The Water Quality and Health Council is an independent,
multidisciplinary group sponsored by the Chlorine Chemistry Council. Its mission is to promote science based practices and policies to enhance water quality and health by advising industry, health professionals, policy makers and the public.

Seeking Safe Drinking Water

Susan W. Putnam
Jonathan Baert Wiener
Edited by:
John D. Graham and Jonathan Baert Wiener

Cambridge, Massachusetts
London, England

Americans have grown to expect a safe drinking water supply, but achieving safety is not simple. Aquatic pathogens, toxic chemicals, heavy metals, pesticides: the threats to our drinking water are myriad. As the list of chemical and biological risks expands, so do public concerns.

One of the most vexing issues in the effort to provide a safe water supply is that of risk tradeoffs. Efforts to reduce one risk in drinking water may introduce either--intentionally or unintentionally--a different risk to the population using the water supply. A primary example of this type of risk tradeoff involves the chlorination of drinking water. People have come to fear that chlorine in their drinking water may cause cancer. But chemical disinfectants like chlorine are used to combat serious waterborne microbial diseases. If we stopped adding these chemicals to drinking water in an effort to reduce the risk of long term cancer, would we thereby increase the risk of waterborne microbial disease? Would we just trade one form of risk for another? This dilemma is at the core of modern drinking water policy (Craun 1993; EPA 1994a, 1994b).

The quandary of chlorination in drinking water is largely an issue of risk substitution. In general, the target population remains the same: everyone must drink water to survive. The disease outcomes posed by the target and countervailing risks, however, are very different. The concern about chemical disinfectants such as chlorine focuses on their chronic effects, most notably cancer. The health effects from waterborne pathogens, by contrast, involve acute diarrhea and gastrointestinal disease. (Because carcinogens and microbial disease may affect different sensitive subpopulations--in particular, waterborne disease can be especially devastating to the elderly and those with immune system deficiencies such as AIDS--issues of risk transfer and transformation are also present here.) Whereas cancer is now one of the major causes of death in the United States (accounting for about one of every four deaths each year), acute microbial diseases used to be among the primary killers in this country (causing one in every five deaths in 1900) but have since been largely controlled (CDC 1991). Still, microbial diseases remain the cause of millions of deaths worldwide, especially in poorer countries where clean water systems have not been constructed (World Bank 1992), and are again increasing in the United States in part because of AIDS. The incidence of acute disease from waterborne microbes is well documented; the relationship between chemical disinfection and cancer is far less certain.

In this chapter we explore the countervailing risks of water borne microorganisms and the major chemical disinfectants used to control them. We focus on the health effects associated with these risks and examine some of the tradeoffs that must be considered in trying to compare and weigh these risks in the policy arena. Although the scientific data do not often provide certainty, it is essential for policymakers to consider the kinds of tradeoffs involved if they are to make sensible policy decisions for the protection of public health.

The Imperative of Safe Drinking Water

Safe drinking water has long been a key public health and environmental issue. Recognition of the importance of water quality to health dates hack to ancient times: Sanskrit writings advocated heating and filtering of water as early as 2000 B.C. (EPA 1986). Throughout history, water was known to be an essential element for survival and was hailed as the "scarce elixir of life" (Carpenter 1991).

With the advent of germ theory and greater understanding of the role of water in spreading infectious disease in the nineteenth century (NRC 1977), attention increasingly focused on the protection and purification of the drinking water supply. In the nineteenth century health minded American communities began to separate drinking water, delivered to users from reservoirs and wells, from household and industrial wastes, discharged by users into sewage water systems. (Today many people in developing countries still do not have separate drinking water and sewer systems.) As early as the 1890s, U.S. municipal water companies began to establish filtration systems to remove bacterial microorganisms from the water supply. Chemical disinfection followed in the next decade with the introduction of chlorination technology in water treatment plants (EPA 1986).

The first health standard for drinking water was established in 1914 by the US Public Health Service to protect against acute bacterial diseases. Standards have since been added to include water source protection as well as regulation of radioactivity and a host of chemicals (Walker 1989), With the congressional enactment of the Safe Drinking Water Act in 1974, "drinkable" water joined "fishable and swimmable" water--the latter delineated by the Clean Water Act of 1972--on the national agenda. The goal of the drinking water legislation was to protect the nation's groundwater system from contamination by organic and inorganic chemicals, radionuclides, and microorganisms. The Act was amended in 1986 to reinforce the federal government's commitment to protecting the water supply from toxic contaminants both in the distribution system and at the source.

Despite the scientific and technological advances made in water treatment processes over the last century, concern persists about drinking water contamination. Few areas of the country are without some form of current water quality problems or apprehension about safe sources for the future. Potentially toxic substances are continually being detected in both surface and groundwater supplies, in residential water pipes, and in agricultural runoff, underground storage and septic tank leakage, and toxic waste site leachate. Most recent concern has focused on toxic substances such as lead and chlorine. But microbes have not vanished: in 1993, a protozoan called cryptosporidium infiltrated public water systems, causing 400,000 cases of diarrhea and numerous deaths in Milwaukee. Residents of Washington, D.C. were forced to boil all their water for several days (EPA 1994b; Terry 1993).

