CDC Perspective on Emerging Chemical Contaminants in Drinking Water
Michael A. McGeehin and Deborah M. Moll
The availability of safe drinking water is a basic cornerstone of public health. Increased access to safe water and better sanitation practices have done more to increase life span and enhance human health than any other advancements in the field of medicine (Last, 1998). Throughout the twentieth century the U.S. Public Health Service (PHS) has taken a leading role in providing safe drinking water for U.S. citizens. PHS advanced the scientific knowledge needed to implement and promote appropriate technologies by sponsoring the development of breakpoint chlorination, linking the levels of coliform bacteria in water with waterborne disease rates, and demonstrating the diarrheal and parasitic burden experienced by rural households without access to safe water or sanitary facilities (HHS, 1997). Following passage of the Safe Drinking Water Act in 1974, the U.S. Environmental Protection Agency (EPA) assumed primary responsibility in the federal effort to protect the public's drinking water supply. However, the Centers for Disease Control and Prevention (CDC), the nation's premier public health agency, remains vitally interested in the safety of the drinking water supply in the United States.
The Subcommittee on Drinking Water and Health of the U.S. Department of Health and Human Services' Environmental Health Policy Committee recently identified several areas of research and public service involving drinking water that require increased federal attention (CDC, 1997). Areas of research included (1) quantify g the association between various contaminants and adverse health effects, (2) developing methodologies and arrangements to better use existing data, (3) developing laboratory and field techniques for measuring hazards, and (4) expanding federal capacity to study waterborne outbreaks. Areas of public service that the committee identified included (1) expanding the breadth of federal responses to outbreaks and disease surveillance and (2) developing techniques for educating the public and the water industry about public health issues related to drinking water.
The CDC is a public health research and support institution that provides technical assistance to state public health agencies on all issues concerning public
health, including those associated with contaminated drinking water. In areas where high exposure to contaminated drinking water is suspected, the CDC may undertake epidemiological studies in collaboration with the state to document the health effects of exposure. As part of linking chemical exposure to health effects, the CDC develops tests of human exposure to a wide variety of toxicants. Working in conjunction with EPA, the CDC also coordinates surveillance and documents occurrence and characteristics of waterborne disease outbreaks (WBDOs).
The CDC collaborates with state health departments and other federal agencies in investigating unusual occurrences of morbidity or mortality. The scope of these investigations includes those outbreaks that may have an environmental etiology. In recent years state health agencies have asked the CDC's National Center for Environmental Health (NCEH) to help them investigate a number of water-related issues. In a study of the association between elevated sulfate levels and infant diarrhea in South Dakota, the NCEH found a low frequency of tap water use when high sulfate levels were measured in tap water provided by public water systems (CDC, 1998). During an investigation in Delaware of health effects associated with elevated copper levels in drinking water, the NCEH found that gastrointestinal symptoms consistent with copper toxicity were not higher in people living in households with high copper levels than in those in control households (CDC, 1998). The NCEH is conducting ongoing studies to assess the impact that prenatal exposure to disinfection byproducts (DBPs) has on the risk for neural tube defects among newborns and studies to determine the impact of agricultural practices on the level of environmental contamination in ground- and surface waters and the human health risks associated with such contamination. The NCEH is also investigating the possible association between high nitrate levels and adverse reproductive outcomes.
Using epidemiological methods combined with the most advanced monitoring techniques will enable the CDC to provide human health data necessary to EPA and other enforcement agencies for decision making on emerging chemical contaminants.
The NCEH's Environmental Health Sciences Laboratory (EHSL) develops tests of human exposure to toxicants (biomonitoring); when combined with epidemiological studies, these tests provide vital information about how exposures contribute to serious human disease. The EHSL has developed methods for measuring more than 200 toxicants in human biological samples and is nationally and internationally recognized for its application of these methods to assessing exposure in major environmental health studies. The laboratory has developed analytical methods for 10 metals, including lead, cadmium, mercury,
uranium, thorium, and chromium; 144 dioxins and furans; environmental tobacco smoke; 20 polychlorinated biphenyls; 42 pesticides; 32 volatile organic compounds (VOCs); and 19 polycyclic aromatic hydrocarbons.
