By Kelly A. Reynolds, MSPH, PhD

Drinking water standards are set to a level below which the risk of exposure to toxic contaminants is acceptable to the public while being practical for the treatment facility to achieve and the monitoring/regulatory agency to evaluate and enforce. Tools from the science of quantitative toxic risk assessment are used to evaluate the magnitude of human health risks in populations exposed to single and multiple chemical hazards in drinking water. Using of a series of mathematical models, we can estimate an individual’s average daily intake of toxic chemicals in water, evaluate the potential adverse health effects and compare the risks of distinct hazards on the same quantitative scale.

Setting the Standard
Safe drinking water is recognized internationally as a basic need and right for human development, health and well being. The public generally perceives chemical contaminants as a lower priority than chemical contaminants given that low level exposures to chemicals often occur with no immediately apparent effects, while microbial contaminants- even at very low levels- produce effects such as diarrhea, nausea, or other measurable ailment, soon after exposure. The long-term versus short-term endpoints may not seem as severe, however, chronic and prolonged exposure to chemical contaminants can lead to serious and irreversible health concerns, such as cancer.

Chemical contaminants in water may originate from man-made compounds (pesticides, fertilizers) to natural elements (arsenic, fluoride). There are currently over 100,000 chemicals registered with the EPA and more than four million mixtures. Although we could theoretically assess the exposure potential, harmful concentration levels, and health risks for every known chemical, this would not be practical. Thus priority chemicals have been chosen for monitoring programs, standard development and control strategy implementation. In the U.S., priority contaminant guidelines and standards are described in the National Primary Drinking Water Regulations. Similarly, the World Health Organization’s Guidelines for Drinking Water Quality (WHO, 2006) provides selection criteria for chemical contaminants in water considering cultural, social, economic and environmental conditions of a particular country, with nearly 200 chemicals listed1.

The United States Environmental Protection Agency (EPA) regulates more than 90 drinking water contaminants under the authority of the Safe Drinking Water Act (SDWA). The Act was originally passed in 1974 and amended in 1986 and 1996. National Primary Drinking Water Regulations (NPDWRs or primary standards) are legally enforceable standards that apply only to public water systems. These standards were designed to protect public health by limiting the levels of contaminants in drinking water. Contaminants such as arsenic, uranium, and cryptosporidium, are all currently regulated.

EPA standards are set as Maximum Contaminant Levels (MCL). These are the highest levels of a contaminant allowed to be present in drinking water and are based on known or suspected doses where an adverse health effect might occur but also considers feasibility of achieving and enforcing the limit. The term, the Maximum Contaminant Level Goal (MCLG) is the level of a contaminant where health effects are known to occur. MCLs may be greater than MCLGs but the goal is for MCLs to be as close to the MCLs as possible. MCLGs often allow for a margin of safety but, unlike MCLs, they are non-enforceable public health goals.

Efforts to improve the regulations include a review and if necessary, revision, of each National Primary Drinking Water Regulation (NPDWR) at least once every six years. Any revision must either maintain or increase public health protection. In addition, the Drinking Water Contaminant Candidate List (CCL) requires EPA to identify additional conaminants that do not yet have health based standards but may require additional research or occurrence data for the development (or not) of a standard. (see

The Risk Assessment Approach
The risk assessment paradigm has been described by the National Academy of Sciences (US NAS, 1983) and includes the evaluation of the human health impact of a chemical based on 1) hazard identification where an agent is described as a known or suspected hazard to humans and under what specific circumstances the agent is a hazard; 2) dose-response assessment is the process of establishing the particular dose of a hazard produces an adverse effect and whether or not there are tolerable (threshold) levels or if even small doses over time are eventually hazardous; 3) exposure assessment determines the nature of an individual’s or population’s contact with the hazard and the extent of that contact; 4) risk characterization is the final step in a risk assessment where the risk is quantitated or qualitatively assessed under relevant exposure scenarios and the information is compiled in understandable language for risk managers to utilize.

