Drinking water QMRA

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Quantitative microbial risk assessment (QMRA) is a well established methodology to combine these different pieces of knowledge to assess the health risk of environmental exposure to pathogens. It is used in many fields and routes of exposure: food, water, air, fomites. QMRA allows objective, science based assessment of the health significance of an exposure route and can predict the effects of intervention strategies. QMRA is a framework that integrates the best available scientific knowledge to a level that is understandable for risk managers and supports risk-based decision making.

Key elements of QMRA

Drinking water quantitative microbial risk assessment (QMRA) has four stages:

  • Hazard Identification: Describe a microorganism and the disease it causes, including symptoms, severity, and death rates from the microbe. Identify sensitive populations that are particularly prone to infection.
  • Dose-Response: The relationship between the dose (number of microbes) received and the resulting health effects. Data sets from human and animal studies allow the construction of mathematical models to predict dose-reponse.
  • Exposure Assessment: Describe the pathways that allow a microbe to reach people and cause infection (through the air, through drinking water, by touch, etc.). Determine the size and duration of exposure by each pathway. Estimate the number of people exposed and the categories of people effected.
  • Risk Characterization: Integrate the information from steps 1, 2, and 3 into a single mathematical model to calculate risk -- the probability of an outcome like infection, illness or death. Since steps 1, 2 and 3 will not provide a single value, but a range of values for expsure, dose, and hazard, risk needs to be calculated for all values across those ranges. This is called Monte Carlo Analysis, and the result is a full range of possible risks, including average and worst-case scenarios. These are the risks decision makers look at when choosing policy and that scientists look at to see where we need to run more experiments to find better information. [1]

The risk assessment process requires quantitative information about the exposure of drinking water consumers to pathogens. This is provided by exposure assessment, one of the components of risk assessment. Quantitative information about pathogens in water sources, their removal by treatment and protection of the distribution network and drinking water consumption is collected and translated into an estimate of the exposure of consumers to pathogens through drinking water. To complete the risk assessment, the potential effect (the risk) of pathogen exposure is estimated through known doseresponse models. As will be shown later, the exposure assessment also provides valuable information to aid risk management in the prioritisation of control measures.

Quantitative microbial risk assessment (QMRA) can provide an objective and scientific basis for risk management decisions. Water utilities can use QMRA to assess if they meet the health targets with their water treatment, storage and distribution systems. QMRA should also be used to provide information for setting critical limits in the Water Safety Plans to ensure good performance. Good performance can now be based on a quantitative assessment of the contribution of the Critical Point (such as a disinfection or filtration process) to the overall safety and the limits can be set to ensure that the multiple barrier chain of water collection, treatment and distribution as a whole does meets the desired health target. Risk assessment and risk management should not be regarded as two separate steps in the harmonised framework.


See also