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Perchlorate (ClO4-) is an anion that originates as a contaminant in ground water and surface waters from the dissolution of ammonium, potassium, magnesium, or sodium salts. Because perchlorate is nonlabile kinetically (i.e., the reduction of the central chlorine atom occurs extremely slowly) and sorption or natural chemical reduction in the environment is not significant, perchlorate is exceedingly mobile in aqueous systems and can persist for many decades under typical ground and surface water conditions. Sources for the contamination include chemical fertilizer and various other chemical and industrial uses. One major source of contamination is the manufacture of ammonium perchlorate for use as the oxidizer component and primary ingredient in solid propellant for rockets, missiles, and fireworks. Perchlorate salts are also used on a large scale as a component of air bag inflators.
Perchlorate began to be discovered at various manufacturing sites and in well-water and drinking water supplies within several months following the April 1997 development of a low-level (4 ppb) detection method. There are currently 14 states with confirmed releases in ground or surface water and 44 states with confirmed perchlorate manufacturers or users based on U.S. Environmental Protection Agency (EPA) information request responses. In April of 1997, the existing toxicologic database on perchlorate was determined to be inadequate for quantitative human health risk assessment by an external peer review. A lack of data on the ecotoxicological effects was also noted. By May 1997, a testing strategy was developed based on the known mode-of-action for perchlorate toxicity, the inhibition of iodide uptake in the thyroid and subsequent perturbations of thyroid hormone homeostasis. An integrated approach to risk characterization of perchlorate was necessary because uncertainties existed in the toxicological database to adequately address the potential for perchlorate to produce human health/ecotoxicological effects at low levels in drinking water; the actual extent of the occurrence of perchlorate in ground and surface waters, which is compounded by some uncertainty in the validation of the analytical detection method; the efficacy of different treatment technologies for various water uses, such as drinking water or agricultural application; and the extent and nature of ecological impact or transport and transformation phenomena in various media. There is no National Primary Drinking Water Regulation (NPDWR) for perchlorate. Perchlorate was placed on the Office of Water (OW) Contaminant Candidate List in March 1998, with note that additional research and information are required before regulatory determinations can be made.
The development of the toxicology review document and the risk assessment activities regarding perchlorate have been a model for a full and open public process involving several EPA offices, programs, and regions, many other Federal Agencies, States, the industry and the public. Of particular note is the Interagency Perchlorate Steering Committee (IPSC), a working partnership of government agencies chartered to facilitate identification of the issues and coordinate exchange of scientific information related to potential perchlorate contamination in the environment. The IPSC includes representative from the EPA, Department of Defense , Agency for Toxic Substances and Disease Registry, National Institute for Environmental Health Sciences, Native American Tribes and various state agencies. Additional information on the background and current status of various activities regarding perchlorate can be found on the OW website: http://www.epa.gov/safewater/ccl/perchlorate/perchlo.html
The National Center for Environmental Assessment (NCEA) developed this toxicology review document to revise previous provisional oral reference dose (RfD) values for perchlorate with a more comprehensive data base. The external peer review draft (ERD) of the document presents an updated human health risk assessment of the historical data base together with results of the new health effects studies available as of November 1998 and an ecological assessment of a recent screening test battery. The human health risk assessment model utilized a mode-of-action approach that harmonizes noncancer and cancer approaches to derive a single oral risk benchmark based on precursor effects for both neurodevelopmental and thyroid neoplasia. Both of these are historically established effects of perturbations in the hypothalamic-pituitary-thyroid feedback system. The oral risk benchmark (RfD), as proposed, is protective of potential cancer because of new perchlorate data on the lack of genotoxicity and the reversibility of thyroid hyperplasia. These data allowed perchlorate to be characterized as an indirect anti-thyroid chemical according to EPA guidance thyroid follicular cell tumors. The proposed oral benchmark is 0.0009 mg/kg-day. This value reflects the inclusion of a composite uncertainty factor of 100, although some reviewers suggested that an uncertainty factor of at least 300 would be more consistent with the available data. Although presented as a point estimate, a benchmark value such as this is typically considered to be an average estimate with uncertainty ranging from 3-fold below to 3-fold above. The confidence in the derivation is designated at medium. If standard default body weight (70 kg) and water consumption (2 L/day) values were applied to the benchmark value to derive an action level, the resulting value (32 ppb) would be slightly above the current range of action levels (4 to 18 ppb) based on the previous provisional RfD values. Assessment of ecological screening data suggest that additional research is warranted. The human and ecological assessments may be used in the future to support development of a health advisory or NPDWR and cleanup decisions at hazardous waste sites. No systematic survey of perchlorate occurrence or exposure characterization has yet been made and represents a key data gap in the ability to characterize risk.