Animal Waste, Water Quality and Human Health

Comparative risk analysis 385Question 11 is also framed as a statement to be completed: “The reliability of riskmitigating factors is …” with five possible responses, as follows:completely reliable relative risk = 0virtually “fail-safe” relative risk = 0.01usually reliable relative risk = 0.1sometimes effective relative risk = 0.5unreliable relative risk = 0.9The first response to both questions are unusual compared to all other responses inthe risk model because both have the potential to reduce the risk estimate to zero,that is, no risk. However, a process that completely eliminates pathogens, but isunreliable still has a risk associated with it. Thus, the degree to which a processis unreliable reduces the effective risk reduction. Conversely, a completelyunreliable process that has little effect on pathogen numbers cannot increase therelative risk and the relative risk score must remain as “1”. To model theselogical considerations of the interplay between process efficacy and processoperational reliability the two scores are combined and a logical test applied.Thus, the relative risk due to both of these factors in combination is taken as thesum of the two relative risk scores. However, to prevent the model frompredicting an increase in risk from a low efficacy process operated unreliably,the “MIN” function in Microsoft Excel is used so that if the combined relativerisk score is greater than “1”, the model substitutes a value of “1”. The net effectof this on the relative risk score for mitigations is shown in Table 10.3, below.It is noted that the answers to the above questions may be subjective andcan have a profound affect on the risk estimate. Accordingly, it is advised thatusers make careful considerations and seek guidance as needed, when assessingreliability.10.4.4.4 ExposureThe risk to people from contaminated recreational waters depends not only on thelevel of contamination but also the magnitude of the exposure to that contaminatedwater. This depends on how frequently people are exposed and the manner inwhich exposure occurs. For the sake of this example of the approach to relativerisk estimation we have limited the scope to the risk due to ingestion ofcontaminated water.Risk can be expressed as risk to an individual or risk to an entire population, andcan be affected by the susceptibility of individuals or sub-groups within thepopulation, for example, young children may be more susceptible to pathogensbecause they have not yet experienced the organism nor developed immunity.