The EHSL monitors biomarkers of disease or exposure in long-term, population-based prospective and retrospective studies such as the National Health and Nutrition Examination Survey (NHANES) and the National Human Exposure Assessment Survey. These biomonitoring results provide surveillance and susceptibility data, set background levels of various diseases and environmental exposures, and determine reference ranges for specific analytes. The EHSL also analyzes biomarkers in conjunction with epidemiological studies of drinking water contamination. For example, the EHSL will monitor blood levels of methyl tert-butyl ether (MTBE) for a proposed study; it analyzes trihalomethane (THM) levels in blood for populations exposed to DBPs in water through various exposure routes; and it determined blood levels of chlorinated pesticides, metals, and VOCs and urine levels of organophosphate pesticides, mercury, and cadmium for a recent study of the effects of severe flooding on levels of chemical contaminants in water in Honduras following Hurricane Mitch.
Since 1971, the CDC and EPA have maintained a collaborative surveillance system for collecting and periodically reporting data related to the occurrences and causes of WBDOs (CDC, 1996a). The surveillance system collects data on outbreaks associated with drinking water and those associated with recreational water, including outbreaks of infectious diseases and illness from chemical toxicity. Surveillance is a critical component in the effort to reduce the impact of waterborne illness on public health; it is necessary to document the occurrence of illness, to investigate potential etiological agents, to plan and evaluate the effects of interventions, and to ensure appropriate care for people in need of services.
Public health officials need surveillance methods that allow them to rapidly detect WBDOs and to initiate preventive measures (e.g., advisories to boil water). Surveillance data that identify the types of water systems, their deficiencies, and the etiological agents associated with outbreaks could be used to evaluate the adequacy of current technologies for providing safe drinking and recreational water. In addition, such data are being used to establish research priorities and assist in improving water quality regulations (CDC, 1996a).
State, territorial, and local public health departments are primarily responsible for detecting and investigating WBDOs and for voluntarily reporting them to the CDC. However, because of variations in how they collect data, an improved national collaborative network of state programs is necessary to coordinate the development and application of surveillance methods and uniform methods of conducting epidemiological studies of WBDOs.
Public Health Approach
A public health approach to contaminated drinking water is an essential partner to the regulatory approach taken by EPA. Properly conducted epidemiological, occurrence, and exposure studies provide important data for the decision-making process and for investigators attempting to determine the cause, extent, and impact of WBDOs. The CDC is in a unique position to lead the public health response to emerging contaminants in drinking water. It has the crucial epidemiological and laboratory resources that are necessary to address these issues, as well as an established relationship with state health departments through its role as the federal agency tasked with supporting and interacting with state public health agencies.
Addressing the human health impacts of emerging contaminants in drinking water requires a partnership among federal agencies and state health departments. Once these partners reach a consensus on what substances should be considered emerging contaminants of concern (COC), they should develop and implement a multistate surveillance system to identify and monitor people who are exposed to water likely to contain COC. Such surveillance is a state-based function that requires federal coordination to ensure uniformity and consistency among states. As a complement to public health actions where areas of high exposure are identified, investigators could conduct epidemiological studies to document the health effects of exposure to the COC, and laboratories should develop biomarkers of human exposure to link levels of human exposure to health effects. These activities, both of which are currently being carried out by the NCEH for a wide array of chemical exposures, complement the information gained from experimental research on the effects of exposure of animals to chemical toxicants.
