Herbicides in Drinking Water: A Challenge for Risk Communication

Chapter 24

Herbicides in Drinking Water: A Challenge for Risk

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Communication

David B. Baker Water Quality Laboratory, Heidelberg College, Tiffin, OH 44883

To narrow gaps between public perceptions of human health risks posed by herbicides in drinking water and scientific perspectives of those samerisks,it is necessary to build public confidence in the risk assessment process. To build such confidence, it is essential that the public understand the basic principles ofriskassessment and trust those agencies charged with conductingriskassessments. Because of the wealth of information available on the toxicities of triazine herbicides, and their exposure patterns through drinking water, these compounds provide useful examples for public education regarding (1)riskassessment, (2) the setting of drinking water standards and their interpretation, and (3) the features of the safe drinking water act that permit ongoing consideration of new toxicological and exposure information. This paper illustrates several approaches that have been useful in improving public understanding of risk assessment and of the human healthrisksassociated with the occurrence of herbicides in drinking water. Is our water safe to drink? Concerned citizens frequently direct that question to water supply officials, agricultural and environmental agencies, industries, and environmental research and monitoring organizations. One of their concerns is the occurrence of herbicides in drinking water. It has been known for many years that herbicide residues occur in midwestern drinking water supplies (1-3 ). Until recently this topic has received only limited attention because herbicide concentrations in drinking water derived from both groundwater and surface water sources are generally well below existing federal drinking water standards or lifetime health advisories. The types of water supplies where standards are sometimes exceeded are well known, and programs are being developed and implemented to address those problems. However, in recent years, ©1998 American Chemical Society In Triazine Herbicides: Risk Assessment; Ballantine, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

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304 some environmental advocacy organizations have contended that herbicides pose significant human health threats, even at concentrations well below drinking water standards. These claims have been advanced most strongly by the Environmental Working Group, an environmental advocacy organization based in Washington, D.C. They have recently published two reports on this topic — Tap Water Blues: Herbicides in Drinking Water (4) and Weed Killers by the Glass (5). Both of these reports contain useful information regarding the concentrations of herbicides in midwestern water supplies. However, in my view, both reports are highly biased in their presentation of information and, in addition, contain much misinformation (6,7). Using the services of a public relations firm, they release their reports through media events that are often effective in generating newspaper, television, and radio headlines and in alarming some of those members of the public concerned with environmental and public health issues. Unfortunately, media representatives often lack the background to recognize the biases and misinformation in the reports and, although they often seek and present alternative viewpoints, end up giving the advocacy group undeserved credibility (5). Thus, it is not surprising that the issue of herbicides in drinking water has become a focus of considerable public discussion, as well as an important public policy and political issue. Scientists often lament the wide gap between public perceptions of risk and scientific assessments of risk. Frequently, the public perceives certain risks to be far greater than supported by scientific risk assessment. Since public policies and expenditures generally track public perceptions of risks, rather than scientific assessments of risk, considerable potential exists for inefficient use of resources available for advancing environmental protection and public health (9 ). To narrow the gap between public perceptions of risk and scientific risk assessments, it will be necessary to build public confidence in the risk assessment process. Such confidence requires that the public understand the procedures, benefits, and limitations of risk assessment, and trust those entities charged with conducting risk assessments. Building such understanding should be a major objective of risk communication. The triazine herbicides, and especially atrazine, provide an excellent opportunity for public education about risk assessment because much is known regarding both the toxicity of these compounds and their concentrations in drinking water. The triazine herbicides are currently undergoing Special Review by the EPA in association with possible excess cancer risks associated with their occurrence(70). The knowledge base for triazine risk assessment includes many "state of the science" studies that have recently been completed in connection with the Special Review (77). The challenge in risk communication is to accurately reflect the processes of risk assessment and the data which support current assessments of health risks. It also becomes necessary to counter misinformation regarding herbicide health effects that is communicated by some advocacy groups. In this paper, I will illustrate the approach to risk communication and risk assessment education that I use in our laboratory's environmental extension program.

In Triazine Herbicides: Risk Assessment; Ballantine, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

