Quantitative Microbial Risk Assessment (old)

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While most microbes are harmless or beneficial, some are extremely dangerous – we call these Biological Agents of Concern (BAC). All BAC can cause serious and often fatal illness, but they differ greatly in their physical characteristics, movement in the environment, and process of infection.

QMRA Evolution and Development

Quantitative microbial risk assessment (QMRA) is a framework and approach that brings information and data together with mathematical models to address the spread of microbial agents through environmental exposures and to characterize the nature of the adverse outcomes. Ultimately the goal in assessing risks is to develop and implement strategies that can monitor and control the risks (or safety) and allows one to respond to emerging diseases, outbreaks and emergencies that impact the safety of water, food, air, fomites and in general our outdoor and indoor environments.

Risk by it’s nature is probabilistic and thus relies developing quantitative information.

  • The definition of RISK: chance*hazard*exposure*consequence
  • Risk is the likelihood of identified hazards causing harm in exposed populations in a specified time frame including the severity of the consequences.

The traditional risk assessment process was seen as a four step process, Fig. 1 (NRC, Red Book, 1983) and was adopted for microbial hazards (Haas, Rose, Gerba, 1991).

Risk Paradigm from National Academies of Science
  • Hazard identification: The hazard identification is both identification of the microbial agent and the spectrum of human illness and disease associated with the specific microorganism.
  • Dose response assessment: The dose response analysis provides a quantitative relationship between the likelihood of adverse effects and the level of microbial exposure. Without knowing how different levels of the stressor affect the individual a sizable portion of quantified risk estimate will not be possible. The dose response assessment phase is arguably the most important phase in the QMRA paradigm.
  • Exposure assessment: The exposure assessment identifies affected population, determines the exposure pathways and environmental fate and transport, calculates the amount, frequency, length of time of exposure, and estimates dose or distribution of doses for an exposure.
  • Risk characterization: The risk characterization Integrates dose-response analysis and exposure assessment to estimate the magnitude of risk, uncertainty and variability of the hazard.

Some other important developments of QMRA includes:

  • QMRA Tools: A variety of QMRA tools begin as simplistic tools essentially established and tested Microsoft Excel spreadsheets. Some QMRA tools are being developed as standalone computer applications.
  • Environmental Infection Transmission Systems (EITS): This work advances the conceptual framework for the science of environmental mediation of person to person transmitted infections by stochastic processes.
  • Risk communication: The risk communication is an interactive process of exchange of information and opinion on risk among risk assessors, risk managers, stakeholders and general public.

More recently, according to the NRC’s Science and Decisions: Advancing Risk Assessment (NRC, 2008) the risk assessment-risk management paradigm should be integrated and include several phases.

Three phases for risk assessment-risk management paradigm

The current QMRA framework shown in Figure 2 addresses the assessment-management integration and the need for monitoring to improve exposure assessment, characterization and management.

Integration of Risk Assessment, Communication and Management

  • Understanding Best Use of Risk Assessment in Decision Making
  • Assessment-Analysis Advances
  • Infectious Disease Environmental Transmission Models
  • Modeling How Communities Communicate
Advanced risk assessment-risk management framework


Haas, C.N., J.B.Rose and C.P., Gerba. 1999. Quantitative Microbial Risk Assessment.1st.Ed. John Wiley & Sons, Inc., New York.

NRC (National Research Council) 1983. Risk Assessment in the Federal Government: Managing the Process. Washington DC. National Academy Press

NRC (National Research Council) 2008. Science and Decisions: Advancing Risk Assessment. Washington DC. National Academy Press