Today Americans consume over three and a half billion gallons of treated water every day. There are nearly 250,000 public water supply systems in the United States, serving everything from the smallest towns to major metropolitan centers (AWWA 1984). Ninety percent of the population receives its water through these community water systems, with the rest using private wells or other individual sources. The Environmental Protection Agency (EPA) ranks drinking water pollution as one of the top four environmental threats to health (Carpenter 1991). At the state level, drinking water contamination ranked first among twenty seven environmental health concerns of state public health officials in 1987 (Galbraith 1989), and the issue has also been included on the platforms of several recent major gubernatorial campaigns (Carpenter 1991). Internationally, where over a billion people lack clean drinking water and almost two billion lack sewage systems, waterborne microbial disease presents perhaps the world's single largest environmental health risk, afflicting more than a billion people and killing millions each year (World Bank 1992).

Worried by reports of chemical and biological hazards in the water supply, consumers are increasingly lured to the promise of "pure spring" water for their drinking water needs. Much of the tremendous growth in the bottled water and home filtration industries over the last two decades may be due to consumer anxiety about possible contaminants in the water supply.

The Risks of Chlorination

Chlorine is an element widely used in modern chemistry be cause it readily reacts with so many substances. The vast majority of public water systems in this country use chlorine to disinfect their drinking water. But the widespread application of chlorine as a drinking water disinfecting agent has raised concerns about the risk of human exposure to chlorine and its by-products. Past experience with chlorine based compounds such as DDT, PCBs, and CFCs, which were later found to pose risks to health and the environment, have encouraged some environmental groups to call for a total ban on all uses of chlorine, rather than waiting to study each use of chlorine and its potential alternatives (Amato 1993).

Chlorine is toxic not just to microbes but to humans as well. Chlorine gas escaping at the treatment plant can cause acute health effects in workers, since the chemical is highly irritant to the eyes, nasal passages, and respiratory system. Inhalation of the gas can prove fatal at concentrations as low as 0.1% by volume (1,000 ppm) (AWWA 1984). At the minute amounts used in drinking water, however, the acute toxicity of chlorine is quite low. It is rather the potential long term risk of cancer from chronic exposure to moderate amounts of chlorine that is the major concern with the chemical in the drinking water supply. Much of the cancer risk from chlorine stems from the class of complex chloroorganic compounds, known as trihalomethanes (THMs), that are formed as one of the major by products of the chlorination process. Trihalomethanes are formed when chlorine is added to water containing organic materials, such as the humic and fulvic acids emitted from decomposing plant and animal materials in the water supply (Rook 1974; Bellar et al. 1974). Although THMs occur with chlorination of both surface and ground waters, their concentration is much greater with surface water because of its higher levels of naturally occurring organic materials.

Of particular concern among the THMs is chloroform, the most prevalent and thoroughly studied member of the THM group. Ingestion of chloroform has been associated with an increased incidence of cancerous tumors in multiple laboratory animal bioassays. These tumors occurred at several sites, most notably in the kidney and liver, as well as across several different species and strains of animals (National Cancer Institute 1976; Roe et al. 1979) (see Table 7.1). Although many of the animal studies focused on chloroform administered in corn oil or toothpaste, others also found increased evidence of tumors when the chloroform was administered in drinking water (Jorgenson et al. 1985).

Table 7.1 Summary of animal bioassay data for chloroform



Route of



NCI 1976

Osborne-Mendal rats

Oral gavage, corn oil

M: 0/90/180
F: 0/100/200

Stat. sig.
increase in male kidney tumors

NCI 1976

B6C3F1 mice

Oral gavage, corn oil

M: 0/138/277
F: 0/238/477

Stat. sig.
increase in male
and female liver tumors

Palmer et al. 1979

Sprague-Dawley rats

Oral gavage, toothpaste


No. stat. sig.
increase in tumors

Roe et al.

Multiple-strains mice

Oral gavage toothpaste, arachis oil


Stat. sig.
increase in male kidney tumors

Heywood et al. 1978

Beagle dogs

Oral capsules, toothpaste


No stat. sig.
increase in tumors

Jorgenson et al. 1985

Male Osborne-Mendal rats

Oral, drinking water


Stat. sig.
increase in male kidney tumors

Jorgenson et al. 1985

Female B6C3F1 mice

Oral, drinking water


No stat. sig.
increase in tumors

Source: US Environmental Protection Agency. 1985. Health Assessment Document for Chloroform, Washington. D.C.

  1. Dose level(s) in milligrams of dose per kilogram of body weight per day mg/kg/day). M and F denote sex of test animal.
  2. "Stat. sig." = statistically significant.

Using the mouse liver tumor data (NCI 1976), the EPA originally estimated an upper-bound cancer risk--that is, a worst-case analysis--that indicated that the lifetime risk from exposure to chloroform could reach as high as one cancer per 2,500 members of the population exposed (EPA 1977). On the basis of this analysis, the agency began to regulate chloroform and other THMs in 1979, setting a "maximum contaminant level" of 0.10 milligrams per liter (or 100 parts per billion) for total trihalomethanes in the drinking water supply. The incremental lifetime cancer risk from ingesting chloroform in household water at this level has been estimated to be about two in 100,000 (Maxwell et al. 1991). In other words, if 200 million people drank household water at EPA's maximum chloroform level for their entire lives, about 4,000 would die of chloroform induced cancer.

