Quantitative Microbial Risk Assessment

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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 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.

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 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?

The hazard identification is both identification of the microbial agent and the spectrum of human illness and disease associated with the specific microorganism.

  • Establish parameters associated with the pathogen. Case fatality ratios, transmission routes, incubation times, etc.
  • Research pathogen like before. In addition, there have been large outbreaks of E. coli associated with spinach in the past. It may be useful to research them for reference. Factors like attack rate and corrective actions will be valuable to know.
  • You should first identify the strain you are dealing with. Different strains have different attack rates, effect different population etc. Knowing these unique qualities is important.

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.

  • Visit our dose-response section to look up the best equations for calculating the likelihood of infection based on the dose taken in by individuals.
  • Assuming we are dealing with Enterohemorrhagic E. coli O157:H7, the recommended model based on our criteria uses the exponential equation with a k value of 2.18E-04 and an LD50 of 3.18E+03.
  • The recommended dose-response model according to our criteria would be the beta-poisson equation with an α value of 5.81E-01 and N50 of 9.45E+05.

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.

  • Questions to be answered include; How much water will the child ingest over a period of time? Is the pathogen traveling from a source, is there die-off or grow while the pathogen is in the environment? What is the concentration of pathogen in the water at the time of ingestion?
  • What kind of concentrations of organisms are seen in surface waters in rural farmlands? What concentrations could be expected should these crops be irrigated with this water. How is the crop processed, will there be any growth or die off during this process? What about during shipping and storage? What temperature will it be stored in? How much will the population eat?
  • How many times will a sick individual cough during the day? How many organisms will the ill person excrete with each cough? When they touch a fomite (surface), what concentration of organism will be deposited on that surface? If aerosolized, how will the organisms travel throughout the room and office? If someone touches a contaminated surface, how many organisms will be transferred to their hands and then be ingested?

The risk characterization Integrates dose-response analysis and exposure assessment to estimate the magnitude of risk, uncertainty and variability of the hazard. Assuming you find a range of values for each parameter risk characterization is the process of determining which values within these ranges to use, or how to use the ranges as a probability.

  • If there is a range of concentration in the water, you may consider using a conservative one considering the potential severity of outcome and the relative unimportance of recreational water (i.e. closing a beach does not incur significant economic cost).
  • You may wish to use software to calculate doses based on a probability range as there will be many many potential interactions between pathogen and host, all with possibly slightly different exposure scenarios.
  • There will inherently be wide ranges in excretion rates associated with coughing and other habits. As well as the amount each person coughs or touches objects. The particular scenario may require probability estimates. Or if it's not too serious, a liberal estimate may be acceptable with authorities.

Risk can be managed using many different strategies and is most effective when it is informed through risk characterization. The identification and evaluation of risk management strategies on the basis of cost and effectiveness are integral parts of the process. In addition to quantitative evaluation, an understanding of risk perception and a plan for risk communication are also pertinent risk management activities.

  • Should the lake/beach be closed to swimmers? What is the source of the contamination, can it be stopped?
  • Should there be a product recall? Can you better regulate irrigation of crops? Can you end the source of contamination of the source waters? Can you disinfect the crop during processing?
  • Should you quarantine the office? Improve disinfection techniques by the janitorial staff? Disable the air conditioning? Or perhaps just prepare to absorb the cost of multiple sick days.


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