Animal Waste, Water Quality and Human Health

Comparative risk analysis 389answer is simply calculated as the product of the relative risk factors. Thus, the“Individual Annual Relative Risk” is based on the following calculation:= source relative risk (based on the calculation described under “Question 1”,above)× relative amount of excrement produced per animal of the species selected inQuestion 1 (as described above under “Question 1”, above);× relative risk due to density of animal population (Question 2);× relative risk due to size of herd or flock causing contamination (Question 3);× relative risk due to mode of contamination (Question 4);× relative frequency of contamination (Question 5);× relative risk due to proximity of faecal contamination to recreational water(Question 6);× dilution between source and recreational water (Question 7);× relative risk reduction due to time between source and recreational water(Question 8);× the likelihood of mobilisation (Question 9);× relative risk reduction due to reliability and efficacy of mitigation actions(combination of Questions 10 and 11 as described in Table 10.3, above);× relative risk due to composition of affected population (Question 13);× relative frequency of exposure (Question 14);× relative risk due to type of exposure (Question 15);× relative risk adjustment for possible correlation between infrequentcontamination and infrequent exposure (Question 16) and, if included bythe user;× relative risk adjustment due to other factors not explicitly considered inthe model.The above calculation leads to a number, on an arbitrary scale, based on risk over ayear of potential exposure for an individual. The higher the number, the greater therelative risk.Assuming the most extreme relative risk (i.e., ‘1’) for each factor, andcombining this with the relative risk estimate based on the animal speciesconsidered to represent the greatest hazard, generates a maximum score of5.33 × 10 −4 . Conversely, assuming the lowest relative risk for each factor, leadsto a prediction of “No Risk”. The next lowest predicted relative risk is 2.25 ×10 −40 , obtained if all answers are selected to represent the lowest relative risk,but with Questions 10 and 11 answered as “Very High” and “CompletelyReliable” respectively, or as “Absolute” and “Virtually fail-safe” respectively.These extremes set the scale of relative risk for the model presented. To makethe scale more ‘natural’ to users, the logarithm of the above calculation is taken