UNDER the Safe Drinking Water Act, the U.S. Environmental Protection Agency (EPA) is required to establish the concentrations of contaminants that are permitted in public drinking-water supplies. Specifically, Section 1412 of the act, as amended in 1986, requires EPA to publish maximum-contaminant-level goals (MCLGs) and promulgate national primary drinking-water regulations (e.g., MCLs) for contaminants in drinking water that might cause any adverse effect on human health and that are known or expected to occur in public water systems. MCLGs are not regulatory requirements but are health goals set at concentrations at which no known or expected adverse health effects occur and the margins of safety are adequate. The MCLG for copper will be used by EPA as a basis for establishing the MCL. MCLs are enforceable standards that are to be set as close as possible to the MCLG with the use of the best technology available.
Copper is an essential micronutrient (Underwood 1977; Goyer 1991). The Food and Nutrition Board (FNB) recommends dietary copper intake for adults of 1.5–3.0 milligrams (mg) per day (NRC 1989). The Institute of Medicine's (IOM) FNB is reviewing the recommendations. Acute ingestion of excess copper in drinking water can cause gastrointestinal (GI) tract disturbances and chronic ingestion can lead to liver toxicity in sensitive populations. In 1991, EPA promulgated an MCLG of 1.3 mg per liter (L) for copper in drinking water to protect against adverse GI tract effects. That value is based on a case study (Wyllie 1957) of nurses who consumed an alcoholic beverage that was contaminated with copper. In the study, a dose of 5.3 mg was found to cause GI symptoms. Based on an intake of 2 L
per day, a concentration of 2.65 mg/L was determined to be the minimal dose at which symptoms could occur. That value was ''divided by a safety factor of 2 in recognition of its essentiality" to yield the copper MCLG of 1.3 mg/L (Donohue 1997).
Several U.S. states (such as Nebraska and Delaware) have measured copper concentrations in drinking water that exceed the MCLG for copper because of the leaching of copper from plumbing. On the basis of recent data from epidemiological studies which show no adverse effects at higher levels, questions have been raised about the validity of the science on which the MCLG is based, and whether that level is appropriate. While some have argued that the level might be too conservative, others have argued that some individuals might experience adverse effects with copper levels at or below the current MCLG (Sidhu et al. 1995). A provisional drinking-water guideline of 2 mg/L was proposed for copper by the World Health Organization (WHO 1993). The basis for that value is not clear; however, an interpretation of the derivation is provided by Galal-Gorchev and Herrman (1996). The drinking-water guideline appears to originate from the provisional maximum tolerable daily intake (PMTDI) value established by the Joint FAO/WHO Expert Committee on Food Additives (JECFA). On the basis of the lack of adverse effects or copper accumulation in normal individuals with typical dietary copper intakes and the "considerable margin" between normal intakes and those with adverse effects, JECFA established a PMTDI of 0.05 mg of copper per kilogram (kg) of body weight per day (WHO 1967). Subsequent re-evaluation of that intake dose did not provide any basis for JECFA to change the recommendation. When determining the provisional guidelines for copper in drinking water, WHO assumed that a 60-kg adult would drink 2 L of water per day and that 10% of the PMTDI would come from drinking water. WHO then established the provisional drinking-water guideline of 1.5 mg/L (WHO 1991), which was later rounded to 2 mg/L (WHO 1993). Several states have also recommended exposure limits different from those proposed by EPA (summarized by the ATSDR) (ATSDR 1990). Olivares and Uauy (1996) and Fitzgerald (1998) discuss the drinking water standards for copper established by different agencies and governments.
In response to concern regarding the scientific validity of EPA's MCLG, the U.S. Congress requested that the administrator of EPA enter into a contract with the NRC to conduct a comprehensive study of the effects of copper in drinking water on human health. In response to that request, the NRC convened the Committee on Copper in Drinking Water. The committee's expertise is in the fields of toxicology, epidemiology, pathology, pharmacology, genetics, physiology, medicine, public health, exposure assessment, nutrition, chemistry, biostatistics, and risk assessment. The committee was charged to review independently the appropriateness of the
EPA MCLG of 1.3 mg/L for copper. The specific tasks of the committee were to (1) evaluate toxicology, epidemiology, and exposure data (from food and water); and (2) determine whether the critical study, end point of toxicity, and uncertainty factors used by EPA in the derivation of the MCLG for copper are appropriate. The committee was also asked to identify data gaps and make recommendations for future research. The committee was not asked to address, nor did it address, risk-management issues, and the committee did not attempt to derive an MCL for copper.
CHEMICAL AND PHYSICAL PROPERTIES
Copper is number 29 in the Periodic Table of Elements. Copper has a ground state electronic configuration of 3d104s1 and occurs in the environment in three major valence states: copper metal (Cu0), Cu(I) and Cu(II). As a member of the 3d transition metal series, copper and six other metals in the series—chromium, iron, cobalt, manganese, nickel, and zinc—constitute the bulk of essential metals in biological systems. Its transition metal properties are caused by partially filled 3d orbitals, a characteristic of all metals (with the exception of zinc) in the series. The partially filled (3d9) orbital permits Cu(II) complexes to be highly colorful. The gemstone turquoise is copper aluminum hydroxyphosphate in which Cu2+ ions provide the blue-green color. Metals in the series also have defined spatial geometries. Cu(I) usually exists in a tetrahedral arrangement, whereas Cu(II) complexes most often are square planar. Loss of the single 4s electron gives rise to Cu(I), a weak oxidant with a closed 3d10 shell and a featureless absorption spectrum. Cu(II) is the more stable form. Copper is commonly found in ores. The principal ore minerals are chalcopyrite (CuFeS2), cuprite (Cu2O), and malachite Cu2(CO3)(OH)2.
