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Description of key information

The calculated NOEC of 215 µg/L (geometric mean of the results of 3 laboratories for which iodide concentration data was relevant and reported) for growth and development of amphibians was selected as the key value for the chronic toxicity to these organisms (OECD report, 2007).

Additional information

A lot of studies have been carried out with the South African clawed frog (Xenopus laevis), a species widely used for studying the regulation of growth and development of vertebrates (see the position paper "PNEC freshwater derivation for Perchlorate anion"). Data from literature confirm the high sensitivity of this species when exposed to Perchlorate.

Among the published results, the articles of Goleman et al. (2002a and 2002b) report the lowest NOEC values for the development of amphibians (i.e. NOEC < 59 µg/L and LOEC = 5 µg/L for forelimb emergence and tail resorption). Nevertheless, these studies have been performed in FETAX medium which is free of iodine. In addition, any iodide input was considered to come uniquely via the feeding.

In the OECD guideline 231 (Amhibian Metamorphosis Assay) it is clearly indicated that based on the available data from the validation studies, the relevant water iodide concentration ranges between 0.5 and 10 µg/L. Therefore, the studies of Goleman et al. (2002a and 2002b) are considered as not environmentally realistic and may have led to the effects observed at low Perchlorate concentrations. Indeed, Perchlorate is a competitive inhibitor of the uptake of iodine by thyroid: that means that the concentrations and relative proportions of iodine and Perchlorate are of paramont importance. It is expected that even very low levels of Perchlorate can affect development, when the available level of iodine is also very low. Conversely, the presence of sufficient iodine in natural waters should compensate the antithyroidal effects of Perchlorate. Recent experiments have confirmed that natural waters as test medium, containing iodine, strongly mitigate the effects of Perchlorate.

Therefore, the studies of Goleman et al. (2002a and 2002b) have been disregarded.

An OECD report (2007) summarises the results from an inter-laboratory study to assess the reliability of the amphibian metamorphosis assay (AMA) for the detection of thyroid system-disrupting substances acting through different pathways. Sodium Perchlorate was used as a reference compound. The biomarkers of thyroid activity were the most sensitive endpoints but no NOEC was determined because effects were observed already at the lowest tested concentration of 62.5μg/L Perchlorate. Due to auto regulation mechanisms in the organisms, even if effects are observed at the cells level in the thyroid, this does not necessarily translate to macroscopic effects in the organism. On the contrary, macroscopic effects observed concern the development of organisms and their behaviour. They will have an impact on the survival of organisms (difficulty to move) and their reproduction (difficulty to breed). As a consequence, macroscopic effects on amphibians (determined in three laboratories, with iodine concentrations measured to be > 1.5 µg/L) have been taken into account to identify the chronic toxicity of Perchlorate on these organisms. A geometric mean of three NOEC was then calculated from these results and was determined to be 215 µg/L.

The published study of Olmstead (2009) determined the effects of Perchlorate on X. tropicalis after 40 weeks of exposure (Larval amphibian growth and development assay - LAGDA). No apical effects (i.e. metamorphosis, body size) were affected at concentrations up to 1500 µg/L whereas a NOEC of 170 µg/L was observed for thyroid histopathology. These data confirm that larvae are able to cope with a certain degree of thyroid disruption without long-term adverse consequences.