## Quantitative Microbial Risk Assessment

## Related Topics

- Risk Assessment Home
- Clean Water Fund
- Cleaners, Sanitizers, and Disinfectant Products
- Minnesota One Health Antibiotic Stewardship Collaborative
- Contact Health Risk Assessment

## Environmental Health Division

# 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:

## 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.

**Last Updated:**10/03/2022