By Kelly A. Reynolds, MSPH, Ph.D.

At first glance, there appear to be two diametrically opposing views when it comes to assessing the risk of waterborne disease in the United States. There is the school of thought that tap water is acceptable and safe and the view that point-of-use (POU) treatment of tap water is necessary. In reality, the number of persons known to be sickened by drinking tap water remains the same in discussions on both sides of the issue, it is the perception of what is acceptable and safe that varies.

Risk assessment
Microbial risk assessment is the process of first determining the probability that a specific event will occur, then calculating the expected level of the adverse effects resulting from that event, over a period of time. Throughout the last decade, methods of microbial risk assessment have emerged to measure acute and chronic health-based risks of exposures to human pathogens. Mathematical models, taking into account specific pathogen characteristics and exposure levels, estimate the risk of a pathogen’s presence to an individual or population. From this information, regulatory agencies evaluate a level of acceptable risk and set standards aimed at protecting public health and the environment.

The risk assessment process includes four basic steps:

  1. Hazard identification combines known information on the nature of a pathogen and evaluates the threat of that agent (i.e., acute or chronic health effects). Human volunteer, epidemiological and clinical study data may be used to determine the threat of the pathogen.
  2. Exposure assessment estimates the intensity, frequency and duration of human exposures to the hazard. Fate, transport and survival characteristics are used to assess exposure, in addition to consumption data.
  3. Dose-response assessment accounts for the relationship between those exposed and those that experience an adverse response at varying doses.
  4. Risk characterization integrates the information from the first three steps to estimate the magnitude of the outcome. Due to data uncertainties, evaluation of the risk may include a range in parameter values.
    Risk assessment is one of three steps in risk analysis. In risk analysis, the estimation of potentially adverse outcomes is used to develop a risk management plan, where processes for control are weighed. This is the step where limitations of values, engineering, economics, legal and political issues are taken into account.1 Risk communication is the final step in risk analysis, where effective dissemination of information is considered. Here the public’s perception of risk and how the information will be received is again recognized.5

U.S. water supply statistics
Although exact numbers are unknown, water is thought to be a significant route of transmission of infectious diseases in the United States. Morris and Levin2 provide one of the highest predicted values, estimating that 7 million people become ill and more than 1,000 die each year due to waterborne microbial infections. Documented cases number far less, however it is generally recognized that reported outbreaks represent only a fraction of the true total infectious disease burden.

More than 80 inorganic and organic chemicals, radionuclides, and microbes are now regulated under the Safe Drinking Water Act, originally enacted in 1974 and amended in 1984 and 1996. The 1996 amendments require that at least five contaminants be chosen from a Candidate Contaminant List every five years for regulatory consideration. in 2002, the U.S. Environmental Protection Agency (USEPA) reported that 94 percent of the U.S. population served by community water systems received drinking water that met all applicable health-based drinking water standards through treatment and source water protection. Although critics question the accuracy of the data, efforts to improve municipal water supplies have continued to expand. Uncertainties with regard to source water quality impact, treatment plant reliability, distribution system deficiencies and monitoring and reporting violations continue to effect consumer confidence in tap water quality.3

The majority of the U.S. population is served by public drinking water systems (292 million). Although there are 54,000 community water systems, just 7 percent serve 81 percent of the population. During the survey period from 1971-2000, there were 751 documented drinking water outbreaks, causing 574,437 illnesses and 72 deaths. Figures 1 and 2 depict the number of documented outbreaks due to specific water supplies and by cause in public water supplies.

Risk perceptions
Risk is defined as the possibility of an unwanted outcome occurring. Risk means different things to different people and is perceived based on personal value systems (i.e., value of lost time at work due to illness or loss of life) and judgment related to whether or not the unwanted outcome will occur. People’s actual perception of risk is altered by numerous psychological, social and cultural influences.

Preventable risks
It’s interesting when people argue that statistically, you have a greater chance of getting struck by lightning than dying from a waterborne infection in the U.S., implying that we should be satisfied with the status quo regarding water quality. Being struck by lightning is part lack of avoidance and part act of nature. Some of the risks associated with being struck by lightning are unavoidable while others are minimized by taking appropriate precautions. Death due to documented water outbreaks totaled 72 from 1971-2000 and were indeed less frequent than lightning strikes averaging about 124 per year. Our perception of waterborne disease risk is much different than that associated with acts of nature, in part because we have the technology to prevent waterborne disease transmission and control the quality of water that we drink. In addition, remember that estimated deaths due to water are as high as 1,000 per year, thus our perceptions are also affected by known versus unknown risks.