Current NCEH Activities With Emerging Contaminants
The NCEH expects to be increasingly active in responding to health issues related to drinking water contaminated with chemicals such as MTBE and DBPs. Both MTBE and DBPs have the potential to affect large segments of the U.S. population over the next decade. Another high-profile emerging issue is the threat posed by exposure to endocrine disrupters in drinking water. Research is needed on the levels of exposure to and the health effects of chemicals with known endocrine-disrupting activity in order to identify the classes of endocrine disrupters that pose the greatest threat to human health. Extensive use of pesticides and fertilizers may adversely affect drinking water quality throughout large areas of the country; in addition, increased nutrient loading from concentrated animal feed operations (CAFOs) may increase the level of nitrates and phosphates in groundwater. These facilities may also pollute water sources that are resistant to strains of bacteria, antibiotics, hormones, and other pharmaceuticals. Along the U.S.-Mexico border, inadequate infrastructure continues to be the major impediment limiting the availability of potable drinking water. Finally, natural disasters may create both acute and long-term breaches to public water supplies, which pose severe public health problems.
MTBE, so of its use as a gasoline additive, is becoming more ubiquitous as an environmental contaminant. Because the potential health effects of chemical exposure to this contaminant are uncertain, research should be conducted on exposure levels and associated health outcomes. The NCEH is addressing the feasibility of conducting a study to assess MTBE exposure and health effects from contaminated groundwater in two eastern states. The planned study will be a case control epidemiological survey of water use and health effects, and the EHLS will measure blood levels of MTBE to evaluate exposure, and DNA adducts and red blood cell apoptosis to evaluate health effects. The NCEH will also collect water samples at the time of the survey to determine the association between the concentration of MTBE in water and exposure levels and any health effects in the study population.
Research on DBPs and their link to cancer has been conducted over the past two decades. However, recent findings linking DBPs with adverse reproductive outcomes, recent improvements in exposure indices, and the development of human biomarkers for these compounds should yield important results in the near future that will assist regulatory agencies in establishing health-based standards. The NCEH is currently conducting a multisite study of the association of neural tube defects in newborns with prenatal THM exposure. The NCEH is also investigating the incidence of bladder cancer in pet dogs exposed to varying levels of DBPs in their drinking water and is planning a study comparing THM exposure from drinking, showering, and bathing.
While the regulatory focus for halogenated organic DBPs is currently on THMs and haloacetic acids, approximately 50 percent of the chlorination DBPs measured as total organic halide have yet to be identified, and only about 25 percent of the total mutagenicity of chlorinated drinking water has been identified (Richardson, 1998). Further identification of DBPs is essential, along with assessments of the health effects and exposure levels of known and newly identified DBPs.
In the past few years there has been increasing media interest in the possibility that chemicals in our environment can disrupt normal hormonal function in humans. Implicated chemicals include industrial chemicals such as PCBs, as well as a wide variety of pesticides, including herbicides, fungicides, nematocides, and insecticides. Potential health effects of exposure to endocrine-disrupting chemicals include adverse reproductive outcomes, birth defects, breast cancer, developmental disabilities, endometriosis, thyroid problems, and testicular cancer.
The NCEH has been at the forefront in investigating some of the health outcomes associated with exposure to endocrine disrupters. For example, it
examined the relationship between exposure to chemicals that can act like estrogen and the risk of breast cancer in a case control study of 63 Alaskan native women with breast cancer and an age-matched control group. However, there are still major areas where lack of data is limiting our ability to determine the health risks posed by endocrine disrupters. For example, there is inadequate information regarding levels of prenatal exposure to these substances and the correlation between such exposure and subsequent adverse health outcomes among children.
The NCEH has more than a dozen ongoing research projects involving endocrine disrupters. They vary from large analyses of datasets (including NHANES data) to laboratory/epidemiology studies that use biomarkers to define exposure. It is vitally important that we define the hazards that may be associated with endocrine disrupters as soon as possible and devise appropriate measures to prevent exposure and adverse health effects.
