Cover of Clean Water: What Is Acceptable Microbial Risk?

Access to clean water is essential for life. In recent decades, technology, civic progress and an abundance of resources have enabled developed countries to cultivate high-quality water sources and distribution systems. As a result, people in these countries now enjoy lower infectious disease rates, higher hygiene standards and a higher quality of life than has ever been witnessed in history.

It is a familiar scenario: an outbreak of gastrointestinal illness suddenly emerges in a community, and no one knows where it came from or how to stop it. At the start of an outbreak, only a few people are affected with the uncomfortable consequences: nausea, vomiting, cramping and diarrhea. Sick people trickle into doctors' offices and clinics for help. Among them are elderly patients and small children, some of whom are admitted to the hospital. As the outbreak worsens, more and more people fall ill, and individuals who were weak or unwell before they became infected may develop life-threatening illnesses. Outbreaks like these can originate from a source that most people in the U.S. and other developed countries trust unquestioningly—drinking water.

Although drinking water quality in developed countries is high, a number of outbreaks of waterborne illness are still reported every year (Dziuban, et al., 2006). Worldwide, the statistics are even more alarming; 2 million people die every year from diarrheal illness, most of which can be attributed to waterborne pathogens (Pruss, et al., 2002). Even more alarming is the situation in the developing world, where access to clean water is far from guaranteed, and diarrheal illnesses claim roughly 2 million lives every year, 90% of them small children.

Today, scientists have a relatively new tool for addressing the problems of waterborne infectious disease: microbial risk assessment (MRA), a formal process for quantifying the health risks from pathogenic microorganisms. MRA is guided by a framework, which defines the activities necessary to obtain information required to develop a quantitative model for calculating health risks.

The American Academy of Microbiology convened a colloquium Oct. 6-8, 2006, in Tucson, Ariz., to review the status of microbial risk assessment as it applies to waterborne disease. Experts from diverse fields—including microbiology, public health, engineering, epidemiology, medicine and water science—discussed some of the controversial topics in microbial risk assessment, research subjects that could move the field forward and the need for increased training and risk communication.

The colloquium elicited intense discussion as there is still need to solidify approaches to the microbial risk assessment of water. Numerical water quality standards, for example, are useful in some circumstances, but they are sometimes misapplied or calculated using specious assumptions. The term "acceptable risk" is also controversial. "Acceptable risk" implies that injuries from waterborne illness are expected and fitting, and acceptable risk figures may be appropriately used to derive water quality standards in some instances. For a number of reasons, much of the data available on microbes in water is related to indicator organisms (microbes that denote the presence of fecal material or pathogens), but these organisms are not a substitute for counting the actual pathogen concentrations in water.

Another difficult point is sometimes reconciling the approaches between microbial risk assessment and epidemiological studies. The approaches, although potentially harmonious, often lack coordination. However, epidemiological studies can be extremely useful in identifying risks and every effort must be made to reconcile epidemiological and microbial risk determinations.

An accessible international database of pathogen occurrence in water would be extremely useful. Making data of this kind more widely available would inform microbial risk assessment and risk management and enable implementation of public health initiatives that could save lives.

Microbial risk assessment of water is an evolving field, and a great deal of novel research is needed to fill gaps in the understanding of human exposure to pathogens in water, to determine the current rate of waterborne illness, the dose-response relationships between pathogens and human health and the role of waterborne opportunistic pathogens in human health.

Since the field of microbial risk assessment relies on the skill sets of professionals in many disciplines, education and research in microbial risk should be interdisciplinary and collaborative.

Finally, there is a need to effectively communicate microbial risk principles to consumers and the general public because a lack of information can have serious implications for communities. Risk managers and public health authorities need to make increased efforts to educate the public on everyday matters, like the need to change the filters in water purifiers and the need to upgrade and maintain water and wastewater treatment facilities.

For all the challenges that still exist to advancing the science and application of microbial risk assessment, the effort to do so will offer many benefits. A primary advantage of the iterative process is that it helps to identify data gaps and uncertainties, and it focuses limited research resources towards key parameters that will improve the understanding of risk. When applied correctly, microbial risk assessment can help guide water quality management decisions; identify sensitive subpopulations, spot critical pathogen control points and aid in assessment of the adequacy of drinking water treatment barriers.


Mark Lechevallier, Merry Buckley. 2007. Clean water: what is acceptable microbial risk?

Contact Information

Academy Staff,