A3.1 Introduction to Quantitative Microbial Risk Assessment (QMRA)

Quantitative Microbial Risk Assessment (QMRA) is a formal quantitative risk assessment approach that combines scientific knowledge about the:

• presence and nature of pathogens

• potential fate and transport of pathogens in the water cycle

• rates of exposure of humans

• health effects that may result from this exposure.

It takes into account the effect of natural and engineered barriers and hygiene measures. All of this knowledge is combined into a single assessment that allows evidence-based, proportionate, transparent and coherent management of the risks of waterborne infectious disease transmission (WHO 2016).

QMRA offers a systematic way to use scientific information to help support the water safety management decisions on a utility or regulatory level and prioritise remedial actions and/or research actions. The numerical output of the QMRA addresses the risk management questions in finer detail and allows for more precise comparison between risk management options compared with qualitative and semi-quantitative approaches (WHO 2016). QMRA is a framework or mechanism that allows for quantitative scientific data to be interpreted in the context of estimated health outcomes to support water safety management (WHO 2016).

The key advantages of applying QMRA are that it:

• allows for a quantitative treatment target to be estimated (the treatment target is the magnitude of treatment in $\text{log}_{10}$ units required to achieve the health outcome target)

• enables the level of treatment to be targeted to the level of microbial contamination to ensure that treatment is “fit for purpose”

• can project to low levels of risk - below those observed by epidemiological methods

• is evidence-based, drawing on scientific data from environmental microbiology and human challenge (feeding) studies to predict risk.

QMRA is simply a way of bringing together a diverse range of scientific data to support risk management. Further information on the process is available in the 2016 World Health Organization (WHO) Quantitative Microbial Risk Assessment for Water Safety Management, available at https://www.who.int/publications/i/item/9789241565370.

As with most modelling approaches, QMRA can be useful, but is often highly uncertain and is only as good as the inputs into that model. Uncertainty is addressed by making various assumptions to fill the gaps of limited data and lack of knowledge. There will always be some uncertainty as to how QMRA models represent the reality of the environmental system, and these should be interpreted carefully. In practice, whilst this approach is “quantitative”, in that numerical outputs are delivered from the QMRA models, the reality is that often those numerical values, or ranges in possible values, can be very tentative. In making decisions on public health protection, placing undue reliance on uncertain modelling outputs needs to be done conservatively.