Risk responsibility
Have you ever thought about why airline travel ranks higher on people’s list of fears than automobile travel, even though statistically traveling by plane corresponds to a lesser risk of death or injury than journeying by car (a 19-year average of 136 aviation deaths annually compared to 43,000 deaths per year by car)? One of the reasons for a greater fear of aviation (30 million Americans describe themselves as anxious flyers according to, is that we tend to fear more those things that can adversely affect us but that are the responsibility of someone else.

Historically, municipal water treatment plants have assumed the responsibility to provide drinking water in compliance with current regulatory standards. Looking at the population as a whole, treatment plants have been successful at maintaining the level of waterborne disease to a level determined acceptable, set at a risk of 1/10,000 persons per year by the USEPA. Although others estimate that the waterborne illness rate is closer to 4/1,000 persons per year,1 at some point, the cost to further reduce a particular risk to a population, no longer reflects a practical benefit. In other words, we must then accept a certain level of risk to the population even if that risk is technologically preventable. Perceiving risk on a population level does not evoke the same emotions as occur if one considers the possibility that they, or someone they care about, might be the one impacted by the hazard.6

Consumer perceptions of POU need
What do we have control over as individuals? For individual consumers of POU devices, a cost/benefit analysis of improved municipal water treatment for the entire population is not at issue. Consumers of POU devices have assumed personal control of their water quality. Many purchase POU systems primarily for taste and odor reduction while others utilize systems designed for specific contaminant removal (i.e., arsenic, chlorine, microbes, nitrates, etc.) or multistage systems for removal of a wide variety of contaminants. According to a recent Water Quality Association Survey, 41 percent of homes in the U.S. are reported to have a POU treatment device in place and 39 percent drink bottled water.4

The bottom line is that natural risks are less scary to us and we don’t like risk imposed upon us. When we have a choice about our exposure to risk, it is generally more acceptable. Risk perceptions are effected by the unknown versus known risks and although consumers may agree with experts on the level of risk they may not agree with the acceptability of that risk.

Evident by increasing sales of POU water treatment devices, consumers want better quality water at the tap than is being supplied either from municipalities or their own private water sources. Appropriate treatment at the tap allows the consumer to gauge what level of risk is acceptable to them and what precautions they deem necessary in light of some of the uncertainties associated with tap water supplies (i.e., failed treatments, lack of treatment, distribution system contamination, etc.). Individuals are also able to evaluate their own susceptibility to microbial infections. Sensitive populations (i.e., HIV patients, transplant patients, etc.), for example, require pathogen free water since an infectious disease is more likely to cause severe illness or death in these individuals. While it is important to recognize the practical limitations of further improvements of tap water quality on a population level, we must continue educating individuals on the potential risks and the choices available for POU water treatment. From this information, consumers can make decisions that are in their own best interest.

References and additional reading

  1. Haas, C. N., J.B. Rose, C.P. Gerba. 1999. Quantitative Microbial Risk Assessment. John Wiley and Sons, Inc. New York, NY.
  2. Morris and Levin. 1995. Estimating the incidence of waterborne infectious disease related to drinking water in the United States. In Reichard, et al., eds. Assessing and managing health risks from drinking water contamination: Approaches and applications. International Association of Hydrological Sciences Press. Great Britain.
  3. USEPA, “Factoids: Drinking Water And Ground Water Statistics for 2003,” Office of Ground Water and Drinking Water EPA 816-K-03-001, January 2004.
  4. Water Quality Association. 2001 Statistical & Market Data. Water Quality Association, Lisle, Illinois.
  5. Fischhoff, B., Bostrom, A., & Quadrel, M.J. (1997). Risk perception and communication. In R. Detels, J. McEwen & G. Omenn (Eds.), Oxford textbook of public health (pp. 987-1002) London: Oxford University Press.
  6. Fischhoff, B. (1994). Acceptable risk: A conceptual proposal. Risk: Health, Safety & Environment, 1, 1-28.

About the author
Dr. Kelly A. Reynolds is a research scientist at the University of Arizona with a focus on development of methods for detecting human pathogenic viruses in drinking water. She holds a master of science degree in public health (MSPH) from the University of South Florida and doctorate in microbiology from the University of Arizona. Reynolds also has been a member of the WC&P Technical Review Committee since 1997.


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