Quantitative Microbial Risk Assessment - EH: Minnesota Department of Health

Quantitative Microbial Risk Assessment

Microorganisms in the environment

Microorganisms are viruses, bacteria, protozoa, fungi, algae, and other organisms that are small enough to be considered microscopic. Microorganisms are present all around us. They are in our bodies, as well as in the air, water, soil, plants and every part of our environment. There are actually more microbial cells than human cells in your body! Most microorganisms will not cause any harm. In fact, many of them are helpful. A small portion of microorganisms, however, are capable of causing diseases in humans.

Although only a small portion of microorganisms cause illness, they are responsible for many illnesses throughout the world. When microorganisms contaminate water, the result can be a waterborne outbreak that affects many people in a population. This was the case in Milwaukee in 1993 when the microorganism Cryptosporidium contaminated the drinking water supply and resulted in the largest waterborne outbreak in the country. This outbreak caused an estimated 403,000 illnesses and 69 deaths. It is important to know if disease-causing microorganisms are present in our environment, and at what levels they will cause illness in the population.

Quantitative microbial risk assessment (QMRA) can be used to estimate the public’s risk of infection and illness from disease-causing microorganisms in the environment.


The Minnesota Department of Health is working on several different projects that involve testing for microorganisms in the environment. Testing tells us the levels of microorganisms in the environment, but that alone does not tell us whether or not people will get sick if they are exposed.

This is where QMRA comes in. QMRA is a modeling approach that allows us to estimate the risk of infection in humans from an exposure to a pathogen. QMRA uses a mathematical model to estimate risk. For example, QMRA can be used to estimate the likelihood that someone would get sick from microorganisms in water they drink, irrigation water spray they breathe in, food they eat, water they swim in, as well as other exposure scenarios.

It is important to know what level of microorganisms can be present in the water without causing illness. This information can be used to help in protecting the public from disease. It is also important to know because disinfecting groundwater costs time and money, and can produce unintended chemical byproducts. Knowing acceptable levels of microorganisms in the water ensures that disinfection practices are not used if they are not necessary.

QMRA is a new and growing field! There is a lot more to learn about how the process works and how it can help in protecting the health of the public.



Updated Tuesday, July 17, 2018 at 10:17AM