Chloroform is not the only by-product formed in the chlorination process that has been subjected to animal toxicity testing. Other major trihalomethanes, as well as trichloroacetic acid, dichloroacetic acid, various haloacetonitriles, and chlorophenols have been reported to show carcinogenicity, mutagenicity, or other toxic properties (Cotruvo and Regelski 1989; Simmon and Tardiff 1978; Herren-Freund et al. 1987; Bull 1982). For other by-products, however, such as various chlorinated acids, alcohols, aldehydes, and ketones, there is limited information on their potential toxic effects (Cotruvo and Regelski 1989). There is also a plethora of nonvolatile byproducts formed in the chlorination process for which there are limited data on toxicity and other effects (NAS 1987). Identifying and testing these substances has been a relatively slow process, and much work is continuing in this area (Bull 1982).

In humans, chlorinated water has been associated with an increased risk of bladder, colon, and rectal cancer in multiple epidemiological studies (Crump and Guess 1982; Williamson 1981). These studies, in general, have found the risks of bladder, colon, and rectal cancer associated with drinking chlorinated water to be about 1.1 to 2.0 times higher than the risk for drinking the same quantity of unchlorinated water (Crump and Guess 1982). The aggregate cancer risk from drinking chlorinated water over a lifetime, however, remains modest.

One of the studies finding a significant positive association, the large case control work by Cantor and colleagues (1987), suggests that lifetime consumption of chlorinated water in creases the risk of bladder cancer by a factor of about two. The study compared those residing for more than forty years in communities where drinking water was chlorinated with those who drink unchlorinated water. This association may result in about 1,000 to 3,000 excess bladder cancers per year in the United States and could explain as much as 25 to 30 percent of the occurrence of bladder cancer in adults residing in chlorinated communities (Cantor et al. 1987).

As with many epidemiological studies, however, there are numerous problems with the studies of chlorinated water. Multiple uncontrolled confounding factors (for example, smoking, diet, occupation, lifestyle) could also explain part or all of the observed cancer rates. Inexactness in measuring the chlorine level, water consumption, population migration, and other variables detracts from the definitiveness of the estimated association. Also, because much of the nation's water is chlorinated, the number of individuals who can serve as study controls those who drink unchlorinated water-is limited. Taken together, however, these studies have been interpreted as suggesting a significant association of bladder, colon, and rectal cancer with chlorinated drinking water (EPA 1985). This risk is highest in drinking water contaminated with organic material, but, given the myriad by-products formed in the chlorination process, any of a number of substances or combination of substances could be involved.

Additional health effects from chlorine have also been revealed in human studies. Of particular interest is a suggested relationship between consumption of chlorinated water and an increase in the total serum cholesterol levels of populations suffering from a diet deficient in calcium. While the increase in cholesterol levels was found to be small, it suggests that consumption of chlorinated water could be a possible risk factor for cardiovascular disease as well as for cancer (Wones et al. 1989; Zeighami et al. 1990).

Ingestion is not the only route of exposure to these substances in drinking water. There may also be adverse effects from inhalation or dermal exposure during showering, bathing, or swimming. It has been postulated, for example, that exposure to volatile chemicals in drinking water through inhalation may be as large or even larger than exposure from ingestion alone (Maxwell et al. 1991; McKone 1987).

As with most suspected carcinogens, the adverse health effects associated with chlorine by-products are largely chronic, cumulative, and may involve a long latency period before they appear. The health risks are estimated assuming a constant exposure of many years or often many decades. It may also be several decades before any adverse effects, such as excess cases of cancer, become apparent.

The Risk of Not Chlorinating: Microbial Disease

Our drinking water is living. It is composed of one third green fine moss, one third polliwogs, and one third embryo mosquitoes. (George W. B. Evans, quoted in Carpenter 1991)

When this account was written one hundred and fifty years ago, much of the nation's water supply was teeming with various forms of aquatic organisms. Waterborne diseases, led by cholera, typhoid, and dysentery, were a top health problem in this country in 1900 (CDC 1991). They remain leading concerns abroad; the World Bank (1992) rated drinking water atop its list of preventable environmental hazards worldwide. For example, since 1991 the largest cholera epidemic in recent history has infected over 800,000 people from Peru to Mexico (Brooke 1991; Glass et al. 1992; Food Chemical News 1993).

With the introduction of filtration and disinfection systems during the last century, the risk of disease from drinking water in wealthier countries has been substantially reduced. Despite the enormous progress that has been made, however, there are still significant numbers of disease cases resulting from contaminated drinking water in the United States. Health risks from aquatic pathogens range from mild gastrointestinal distress to systemic disease and, in severe cases, death (Fowle and Kopfler 1986).

From 1971 to 1988, there were nearly 137,000 cases of waterborne disease--or an average of 7,600 cases per year--reported in this country (Levine and Craun 1990). It is suspected that there were numerous undocumented cases as well, because many cases of gastrointestinal illness are not recognized as part of a pattern of waterborne disease. It has been estimated that only half of waterborne disease outbreaks in community water systems and about one third of those in noncommunity systems are detected, investigated, and reported (Craun 1986). Microbes in tap water may be responsible for as much as one in three cases of gastrointestinal illness in the United States (Carpenter 1991). Rates of waterborne illness as high as 900,000 cases and 900 deaths per year have been estimated by the Natural Resources Defense Council (Lee 1993).