When dealing with non-threshold (no tolerable dose) chemical exposures, guidelines are often set to limit exposure to the level most reasonably practical. For threshold chemical exposures, information is obtained from experiments or experience with animal, human or other types of biological assays to determine the lowest observed adverse effect dose (LOAEL) or the lowest dose tested where no adverse effects (NOAEL) occurred. In order to extrapolate from controlled experiments with animals to humans or to account for more sensitive populations that were not part of the test group (i.e., children), 10 to 1,000-fold safety factors (uncertainty factors) may be applied to the estimated acceptable dose value. These values are utilized in risk assessment models to determine acceptable human health risks.

The NOAEL divided by determined uncertainty factors (UF) gives us a reference dose value (RfD) which is the daily dosage an individual can be exposed to over a prolonged period of time with no adverse effect. The formula therefore is:


Exposure is assessed by evaluating the average daily dose (ADD). ADD is estimated by calculating the concentration of the hazard (C; obtained by monitoring data) by the ingestion rate (IR; for drinking water we can assume 2 L/day) by one’s exposure frequency (EF; days per year) by the exposure duration (ED; in years) in the numerator. The equation denominator is simply one’s body weight (BW; in kg) by the average time a person is expected to live AT (in years).

The equation therefore is:

ADD=C x IR x EF x ED / BW x AT

Quantitative Risks
Now that we have our exposure dose (ADD) and our adjusted acceptable dose above which unacceptable health risks may occur, we can calculate a more accurate assessment of the potential health risks, using the Hazard Quotient equation.


If your HQ value is greater than one, the risk of exposure to that particular chemical in water is unacceptable and needs to be mitigated. RfD values for priority contaminants have been previously developed and are available from the EPA IRIS (Integrated Risk Information System) database at A hazard index value (HI) has also been developed to evaluate the risk when more than one potential toxicant is present. In this case, the interactions are assumed to be additive and thus HQ is calculated for every chemical and all HQ values are added together to produce an HI. An HI above one is considered a significant and unacceptable health risk.

For chemical contaminants that have no threshold, meaning that risks are known or suspected to occur at any and all exposures and doses, values related to increasing doses are plotted against a response matrix. The slope of the line (higher slope equals a greater toxic effect) is used in the development of a slope factor (SF). SF values can also be obtained for select contaminants at the IRIS website. These effects are calculated as cancer risks and are equal to ADD in one’s lifetime multiplied by the slope factor:

Cancer risks- ADDlife x SF

In 1986, the EPA published guidelines for performing risk assessments of human health impacts related to exposure to contaminants in the environment. Since then, scientific knowledge of the hazards, exposures, and dose-response results have continued to broaden and improve. Following extensive public and scientific peer review, the EPA’s final Guidelines for Carcinogen Risk Assessment and a second document entitled Supplemental Guidance for Assessing Susceptibility from Early-Life Exposure to Carcinogens were issued in March, 2005 (

Quantitative Toxic Risk Assessment (QTRA) fills the need to accurately quantify the toxicological risks associated with human exposure to mixed drinking water contaminants. The same procedure can be used to assess the total human exposure condition to toxic pollutants in air, food, dust, etc., in addition to water. The science of risk assessment bridges the gap between environmental monitoring and endpoint analysis. The extensive database of information on toxic chemicals in the environment is not useful, from a public health standpoint, unless we take further steps toward translating this information to determine the impact these exposures have on the human condition. In particular, risk assessment is useful for determining the adverse impacts of multiple exposure routes and mixed compound exposures- parameters that are difficult to evaluate in the general population.


  1. WHO (2006) Guidelines for Drinking Water Quality. World Health Organization ISBN 92 4 154696 4

About the author
Dr. Kelly A. Reynolds is an associate professor at the University of Arizona College of Public Health. She holds a Master of Science degree in public health (MSPH) from the University of South Florida and a doctorate in microbiology from the University of Arizona. Reynolds has been a member of the WC&P Technical Review Committee since 1997. She can be reached via email at [email protected].


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