Quantitative Microbial Risk Assessment

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Environmental Health Division

Contact Info
Health Risk Assessment Unit
651-201-4899
health.risk@state.mn.us

Contact Info

Health Risk Assessment Unit
651-201-4899
health.risk@state.mn.us

Quantitative Microbial Risk Assessment Basics

Quantitative microbial risk assessment (QMRA) is a mathematical modeling approach used to estimate the risk of infection and illness when a population is exposed to microorganisms in the environment. Through QMRA, we can understand how much of an impact microorganisms in the environment will have on the health of the population.

There are four steps in QMRA:

The first step in QMRA is hazard identification. This step involves deciding which microorganisms are of interest in the study and finding out what diseases these microorganisms cause.

The next step is exposure assessment. In this step, we measure the dose, or the amount of a microorganism individuals are exposed to. The dose is a combination of the amount of the microorganisms in the environmental medium (such as soil or water) being studied and the amount of that environmental medium a person is exposed to. For example, if there is a concentration of 1 bacterium per liter of water, and a person drinks 1.2 liters of water per day, the dose is 1.2 bacteria per day. Once a dose has been calculated, it is put into an equation created specifically for the microorganism we are studying.

Selecting and using the right model is the next step in the QMRA process. Dose-response models have been created for a variety of microorganisms and each model is specific to a microorganism and route of exposure. Dose-response models used in MDH’s analysis were all created by other researchers. Researchers created these models using data from studies that investigated how many people were infected when exposed to a certain amount of microorganism of interest.

Once a dose is calculated and a dose-response model has been selected, the next step is risk characterization, which puts all of the information together. The calculated dose is put into the dose-response model, and the answer to the equation tells us how likely a person is to get infected from exposure to this microorganism.

Interpreting results

The results from the risk characterization step will be a number from 0 to 1. A risk of zero means no chance of infection and a risk of 1 means certain (100% chance of) infection. For example, if we calculated a risk of 0.18, this means that if someone is exposed to the environmental medium, they will have an 18 percent chance of becoming infected.

It is important to remember that infection and illness are not the same thing. Becoming infected by a microorganism does not mean that you will feel sick and show symptoms. Many people become infected by microorganisms without having any symptoms.

After we have calculated a probability of infection, we can estimate the probability of illness. Based on the work of other researchers, the risk of infection is divided by two or another appropriate factor to give the risk of illness. Using the same example as before, if the estimated risk of infection is 0.18, the estimated risk of illness is about 0.09. Another way to look at this is if 100 people were exposed to this same medium, it is likely that about 18 of them would become infected and 9 of them would get sick. We also estimated risk per day, meaning that, if a person was exposed to this amount of microorganism every day for a year, they would likely become infected from about 66 of these days and sick from about 33 of these days.

For more information on QMRA methods, see Quantitative Microbial Risk Assessment (QMRA) Wiki, maintained by Michigan State University.

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Last Updated: 08/21/2024