Waterborne microorganisms encompass a broad range of pathogens, including coliform and heterotrophic bacteria, viruses, and protozoa. These organisms range in size over nearly three orders of magnitude, from extremely small viruses to relatively large protozoan cysts. They also vary greatly in the nature of their surface, living, and replication characteristics (Hoff and Akin 1986). These pathogenic microorganisms are naturally ubiquitous in lakes, streams, reservoirs, and most surface water sources. Groundwater supplies are also a subject of increasing concern, because enteric viruses and other organisms can leach into the groundwater system from the land application or burial of sewage sludge and other treatment wastes (Gerba and Haas 1988).

Although the "traditional" bacterial diseases of cholera and typhoid have largely been brought under control in this country, other microorganisms are constantly being identified and connected to waterborne illness. For example, the Legionnaires hemophile bacteria--the cause of legionnaires' disease-has recently been found in community water supplies (Stout et al. 1992), while tiny waterborne rotaviruses have been shown to be a major cause of acute gastroenteritis in infants and young children (Craun 1986).

The microbial agent most commonly identified and implicated in outbreaks of waterborne disease in recent years is the protozoan cyst Giardia lamblia (Levine and Craun 1990). Found in water as a result of deposition of fecal material from both humans and animals, Giardia is the most common pathogenic parasite in the United States. Surveys of various water supplies, for example, indicate that 26 to 43 percent of surface water is contaminated with Giardia cysts ranging in concentrations from 0.3 to 100 cysts per one hundred liters. Giardia strains are known to cause infection even at low doses, and outbreaks of disease have been associated with Giardia levels of 0.6 to 21 cysts per 100 liters (Rose, Haas, and Regli 1991).

The health risks associated with these types of microorganisms have been well established. The primary risks involve intestinal and gastroenteric diseases, such as diarrheal infections and dysentery. Giardia, for example, causes symptoms that are flu like in appearance but are usually more severe, such as diarrhea, nausea, and dehydration that can last for months in some cases. The bacteria Legionella invokes severe pneumonia-like symptoms, especially in a less resilient population such as the elderly. Other health risks are present as well, such as hepatitis A or poliomyelitis from waterborne viral infections. In most instances, the adverse effects are acute, immediate, and readily apparent. Table 7.2 provides a summary of the diseases caused by waterborne microorganisms.

While many cases of waterborne disease are short term and relatively minor, some are fatal and others drag on for months and can be chronically debilitating. The effects are particularly serious for the more vulnerable groups of the population, such as the very young, the very old, and those already weakened from other health problems. People with suppressed immune systems, such as those undergoing therapy for cancer or AIDS, for example, can be burdened with waterborne illnesses for many months, with often devastating fluid loss.

The infectious dose for waterborne microorganisms varies tremendously (see Table 7.3). The risk of infection for some of the viruses and protozoa is estimated to be 10 to 1000 times greater than for the bacterial organisms at a similar level of exposure (Rose and Gerba 1991). In general, viral and protozoan pathogens, such as Giardia, appear to be highly potent, with small numbers of the organism being capable of causing infection and disease in susceptible human hosts (Gerba and Haas 1988). Moreover, many of the protozoa and viral agents are more resistant to disinfection than pathogenic bacteria, requiring larger quantities of chlorine and other disinfecting agents to successfully reduce their numbers in the water supply (Hoff and Akin 1986).

Table 7.2 Waterborne diseases

Waterborne disease

Causative organism

Source of
organism in water




Animal or human feces

Acute diarrhea and vomiting


Salmonella typhosa (bacteria)

Human feces

Inflamed intestine, enlarged spleen, high temperature; can be fatal


Vibrio cholerae

Human feces

Diarrhea: rarely fatal


Vibrio cholerae (bacteria)

Human feces

Vomiting, severe .diarrhea, rapid dehydration, mineral loss: often fatal

Infectious hepatitis


Human feces, shellfish grown in polluted waters

Yellowed skin, enlarged liver, abdominal pain: lasts up to 4 months, seldom fatal

Amebic dysentery

Entamoeba histolytica(protozoa)

Human feces

Mild diarrhea, chronic dysentery


Giardia lamblia (protozoa)

Animal or human feces

Diarrhea, cramps, nausea and general weakness; lasts 1 week to 30 weeks, not fatal

Source: Reprinted by permission from Introduction to Water Treatment: Principles and Practices of Water Supply Operations, vol. 2, p. 284. Copyright 1984, American Water Works Association.

In addition to primary infection from direct ingestion of water laden with microbial contaminants, there is also the inherent risk with these pathogens of secondary or tertiary infection. Disease may be spread by person to person contact from infected individuals or by contamination of other materials, such as food, which can then expose previously noninfected members of the population.

