOSE PARTICIPATING in the workshop endorsed theeffort that EPA is making to tease out the special characteristicsof metals for risk assessment purposes. But they also indicatedthat EPA has more work to do before making the document final.

Workshop participant Patrick Doyle, senior science adviser in EnvironmentCanada's Existing Substances Branch, expressed concernthat the draft document overemphasizes the differences betweenmetals and organic chemicals. He mentioned also that the draftin places oversimplifies the behavior of organic compounds whilestressing the complexity of how metals act. Charles A. Menzie,president of Menzie-Cura & Associates, a Severna Park, Md.,company that specializes in risk assessment, agreed that the document'smany comparisons between metals and organic compounds are not useful.

At the same time, the document doesn't elucidate the particularcharacteristics of metals enough, other workshop participants said.The mathematical and computer models that EPA uses in risk assessmentwere developed for organic chemicals, pointed out Keith G. Sappington,an ecotoxicologist with EPA'sNational Center for Environmental Assessment. These modelsdo not take into account factors such as speciation of metals,which can affect, for example, whether a metal dissolves in water,Sappington said. Solubility of a metal can affect its toxicity,said Herman J. Gibb, vice president of Sciences International,a risk assessment consulting company in Alexandria, Va.

Representatives for a number of mining and metal industry groups,while not official participants at the workshop, offered theirviews on EPA's draft document to the reviewers at the conference.They had a number of problems with the document.

For example, the draft recommends that risk assessors assume that100% of a metal in the environment is bioavailable-able tobe absorbed and available for use in organisms-unless thereare data available supporting a different percentage. Robert L.Dwyer, associate director of the environment program at the NewYork City-based InternationalCopper Association, took issue with this argument.

"Metals do not exist in 100% bioavailable forms in the naturalenvironment," he told the workshop participants. "Thereare always dissolved or particulate binding ligands, substancesthat compete at various organism uptake pathways, or other factorsthat reduce the intake of metals by organisms."

The idea that 100% of a metal in the environment is bioavailable"was a good assumption 20 years ago," Dwyer continued.But today, "for almost all situations, there exist enoughphysical-chemical data to make reasonable accurate predictionsof bioavailability," or such information can be readily collected,he said.

Andrew Green of the InternationalLead Zinc Research Organization also called for EPA to stateclearly in the document that its strategy for persistent, bioaccumulative,and toxic (PBT) chemicals is not applicable to metals when theagency ranks chemicals by hazard. Members of Green's ResearchTriangle Park, N.C., research organization include most of themajor producers of lead, zinc, and silver, as well as many endusers of these metals.

Luxton, a representative for the North American Metals Council,agreed with Green and said the agency's PBT strategy is notappropriate for metals. Persistence, while helpful for examiningorganic chemicals that generally break down over time, is not avalid metric for metallic elements. "Of course, metals arepersistent," she said. Bioaccumulation is also a problematicmeasurement because many organisms have adaptive responses foreliminating or sequestering metals and because some metals areessential nutrients, she added.

SOME ACTIONS by EPA have segregated metals fromother substances when conducting hazard rankings, Luxton said,but others have not. For instance, EPA'sOffice of Solid Waste has ranked a number of substances forwaste minimization efforts. It used the agency's PBT strategyto rank organic compounds and, separately, identified some metals-includinglead, cadmium, and mercury-as priorities for waste minimization.But the agency's research efforts on the Chesapeake Bay stilluse the PBT strategy on metals as well as other chemicals to identifypriorities, Luxton and Green said.

Also at the workshop, Kevin Bromberg, assistant chief counsel forenvironmental policy advocacy at the U.S.Small Business Administration, faulted the draft report foruse of the term "toxic metal." Noting that some of thesame metallic elements that are considered environmental hazardsare often essential for health in many organisms, Bromberg saidEPA should eliminate the word "toxic" because "dosemakes the poison."

EPA expects to revise the document in light of the informationand comments from the workshop, then submit it to the agency'sScience Advisory Board forformal review later this year. The advisory board, made up of scientistsfrom outside the agency, is currently seeking nominations for reviewers.

The agency plans to make the document final in 2005.

EPA's draft "Framework for Metals Assessment" canbe downloaded at http://cfpub2.epa.gov/ncea/cfm/recordisplay.cfm?deid=56752.