Guidelines for Canadian Drinking Water Quality: Guideline Technical Document - Enteric Viruses - Figure 1

Examples of a risk assessment under specified conditions

The steps in a quantitative microbial risk assessment for enteric viruses are presented as a flow chart along with an example risk assessment calculation. On the left side of the flowchart, in boxes, are the 5 steps; on the right side, parallel to each box, is the corresponding information from the example calculation. For step 1), source water, the example uses a mean concentration of 1.4 viruses per 100 litres of water. For step 2), treatment impact, the example calculation estimate for log removal or inactivation is 4 log and therefore the resultant concentration of viruses in drinking water would be 1.4x10^-4 viruses per 100 litres, or 1.4x10^-6 viruses per litre. For step 3), water consumption and pathogens ingested, the example calculation includes the assumption that 1 litre of water is consumed daily, therefore the number of viruses ingested per day would be 1.4x10^-6. For step 4), dose-response, the information from step 3 and equations 1, 2 and 3 from the text, are used to calculate the probability of infection per person per day, the probability of infection per person per year, and the risk of illness per person per year which correspond to 3.8x10^-7, 1.4x10^-4, and 1.2x10^-4, respectively. For step 5), disease burden, equation 5 from the text is used to calculate a disease burden, which for this example is 1.0x10^-6 disability adjusted life years per person per year.