Reducing Microbial Risk through Chlorine Disinfection

Chemical disinfecting agents, most commonly chlorine, have been used successfully to combat waterborne microorganisms since the early 1900s. The chlorination process proved so effective and easy to administer that by 1914 most of the drinking water supplied to cities in the United States was chlorinated in some manner (Fowle and Kopfler 1986).

Table 7.3 Probability of infection from waterborne microorganisms


Probability of infection from exposure to 1 organism

Number of organisms for 1% likelihood of infection

Salmonella (bacteria)

2.3 x 10-3


Salmonella typhi (bacteria)

3.8 x 10-5


Shigella (bacteria)

1.0 x 10-3


Vibrio cholerae classical (bacteria)

7.0 x 10-5


Poliovirus 1 (virus)

1.5 x 10-2


Entamoeba histolytica (protozoa)

2.8 x 10-1


Giardia lamblia (protozoa)

2.0 x 10-2


Source: Adapted from Joan B. Rose and Charles P. Gerba. 1991. "Use of Risk Assessment for Development of Microbial Standards." Water Science Technology, vol. 24. p. 31.

Chlorine is the most widely used disinfectant in the nation, with over half a million tons used annually by the water treatment industry (Manwaring 1985). It has become the predominant disinfecting agent for all types of potable water treatment operations in locations ranging from small rural communities to major municipal centers.

The addition of chlorine to the water supply is economical, convenient, and effective in virtually eliminating the transmission of bacterial and viral diseases from drinking water. Not only is it successful in destroying pathogenic microorganisms during the treatment process-the primary reason for using a disinfectant-but because it persists in the water distribution system, chlorine helps to prevent the regrowth of nuisance microorganisms (EPA 1981). It also is effective at reducing noxious odors and tastes in the water supply. The addition of chlorine has markedly expanded the drinkability of the nation's water resources by permitting the use of water not considered pristine and protected (Calabrese and Gilbert 1989).

The water treatment process involves a series of different steps. Some of the major steps include flocculation and coagulation (the joining of small particles of matter in the water into larger ones that can more readily be removed), sedimentation (the settling of suspended particles in the water to the bottom of basins from which they can be removed), and filtration (the filtering or straining of the water through various types of materials to remove much of the remaining suspended particles), as well as chemical disinfection. Chlorination is usually performed at several stages of the treatment process. Prechlorination may be performed in the initial stages to combat the algae and other aquatic life that may interfere with the treatment equipment and later steps. The major chlorination stage, however, occurs as the final treatment step after the completion of the other major processes, where the concentration and residual content of the chlorine can be closely monitored (AWWA 1984). In this postchlorination phase, the chemical is more effective in the filtered water, and less contact time is required for the chemical to disinfect the water supply (Clark et al. 1984-85).

Chlorination can deactivate microorganisms by a variety of mechanisms, such as damage to cell membranes, inhibition of specific enzymes, destruction of nucleic acids, and other lethal effects to vital functions (Hoff and Akin 1986). The effectiveness of the chlorination process depends upon a variety of factors, however, including chlorine concentration and contact time, water temperature, pH value, and level of turbidity (AWWA 1984). Disinfectant concentrations and contact times used by different water utilities vary widely, usually depending on the characteristics of the water being treated. Several states and advisory groups suggest minimum requirements or recommendations for these parameters, but there are no federal standards for them (Hoff and Akin 1986).

Alternatives to Chlorination?

The potential adverse health effects associated with the use of chlorine, the regulation of trihalomethanes (THMs) by the EPA, and the recent crusade to end chlorine uses (Amato 1993) have encouraged the water treatment industry to re examine the disinfection process. The use of disinfectants other than chlorine is currently being explored to find ways of controlling pathogens in drinking water without forming halogenated by-products. Several alternative disinfectants are increasingly coming into use; three of the most common are chloramines, chlorine dioxide, and ozone. While other alternatives to chlorine are under focus as well, these three have received the greatest attention because they are effective, relatively inexpensive, and easy to use (Bull 1982).

One alternative to using straight chlorine in the disinfection process is to combine the chemical with another substance, such as ammonia. The mixing of ammonia with chlorine to form chloramines not only retards THM formation, but it also slows the reaction of chlorine with soluble organics and other compounds in the water supply (Moore et al. 1980). Chloramines have the advantage of being easy to generate and feed into the existing technology used for chlorination. They also produce a disinfectant residual that persists throughout the distribution system, preventing regrowth of microbes. In addition, they help to reduce unpleasant odors and tastes apparent with chlorine disinfection. The disadvantage of this alternative, however, is that chloramines are a weaker microbicide than chlorine, and their biocidal action becomes even weaker in water with high pH levels (EPA 1981). They must be used at higher concentrations and for longer periods of contact to achieve sufficient disinfection. Because of this, chloramine may not always be as successful as chlorine as the primary disinfectant in a treatment system, especially where more resilient viral or parasitic cyst contamination is potentially present (NAS 1987).

In contrast to chloramines, chlorine dioxide is a strong microbicide whose biocidal activity is consistent over the pH range usually occurring in water treatment and thus is useful in most systems. Chlorine dioxide is also easy to generate and feed into existing systems, but care is needed to monitor the chlorine concentration in generated chlorine dioxide. Like chlorination, it also produces a residual disinfectant that can persist throughout the water distribution system (EPA 1981).