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The Concentration Makes the Poison To answer the question "Is our water safe to drink?" scientists use the procedures of risk assessment. This procedure involves comparing two major sets of factors that affect risks ~ the concentrations of particular contaminants and the toxicities of those contaminants (Figure 1). Even for herbicides with relatively high toxicity, based on laboratory animal studies, if the concentrations are low enough, no significant adverse health effects will occur. Conversely, even for herbicides with very low toxicities, if the concentrations are high enough, adverse health impacts will occur. Thus, it is always necessary to compare concentrations and toxicities in making risk assessments. The concentrations of herbicides in drinking water are usually measured and reported in micrograms per liter (pg/L). One pg/L is the same as one part per billion (ppb). If a person was to drink two liters of water per day containing a pesticide at a concentration of 1 pg/L for 365 days per year for 70 years, that person would consume a total of 51.1 mg of pesticide over the 70-year period. This amount of pesticide is equivalent in weight and size to about 14% of one aspirin tablet. Thus, for atrazine, the question is whether or not atrazine is sufficiently toxic that consumption of an amount equivalent to less than an aspirin tablet over a lifetime poses significant human health risks. Normally, to assess the risks of herbicides in drinking water, it is not necessary to directly evaluate the toxicological literature. Instead, we can compare drinking water concentrations with federal drinking water standards for the herbicides (Figure 1). The U . S. Environmental Protection Agency (EPA) has been charged with evaluating the toxicological literature and setting drinking water standards such that consumption of drinking water containing herbicides at concentrations equal to or less than their drinking water standards should not adversely impact human health. Since the adequacy of current drinking water standards has been questioned, the public needs to be familiar with the methods used by the EPA in setting drinking water standards. How Drinking Water Standards Are Set As part of the registration process, either to bring a new pesticide onto the market or to maintain registration of an existing pesticide, the EPA requires that a battery of toxicological tests be completed. The EPA specifies the testing protocols that are to be used. The tests look at both acute effects, which are associated with short term exposures at relatively high concentrations, and chronic effects, which are associated with long-term exposures to relatively low concentrations. Just as in the development of new medicines, toxicological testing for pesticides starts with testing on laboratory animals, such as mice, rats, rabbits, and dogs, and on various bacterial or cell cultures. In contrast with medicines, where clinical trials on human subjects generally follow the animal testing, toxicological testing for pesticides stops with the animal testing. The toxicity of a pesticide to humans is then predicted based on the toxicity of that pesticide to animals. Because of the uncertainty in extrapolating animal test results to humans, safety factors are



306 incorporated into estimates of human pesticide doses that should pose no significant human health risks. Virtually all toxicity testing incorporates dose-response testing (Figure 2). In such testing, the relationships between the sizes of pesticide doses contained within food and adverse health impacts are investigated. A control group of animals receives food lacking any of the pesticide, while other groups of animals receive food with increasing concentrations of the pesticide (Figure 2). Doses are reported in mg pesticide per kg body weight of the test animal per day. This method of reporting doses facilitates comparisons of doses among animals of differing sizes, such as mice, rats, and dogs, and even extrapolation to humans. The high doses are chosen such that obvious adverse health effects are apparent. Two particular doses are important for the setting of drinking water standards. These are the lowest observed adverse effect level (LOAEL), and the next lower dose, the no observed adverse effect level (NOAEL). Somewhere between these two doses lies a threshold dose, a dose at which the onset of adverse effects occurs. Often there is a five- to ten-fold difference between the L O A E L and N O A E L doses. Because of the high costs associated with such testing, no attempts are made to zero in on the threshold value. Instead, the NOAEL is generally used as the starting point for setting drinking water standards. A wide variety of "adverse health effects" are examined in the various toxicological studies (72). These include determination of lethal doses, dermal/ocular effects, growth rates, organ weights, blood chemistry, multigenerational reproductive studies, developmental effects, mutagenicity, and carcinogenicity. As a first step in setting drinking water standards, the EPA's Office of Drinking Water decides which adverse effect appears to pose the greatest threat to human health. Subsequent standards are set in two different ways, depending on whether the greatest threat is associated with carcinogenic effects (cancer causing effects) or non-carcinogenic effects. Standards Based on Non-Carcinogenic Effects. If non-carcinogenic effects are thought to pose the greatest threat, then the EPA identifies the N O A E L for the most sensitive animal species and adverse effect, and that N O A E L becomes the starting point for the incorporation of a variety of safety factors that lead to the drinking water standard. This procedure is illustrated in Figure 3 for chronic effects from atrazine. A 100-fold safety factor is incorporated into the drinking water standard for virtually all pesticides. This factor includes a 10-fold safety factor based on the uncertainty in extrapolating from one animal species to another, and a second 10-fold safety factor to allow for variable sensitivities among individuals of the human population. Because the triazine herbicides are classified as Class C carcinogens (possible human carcinogens), the EPA's Office of Drinking Water incorporates an additional 10-fold safety factor into the drinking water standard. To allow for alternate pathways of pesticide entrance into humans, such as via food, an additional 5-fold safety factor is added. These separate safety factors yield a combined 5,000-fold safety factor for drinking water. The drinking water dose deemed safe for chronic exposure to humans is therefore 5,000 times smaller than the dose which has no observed adverse effect


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Risk Management


Health Risk Characterization

Figure 1. Basic components of risk assessment and management for herbicides in drinking water.

threshold

no observed adverse effect level (NOAEL)

&

1 jf

Γ

lowest observed adverse effect level (LOAEL)

Herbicides in Drinking Water: A Challenge for Risk Communication

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