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Quantitative Microbial Risk Assessment

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. 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. 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 its 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.

Some other important developments of QMRA include:

  • QMRA Apps and Calculators: 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.
  • Social Science of Risk: The risk communication is an interactive process of exchange of information and opinion on risk among risk assessors, risk managers, stakeholders and the general public.


First step, state your problem

The first step in any risk assessment is to start with a question you want to answer. This can be quite broad or very narrow. Below are three examples of problems that would benefit from a risk assessment approach. We will follow these examples throughout the page so that you can get a sense of how these steps work together.

  • What is the likelihood a 12 year old child will get sick with shigellosis from swimming in a contaminated lake. How should this situation be approached?
  • If a crop of spinach sprayed with biosolids, how will that effect the end product being consumed in the home? Is there a risk of E. coli infection? Where in the chain can we act to reduce risk?
  • If a person sick with the flu goes to work in an office building, how will they effect their coworkers. How will the virus move through the building?