Ozone is a powerful but unstable disinfectant, already in use in about 1 percent of U.S. drinking water systems (Amato 1993). It is an excellent microbicide whose biocidal activity is not affected by the pH of the water. Because of its instability, however, ozone does not produce a disinfectant residual to re main in the system after the water leaves the treatment plant, and there is thus a danger of biological regrowth as the water passes through the distribution system (Cotruvo and Regelski 1989). Moreover, the equipment that is required to generate ozone on site is more elaborate and expensive than the technology required for either chlorine or the other two alternatives (EPA 1981; Amato 1993). The manufacturing of ozone requires a great deal of energy; rising energy costs make ozonation more expensive to use relative to the other alternatives, and high energy intensity potentially increases air and water pollution at the energy generating stations. In addition, because the ozone residual does not persist. The plant operator has no simple way to determine whether enough ozone has been added to the treatment process to destroy all the pathogens in the water (AWWA 1984).

The primary advantage to the use of chloramines, chlorine dioxide, or ozone as alternative disinfectants is that the THM concentration in drinking water is substantially reduced when these alternatives are used in place of chlorination. However, there are limited toxicological data on these chlorine alternatives. Currently there is no evidence to suggest that alternative disinfectants are any less (or more) toxic than chlorine (Bull 1982).

Although they do not produce THMs, the alternatives to chlorine do form their own organic by-products. Chlorine dioxide, for example, spawns the products chlorate and chlorite, which have been shown to cause consistent hematological (blood) effects--such as decreased red blood cell counts and glutathione levels--in multiple laboratory animals (Abdel-Rahman, Couri, and Bull; Bull 1982). In addition to similar hematological effects in rats (Abdel-Rahman, Suh, and Bull 1984), chloramines may also pose a danger for hemolytic anemia in kidney dialysis patients where the dialysate water was treated with chloramines (Kjellstrand et al. 1974; Moore and Calabrese 1980). Monochloramine has also been shown to be a weak mutagen (Moore and Calabrese 1980; Shih and Lederberg 1976). At present, little is known about the types of by-products produced by ozonation of natural organics (NAS 1987).

In general, then, less is known about the possible by-products formed by the alternative disinfectants and about their toxicological implications than is known about chloroform and the other chlorine by-products. Even though these alternatives are increasingly being used in disinfection processes, attention is only beginning to focus on their toxicology and possible health effects.

Moreover, as long as substantial quantities of background organic material are present in the source water, all chemical disinfecting agents will produce some type of by-product which, in all likelihood, will include certain chemicals that will be found to affect human biological activity (Bull 1982). Disinfectants are effective precisely because of their ability to kill or deactivate microbes and their ability to disrupt biological material (Fowle and Kopfler 1986). By their nature, they are reactive molecules capable of altering the chemical structure of organic substances present in the water supply (EPA 1981). The question then becomes whether their intended deadly effects on smaller life forms pose an unacceptable risk tradeoff for human health.

Weighing Risk vs. Risk

How do we compare the health risks of microbial disease and chemical disinfectants? The risks of microbial and chemical hazards differ substantially with respect to their outcomes, severity, certainty, timing, and distribution among population groups.

The adverse health effects of microbial disease are uncomfortable, debilitating, and potentially fatal. The diarrheal and other disabilities can last for only a few days, or they can drag on for many months. The effects occur promptly after exposure, often in a population already in a vulnerable state due to age (children and the elderly) or disability. For the most part in the United States, however, the disease effects are nonfatal and usually reversible. The issue is primarily one of morbidity, not mortality. (This is not true in developing countries, where diarrhea and dehydration due to poor water quality kill mil lions of children each year.) For example, mortality rates from enterovirus infections have been reported to range from less than 0.1 percent to 1.8 percent (Assaad and Borecka 1977). Since this may only reflect hospitalized cases, the actual mortality rate for all cases may be even lower.

Chlorine risks, on the other hand, present a very different picture. Disease occurs only after long term, cumulative exposure of often many decades. The affected population is usually middle aged or older; rarely are children afflicted with the types of cancer thought to be associated with chlorinated water. Here, as with most suspected carcinogens, the end points of interest include mortality as well as morbidity.

Microbial risks can be estimated and foreseen with a high degree of certainty. Waterborne microbes can be isolated, identified, and studied to assess their risk level, and specific effects in humans can be shown to be caused by specific organisms. There is a wealth of historical data indicating the types of diseases that result from consumption of pathogen infested water supplies. Although the dose response relationship is not clearly defined for microbial disease, it is suspected that even a modest number of microorganisms can infect much of the population--particularly the most vulnerable groups--that drinks from the contaminated water supply. With the high prevalence of pathogens naturally occurring in water sources, the likelihood of disease resulting from untreated (or poorly treated) drinking water is high.

