Public Health Quantitative Microbial Risk Assessment
Patrick Gurian
  • LAST REVIEWED: 26 July 2017
  • LAST MODIFIED: 26 July 2017
  • DOI: 10.1093/obo/9780199756797-0166


Quantitative microbial risk assessment (QMRA) is the application of mathematical models of exposure and dose response to predict the likelihood of adverse outcomes due to exposure to pathogens. These adverse outcomes include infection (the microorganisms replicate in or on the host organism), morbidity (illness, the microorganisms induce disease in the host), and mortality (the host dies due to the effects of the microorganisms). QMRAs have addressed a variety of pathogens, including viruses, bacteria, protozoa, and prions, and produce probabilistic estimates of harm. In other words, QMRA generally does not indicate if an adverse outcome will occur or not, but will instead indicates the probability that it will occur. As pathogens are present in many environmental media, avoiding all potential exposures with pathogens is not a feasible goal. QMRA provides a way to assess the impacts of these many potential routes of exposures in order to inform decisions about which risks are significant enough to merit efforts to avoid or mitigate them. The term QMRA is generally applied only to the calculation of the probability of harm, while efforts to prioritize and make decisions about risk are referred to as “risk management.” QMRA is often used synonymously with the term “microbial risk assessment,” given that the microbial risk assessment framework (described below) includes inherently quantitative steps. However, non-quantitative approaches to risk are recognized as an important component of risk assessment and management (see Hazard Identification section below). Microbial risks may also be identified, and some cases quantified, using epidemiological methods that correlate exposures and risks without employing specific models of exposure and dose that are part of the QMRA framework (see below). QMRA approaches have been applied to inform standards for microbiological quality of food, water, air, and touched surfaces, such as counters, doorknobs, and so on. QMRA approaches are seen as valuable because they allow for hypothetical cases to be considered (i.e., scenarios for which there are no available data. QMRA also allows for an assessment of the exposures associated with very low risks, such as 1 in 1 million or 1 in 10,000, that may be desired targets for risk mitigation efforts but are too low to be realistically measured. QMRA approaches may be criticized as being frequently applied without validation to scenarios substantially different from the circumstances under which the models were developed. Consensus views see a role for QMRA in decision making, while also recognizing that in many cases QMRA estimates are subject to substantial uncertainty and should be interpreted cautiously.

Risk Assessment Framework

A common conceptualization of the framework for developing quantitative microbial risk assessments includes four steps: hazard identification, exposure assessment, dose response, and risk characterization (QMRA Wiki 2016). These steps are essentially the same as those commonly used for chemical risk assessment. Each of these steps is described in more detail below.

  • QMRA Wiki. 2016. Quantitative Microbial Risk Assessment (QMRA) Wiki. East Lansing, MI: Center for Advancing Microbial Risk Assessment.

    This website provides information on QMRA methods and serves as a repository of inputs and data for QMRA. Both raw data and fitted parameters are available for dose response models for a large number of pathogens and host species. An archive of case studies provides many examples of how QMRA methods can be applied in different contexts.

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