Abstract
This paper applies cumulative and aggregate risk methods and weight of evidence determination to re-analysis of epidemiological and clinical studies of exposure to perchlorates. The implications of cumulative and aggregate risk are considered for 28 epidemiological studies on IUI, serum thyroid hormone levels and clinical indicators. Consideration is given to simultaneous exposures to perchlorates, nitrates, thiocyanates and organohalogens in the study populations. The elevation of effects by perchlorates alone is found only in the studies that use urinary perchlorate as the metric of exposure. These studies are beset by a problem with use of urinary perchlorate concentration in that there is large inter-subject variability in the relationship between intake and urinary concentration due to differences in metabolism and disposition of the compounds following ingestion. As a result, an individual placed into the “high urinary concentration” group may be there due to high values of exposure, to long biological clearance halflives, or due to high transfer fractions from the serum into the urine. The influence could be removed by correcting urinary levels by measured clearance half-times for individuals in a study, but that has not been done in the case of the studies examined here. It is of interest therefore that the studies that use direct measures of intake of perchlorates rather than urine concentration fail to display the hormone effects. The current study uses a “weight of evidence” approach for perchlorates, employing all 28 studies. The result is a slope of the exposure response curve (percentage change in hormone effect per unit exposure) of 0.3% per μg/kg-day, with 95% confidence interval of (?0.05%, 1%). This confidence interval for the slope encompasses 0, indicating no statistically significant slope when all data are combined in a weight of evidence determination. This is consistent with the conclusions of the USEPA and EFSA that the epidemiological studies do not provide compelling evidence for a causal association between exposures to perchlorates and either hormone effects or clinically adverse effects. The conclusions are 1) that current epidemiological results do not provide evidence of effects induced by perchlorates apart from the IUI effects, 2) that the same results provide evidence that the IUI effects induced at environmental levels of exposure are associated with down-stream adverse effects and 3) that effective risk management requires the cumulative and aggregate risk framework adopted here, suggesting a need for risk assessors to return to the original studies and provide separate estimates of exposure response relationships for all four compounds or at the least to control for exposures to nitrates, thiocyanates and organohalogens.
Highlights
Establishment of regulatory limits on exposure to environmental risk agents has traditionally focused on one compound at a time
Aggregate risk assessment refers to the same compound acting through different pathways of exposure, while cumulative risk refers to the combined effects of different compounds with the same mechanism of action
The effects of perchlorates at the low levels of regulatory limits proposed have been significantly over-stated through failure to account for cumulative and aggregate risk in interpreting the primary data of Greer et al [14] using PEC. This raises the primary research question of the current paper: Does consideration of cumulative and aggregate risk significantly alter the conclusions of those studies with respect to risks of exposure to perchlorates, and does the weight of evidence support claims that exposure to perchlorates at environmental levels are associated with changes in Iodide Uptake Inhibition, serum thyroid hormone levels and/or clinically adverse effects? Before turning to this question, it is useful to review the summaries of epidemiological studies by the USEPA [20] and EFSA [21]
Summary
Establishment of regulatory limits on exposure to environmental risk agents has traditionally focused on one compound at a time. While this reinterpretation of primary data from Greer et al [14] through the lens of cumulative and aggregate risk has been carried out for perchlorates and their effect on IUI [18] [19], it has not been the norm for epidemiological studies. From the above two summaries [20] [21], it is evident that effects of exposures to perchlorates at environmental levels either are absent from epidemiological studies or are being masked by failure to include control for confounding by compounds such as nitrates and thiocyanates that act by the NIS mechanism of action to influence IUI, or by compounds such as organohalogens [23] that are known to induce effects on serum thyroid hormone levels through endocrine disruption. The following analysis explores these possibilities to understand the apparent discrepancy between the more stringent proposed regulatory limits on exposures to perchlorates and lack of evidence for effects in epidemiological studies conducted at environmental levels of exposure
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