The certainty of cancer predictions from exposure to chlorinated drinking water, on the other hand, is much lower. With chlorine, as with many chemicals, it is more difficult to isolate the health effects and to prove a causal relationship between the chemical and human cancer. The data are far from conclusive. Risk assessments of chlorine's carcinogenic potential necessitate multiple extrapolations from animal bioassays (from high dose experimental exposure to low dose actual exposure, and across species from laboratory animals to humans) and various assumptions and choices made in the mathematical modeling processes. Similarly, the epidemiological data on human exposure are plagued by multiple confounding sources of cancer (such as smoking or diet), as well as by imprecise measurements of chlorine and chloroform levels, water consumption, and other elements. There are no dose response data for humans, and the actual number of excess cancers seen in the studies is small. In addition, the data on other exposures to chlorine in drinking water, such as from swimming, showering, or washing fruits and vegetables, are very limited.

The issue remains how to weigh sensibly the target and countervailing risks. It is difficult to compare sudden, predictable episodes of diarrhea among children to latent, uncertain cases of cancer among adults. It is hard to balance an almost certain level of morbidity (and an uncertain level of mortality) in the present against a highly uncertain chance of cancer morbidity and mortality several decades in the future. Disease at hand demands action, but there is also strong support for protecting against future adverse outcomes; and the particular risk of cancer may provoke special public anxiety.

The challenge of trying to manage and reduce these diverse risks is further complicated by difficult economic, ethical, and attitudinal issues. For example, some say that a risk may be more acceptable to the public if it is naturally occurring (waterborne microorganisms) rather than technologically imposed (chemical disinfectants). But an immediate and present risk-the effects of which are readily apparent, unpleasant, and clearly consume resources in the form of medical treatment, work days lost, and so forth--may be less acceptable than the nebulous possibility of risk transpiring many decades down the road. The immediacy of disease raises the call for action--witness the recent clamor over microbial infections from fast food hamburgers. Another factor is that some types of risk appear to be particularly dreaded, regardless of what the data show about likelihood and mortality rates (Slovic 1987). Many people in this country, for example, appear to have a deep seated fear of cancer. Any time that the term "cancer" is even vaguely associated with a risk, no matter how uncertain the data may be, some people will demand that all possible steps be taken to eliminate that risk. The cancer risk associated with chlorine and its by-products triggers a demand from certain sectors of the population for removal of the chemical from public water supplies, even though such removal would increase the risk of immediate microbial disease.

An additional critical element in weighing these risks is the issue of who suffers each risk. Although risks in drinking water are borne by the same population in general--everyone must consume water to survive (though some can afford to purchase bottled water)--there is a difference in the subpopulations most vulnerable to each risk. Microbial disease would be injurious to many people, but especially to children, the frail elderly, and those with immune system deficiencies such as AIDS. The fatalities in the Milwaukee cryptosporidium outbreak in 1993 were concentrated among people with AIDS. Cancer from THMs, by contrast, is principally a threat to middle aged adults who have consumed chlorinated water over many decades. As our society ages. cancers later in life may be of greater concern to a growing share of the electorate.

Similarly, the burden of risk alleviation may not be shared equally by different subgroups of the population. Were restrictions to be placed on the amount of chlorine that could be added to water supplies, lower income and smaller communities and households, with more limited resources, would be less able to expend the necessary funds to switch to an alternative disinfection system or to purchase bottled water. These communities and households could be forced to lower the chlorine level to comply with the regulations without adequate protection against microbial disease. In addition, the solutions for the water systems of large communities may not be easily transferable to small systems. Given a choice, some communities might prefer to accept the long term risk of chlorination rather than face the immediate risk of waterborne disease; others might decide differently.

Even in communities that could implement new alternatives to chlorine, the economic burden would not be easy to bear. Meeting federal requirements from the Safe Drinking Water Act and its amendments has been estimated to cost the nation's water suppliers more than $14 billion per year (Carpenter 1991), which is ultimately passed on to the consumer or taxpayer. Converting the nation from chlorination to ozonation might add another $6 billion per year (Amato 1993). Historically, Americans have enjoyed cheaper water than in many other countries, but many families will find their water bills rising as alternatives are sought to the present treatment technology. Water is not a commodity that consumers can do without, so a rise in price would most likely cause hardship for some American families. And increased regulations and restrictions placed on chlorination systems may induce the outlays of additional funds for drinking water that would other wise have been spent for other public health activities or household purchases.

These dilemmas are not trivial in poorer communities. The devastating epidemic of cholera that began in Peru in 1991, for example, appears to have been unleashed (once cholera arrived in South America on ships from Asia) by a Peruvian government decision to stop chlorinating the urban water supply in response to fear of cancers from chlorination (Anderson 1991; Dowd 1994, p. 98). But in the absence of an affordable alternative disinfectant, the result was a dramatic in crease in the risk of microbial infection. In short order more than 1,000 people died of cholera, and 150,000 were afflicted in Peru alone (Brooke 1991; ILSI 1991; Nature 1991). The epidemic then spread up the coast to Mexico; by 1993 it had killed more than 7,000 and afflicted over 800,000 people (Anderson 1991; Glass et al. 1992; Food Chemical News 1993).

Managing the Risk

Caught between the risks of cancer and microbial disease, the federal EPA has begun negotiated rulemakings, involving industry as well as community leaders, aimed at managing the risks of disinfection without undermining the control of microbial disease (EPA 1994a, 1994b). Many communities are beginning to search for "risk superior" options for obtaining clean drinking water. At the technical level, there are currently various options available to Americans for managing the port folio of drinking water risks. Chlorination as a disinfecting treatment has long been demonstrated as an effective system for achieving a substantially microbial free drinking water supply. Many of the cases of waterborne disease that do occur every year can be attributed either to breakdowns or inadequacies in the treatment system or to areas where no disinfection has been implemented (Akin, Hoff, and Lippy 1982; NAS 1987)--though some, such as cryptosporidium, may be unaffected by chlorination (EPA 1994b).