The southwestern United States is one of the world's largest producers of copper. Because minerals such as malachite are plentiful and cheap, metallic copper has been used in many industrial applications, ranging from coins and ornamental jewelry to relatively inexpensive plumbing fixtures. The metal has the properties of malleability, ductility, and electrical conductivity, which make it a preferred choice in the building industry for hot- and cold-water pipes, electrical wires, hose nozzles, and castings. Brass, an alloy of copper and zinc, has been used in cooking ware and musical instruments. Bronze, an alloy of copper with about 5–10% tin, is used in castings and marine equipment. Metallic copper is basically unstable and is subject to corrosion and leaching. It is a mistake, therefore, to consider copper metal or any of its alloys as impervious to environmental conditions. Oxygen in the air reacts slowly with brass, forming the familiar green coating on fixtures. Although copper metal tends not to leach in
neutral solutions, organic solvents, and detergents, acid solution effectively leaches traces of the metal as cupric ions. Thus, valves in soft-drink dispensing machines are subject to the corrosive effects of carbonic acid and have been shown to be a source of toxic cupric ions. Cupric ions are much less soluble in alkaline solution because of the formation of highly insoluble cupric hydroxide, Cu(OH)2. Amine compounds and ammonia, however, form complex tetracupric amines (e.g., [Cu(NH3)4]2+), which are highly soluble in both acids and bases.
SOURCES OF COPPER IN DRINKING WATER
Copper is a natural element with widespread distribution. It is present in the environment in different valence states and in different complexes. The form of copper affects its solubility; therefore, the copper forms present in water will be different from those found in food. In rivers, copper is generally adsorbed to insoluble particles or complexed with inorganic ligands (Florence et al. 1992). In drinking water, copper is generally free in solution.
Human activities can release copper into the environment, especially to the land. Mining operations, along with incineration, are the main sources of copper release. Release into water occurs from weathering of soil, industrial discharge, sewage-treatment plants, and antifouling paints (IPCS 1998). The concentrations of copper in drinking water can be greatly increased during the distribution of drinking water. Many pipes and plumbing fixtures contain copper, which can leach into the drinking water. Characteristics of the water that can increase the leaching of copper include low pH, high temperature, and reduced hardness. Electrolysis of copper from pipes can result from using household pipes to ground appliances. The length of time that the water has been sitting stagnant in the pipes can also greatly increase the concentration of copper to several milligrams per liter in the water (EPA 1994). The concentration of copper is much higher in first-draw water than in water after the tap has been flushed.
COMMITTEE'S APPROACH TO ITS CHARGE
The committee evaluated data related to hazard identification, dose response, and risk characterization-key elements of the risk-assessment process-that address the protective nature of the current MCLG. Specifically, the committee reviewed information on the health effects of excess copper exposure in humans following acute and chronic oral exposure. The current MCLG is based on GI effects following acute exposure rather than
chronic exposure. However, chronic-exposure effects in the liver have been observed in sensitive populations. Therefore, in this report, the committee addresses the effects of acute and chronic exposure to copper. The committee also evaluated information that could affect the risk assessment. That included data on the mechanism of action of copper toxicity, the health effects associated with copper deficiencies, and factors affecting the bioavailability of copper. The toxicity of copper was used as the basis for evaluating a "safe" level of copper in drinking water, but the essentiality of copper was taken into account when considering uncertainty factors.
To gather background information on relevant issues in copper toxicity, various government representatives and trade organizations gave presentations to the committee. Those presentations included representatives of the EPA, who sponsored the report, the office of Senator J. Robert Kerrey of Nebraska, and the International Copper Association. The committee also heard from scientists with relevant expertise in copper toxicity and from the IOM's Food and Nutrition Board regarding the essentiality of copper.
The committee recognized that exposure to copper can occur from multiple sources and considered the role of food in copper intake. Exposure via inhalation and dermal routes, although possible from copper in drinking water were not addressed.
STRUCTURE OF THE REPORT
The remainder of this report is organized in five chapters. Chapter 2 discusses the physiological role of copper, including its essentiality, biochemistry, and physiology. Factors affecting the bioavailability of copper are also identified. In Chapter 3, the health effects associated with copper deficiencies are presented. Disorders of copper homeostasis are described in Chapter 4. Chapter 5 addresses the health effects following acute and chronic exposure to excess copper. Particular attention is given to the health effects in sensitive populations, including individuals who are heterozygous for genetic disorders of copper homeostasis. In addition, Chapter 5 presents toxicity data from experimental animals and discusses the appropriateness of animal models for studying the underlying mechanism and toxicity of copper in humans. Chapter 6 characterizes the risks associated with acute and chronic exposure to excess copper and provides a discussion on the appropriate use of uncertainty factors and the public-health implications of the narrow margin of safety of the MCLG for copper.
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