In the assessment of health risks of environmental pollutants, both conventional methods and probabilistic methods based on the use of cumulative probability plots have been used. Probabilistic risk assessment has the advantage that the statistical distributions of both the exposure data set and the adverse effect data set are taken into account in risk characterization. In addition, the method of overall risk probability (ORP) has been used to quantify the overall risk probability of adverse effects for the population samples. The disadvantage is that many more data points are needed to obtain the cumulative probability plots and the calculation of the ORP from the probability plots is more complicated. In this paper, a simplified computational method for the estimation of the ORP was developed. In this method, an overall risk factor (ORF), as an estimated value of the ORP value, was obtained from the mid-point of an exposure exceedence curve. The ORF can also be obtained from an inversed cumulative probability plot of the exposure curve, thus eliminating the needs for the use of the exposure exceedence curve and the numerical integration of the exceedence curve in ORP computation. The use of ORF method was illustrated with a practical example and the results indicated that the values of the ORF and ORP are about the same.
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