Application of probabilistic risk assessment in establishing the perchlorate and goitrogen risk mitigation strategies

This paper applies a methodology of probabilistic risk assessment to the task of identifying risks from cumulative and aggregate risk pathways for selected goitrogens in water and food. The methodology develops a Hazard Index calculation for exposed populations in the US under three approaches to established the Reference Dose (RfD) for use in probabilistic risk estimation, producing probability density functions (PDFs) for both total Average Daily Rate of Intake (expressed in PEC or Perchlorate Equivalent Concentration) for the three goitrogens considered, and for the Hazard Index (HI). The results are then used to calculate the percentages of individuals in a population with an HI value above 1.0, sorted by age groups (1-3 months; 2-3 years; 6-11 years; 18-21 years; >21 years (full adult); and a separate category of Pregnant Women). A Contribution to Variance calculation is performed to determine how these percentages with HI>1 are influenced by the separate numerical values of the 8 variables that influence exposure and risk:  Perchlorate concentration in water  Perchlorate concentration in food  Nitrate concentration in water  Nitrate concentration in food  Thiocyanate concentration in water  Thiocyanate concentration in food  Water exposure factor  Food exposure factor The results show that the percentages of individuals with an HI above 1 ranges between 30.2 and 50% both with and without half-life correction when a NOEL-based approach is used. When an exposure-response curve is instead used and a threshold of 50% inhibition is assumed, 1.1% or less of the population exceeds an HI value of 1 with no half-life correction, rising to as high as 11% for some age groups when half-life correction is applied. If 0% or 5% inhibition threshold is assumed (results are the same at the level of significant digits used here), the percentage of the population with an HI above 1 is 46.2% or less with no half-life correction, and 47.2% or less when half-life correction is applied While there are exposed groups and/or sensitive subpopulations where perchlorate exposures are the primary cause of individuals having HI values above 1, these pockets constitute significantly less than 1% of the population. For individuals with an HI value above 1, the potential risk from exposure to goitrogens is dominated by nitrates without half-life correction and thiocyanates with half-life correction.