Extensive use of pesticides and fertilizers may adversely affect drinking water quality throughout large geographic areas of the country. These pesticides include organophosphates, carbamates, and pyrethroids. Some preliminary data indicate that intensive agricultural practices may increase the levels of nitrate and phosphates in groundwater. High levels of nitrate leaching into drinking water supplies increase users' risk of methemoglobinemia (excess of methemoglobin in the blood). The NCEH recently investigated a cluster of spontaneous abortions in Indiana suspected of being associated with high nitrate levels in wellwater (CDC, 1996b). The report on the results of this investigation indicated a need for further assessment of a possible relationship between ingesting nitrate-contaminated water and spontaneous abortion. The NCEH is developing a study to investigate this issue.
As mentioned above, CAFOs may pollute water sources with antibiotics, hormones, and other pharmaceuticals as well as resistant strains of bacteria. The intensive use of antibiotics is an integral feature of large-scale animal agriculture. Over 40 percent of the antibiotics sold in the United States are used in agriculture. There is growing evidence that animal use of antimicrobials is tied to the evolution of multiple drug resistance in foodborne disease agents and the loss of efficacy of drugs important in human medicine (Levy, 1998). Limited research has been conducted into the environmental fate of antibiotics given to animals, such as those in the fluoroquinolone group, which includes synthetic antibiotics that are licensed for use in poultry and are excreted in the feces largely unmetabolized. The NCEH is conducting a chemical and microbial assessment of ground- and surface waters near large-scale swine-feeding operations and is investigating the environmental effects of large poultry farming operations (CDC, 1998).
The NCEH is also collaborating with the U.S. Geological Survey to develop a cross-sectional survey to determine whether water that serves as a drinking water source contains pharmaceuticals. The NCEH is providing biomarkers of human exposure and epidemiological expertise to complement water analysis capabilities provided by the USGS, which is developing a comprehensive analytical method to determine the composition and concentrations of pharmaceuticals in U.S. waters.
Along the U.S.-Mexico border and in some other areas of the country, inadequate or nonexistent infrastructure continues to be the major impediment limiting the availability of potable drinking water. Some water supplies are contaminated with industrial and household wastes, pesticides, and biologic contaminants. One project currently planned will study the impact of groundwater contamination with arsenic, nitrate, and total and fecal coliforms on the health of the affected population, and another will investigate whether the prevalence of contaminated water affects the prevalence of contaminated produce that is shipped to other parts of the United States.
Natural disasters, such as floods and hurricanes, may cause both acute and long-term breaches to public water supplies that pose severe public health problems. In 1998 the NCEH responded to Hurricane Georges in Puerto Rice and Hurricane Mitch in Honduras and Nicaragua. NCEH investigators found that a clean water supply was the greatest public health need of the affected populations. Short-term lack of adequate clean water was often related to road obstruction mused by hurricane debris, which prevented supply vehicles from reaching towns and shelters. Breaches of water treatment and distribution systems may lead to long-term shortages of potable water in many communities in these localities. In Honduras pesticide contamination was of concern because of severe flood damage to pesticide factories and storage facilities. A severe hurricane or other natural disaster striking the United States may cause similar damage to local water supplies and create subsequent public health problems.
Ensuring the safety of our nation's drinking water supply into the next century is critically important to maintaining public health. The CDC is vitally interested in collaborating with state health departments, EPA, and other interested parties in maintaining the public's confidence in its drinking water.
An appropriate public health response to the possible emergence of new chemical threats to drinking water or the reemergence of old problems will require the cooperation of experts in a broad range of disciplines. A national public health approach, including enhanced surveillance, epidemiological studies, the development of biomarkers, and the maintenance of close working relationships with state health departments, is a central component of a comprehensive assessment of emerging chemical threats to drinking water. We at the CDC will continue to work with state and federal colleagues to address public health issues related to drinking water and to provide human health data to regulatory agencies for consideration during decision-making processes.
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