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ATSDR MOVES TO ENDORSE MORE 'DEFENSIBLE' EPA PERCHLORATE LEVEL
7 November 2005
Superfund Report
Vol. 19, No. 23
Copyright (c) 2005 Inside Washington Publishers. All Rights Reserved. Also available in print and online as part of www.InsideEPA.com.

A federal public health agency is proposing to weaken its informal risk screening level for perchlorate in public health assessments conducted at Superfund and other hazardous waste sites, a move that an agency official says will bring it in line with EPA's recently adopted risk level while also making the standard more scientifically defensible.

The Agency for Toxic Substances & Disease Registry (ATSDR) last month proposed adopting 0.0007 milligrams per kilogram of body weight per day as a chronic oral "minimum risk level" (MRL) for such screenings -- the same level the National Academy of Sciences (NAS) recommended earlier this year as "safe" and EPA subsequently adopted. That corresponds with a 24.5 parts per billion health-protective level, Chris De Rosa, ATSDR director of toxicology and environmental medicine, told Inside Washington Publishers.

Previously, ATSDR had unofficially relied on 1 ppb as a guidance level for protecting health, but "we felt it was probably overly protective," De Rosa said. Because of the lack of supporting data, the agency did not broadly disseminate that number and has applied it to just one site, the Massachusetts Military Reservation, he said.

ATSDR health assessors use MRLs as screening levels to identify a contaminant and potential health effects at sites listed on the Superfund National Priorities List (NPL), as well as other hazardous waste sites, according to the profile. An MRL is the agency's estimate of daily human exposure to a hazardous substance that, over a specified exposure duration, is unlikely to create adverse non-cancer health effects, the profile says.

The proposal is part of a new draft toxicological profile ATSDR developed for perchlorate, which summarizes in one place the existing data on toxicological and adverse health effects on the controversial contaminant. The chemical is used in rocket fuel and munitions, fireworks and explosives and has contaminated numerous drinking water supplies across the United States.

ATSDR prepares toxicological profiles for the most harmful hazardous substances commonly found at sites on Superfund's NPL, and began work on the perchlorate profile in 1998, according to the draft document and De Rosa. The agency says at least eight NPL sites have perchlorate contamination. ATSDR's toxicological profiles are generally recognized as an authoritative resource on toxicology, exposure and epidemiology among scientific experts, including the World Health Organization, he said.

ATSDR's proposal to adopt an MRL follows EPA's decision last February to accept the NAS number as its formal risk level. Based on this risk level, EPA also adopted an exposure limit -- known as a drinking water equivalent level -- for perchlorate of 24.5 parts per billion. This refers to levels of the contaminant considered safe for an adult to consume daily.

ATSDR's public health assessments differ from EPA's site-specific risk assessments in that the former focus more on qualitative health issues while EPA's assessments take a quantitative approach, De Rosa explained. ATSDR to a greater extent relies on community health concerns and health data than EPA does, he said.

If health assessors at a site find perchlorate exceeds the MRL, further evaluations are conducted to find pathways into the environment, human uptake, the site's demographics and make other considerations, De Rosa said. From these, ATSDR can place the site in one of four general categories, determining that it either poses an urgent health hazard, a health hazard, no health hazard or that it needs further site characterization. And it may recommend to EPA and state agencies measures to halt exposure to the chemical, such as switching a community to bottled water, he said. Regulators generally follow ATSDR's advice on these issues, he added.

"It is important to note that MRLs are not intended to define clean-up or action levels," the profile says. They are "intended only to serve as a screening tool to help public health professionals decide where to look more closely."

In terms of health effects, the profile says the thyroid gland is the main target organ for perchlorate toxicity in humans, noting that the chemical may be more likely to harm children and developing fetuses because thyroid hormones are needed for normal growth and development. It says, however, that two studies on babies and children did not find any evidence of abnormal thyroid function among the subjects.

The chemical partially inhibits the thyroid's ability to take up iodine -- a key building block for making thyroid hormones needed for the development of the nervous system and other bodily functions. But, De Rosa noted that iodide uptake must be inhibited at a high level before health is harmed.

But a source with the Environmental Working Group say ATSDR's backing of the NAS reference dose is "entirely unsurprising," since the federal agency typically follows levels recommended by NAS, which is generally considered a "scientific God." Environmentalists, however, contend the NAS-recommended "safe" level does not protect human health, taking issue with how the NAS panel interpreted the study on which it based its risk number.

The Defense Department and environmentalists say they will likely comment on the profile during the public comment period, which runs until Feb. 27, 2006, DOD and environmental sources say. The industry-backed Council on Water Quality says in an email response to questions that it is currently reviewing the toxicological profile. The profile is available on InsideEPA.com. See page 2 for details.

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