Much of the risk associated with chlorine exposure may occur in extraordinary circumstances, not in routine situations. Most municipal drinking water supplies maintain chlorine levels such that the concentrations of chloroform in the systems range from 0.02 to 0.05 milligrams per liter (Wilson 1980), well below the standard of 0.10 milligrams per liter that the EPA has set as a safe level for ingestion of THMs. Trihalomethane levels can vary, however, particularly with seasonal or water quality changes. This is true especially in the summer months, when microorganisms grow more quickly and greater amounts of chlorine are added to the water supply to combat the increased microbial growth. In Washington, D.C., for example, THM levels can rise to 30 percent over the EPA limit, despite the $35 million water treatment plant that the city recently built (Carpenter 1991).

The concern over trihalomethane levels has spurred in creasing use of alternative disinfectants. For example, both the state of Kansas and the Metropolitan Water District of Southern California now use chloramination for the maintenance of a disinfection residual in their distribution systems. Ozone disinfection processes, widely used in Europe, have also been on the increase in the United States. Recent improvements in the reliability and efficiency of ozonation technology, coupled with its high efficacy against resistant protozoan cysts and viruses, have strengthened the desirability of this chlorine alternative (NAS 1987).

New options include modifications to the chlorination process to reduce chlorine by products. Particular attention has focused on reducing the level of organic precursors in the water to protect against trihalomethane production. Once THMs are formed, they are very difficult to remove, so the goal is to prevent their initial formation. One way to try to accomplish this is to pretreat the source water, with technologies such as granular activated carbon or other absorbents, to remove organic materials before the water enters the disinfection process. Another solution under investigation involves moving the chlorination step to a point in time after much of the organic material has been removed through the other treatment stages (AWWA 1984).

Again, however, these technological alternatives pose their own countervailing risks. For example, prechlorination of low quality water is important to maintaining the efficacy of the other disinfection stages. This step is essential in removing algae and other growth from the treatment machinery and equipment and ensuring their optimum function and efficiency (White 1978). Without this step, the effectiveness of the disinfection process in preventing microbial diseases may be severely compromised.


Nothing known to science, including the content of drinking water, is one hundred percent "safe." Zero risk is a quixotic goal, whether in poorer societies or in our highly industrialized and chemical dependent era.

But many people expect perfection--purity--in their drinking water. Not only is water an essential element of life, it is for many people an involuntary risk. Much of the population receives water from large community systems over which it has limited control. Many people believe that the government should be responsible for somehow eliminating all risks of drinking water contamination. The level of risk that society is willing to accept for drinking water may be lower than that for a more voluntary risk such as consuming alcohol or smoking cigarettes.

This case study illustrates the complex problems surrounding two different involuntary risks posed by modern drinking water: chemical versus microbial contamination. Concern over the potential carcinogenicity of chlorine and its by-products has pushed society to explore other disinfection alternatives. But would these options be worse than the process they replace? None of the chemical alternatives has both the biocidal and residual properties of chlorine, nor many of its secondary benefits. Even less is known about the toxicological properties of these alternatives and their byproducts than is understood for chlorine. And reducing chlorination without effective alternatives may unleash deadly microbial diseases. Is the fear of cancer from chlorine pushing us to an inferior or even inappropriate solution for a safe drinking water supply? If we tinker with the present disinfection system, there is a distinct possibility that we may lose effectiveness in pre venting microbial disease. One has only to look at many less developed countries to see the devastation that uncontrolled waterborne disease can cause-and recall that our country has only escaped these diseases relatively recently. It is not inconceivable that currently largely contained microbial pathogens in this country, such as the microbes that cause childhood diarrhea and cholera, could reappear with vigor if disinfection were not kept up to an adequate level.

It is interesting to observe that one era's target risk (water borne disease) is now the modern era's countervailing risk, as target attention has shifted to the cancer risks of chlorination. In part this may reflect the perceived elimination of waterborne disease, in part increasing concern about cancer, and in part the aging of our society. But waterborne enteric pathogens are still responsible for much greater levels of illness in this country than are chemicals or other contaminants in drinking water (Gerba and Haas 1988). On the other hand, even a small excess risk of cancer or other adverse health effects from disinfecting agents could eventually account for a significant amount of latent illness (NAS 1987).

Policymakers need to consider and balance all the salient risks, target and non target, in a coherent and comprehensive fashion. Given the countervailing risks of microbes and chemicals in drinking water, and the technologies available at present, there is no simple solution. The task for decision makers is to manage the portfolio of risks intelligently and comprehensively. Because people's values and preferences for avoiding different risks and their abilities to afford alternatives to the choice of risk management strategies for drinking water may need to be made at the local rather than national level. In any case, we need to weigh all of the risks of drinking water in a thoughtful, sensible manner and search for solutions that reduce overall risk.

Harvard University Press


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