02 March 2021, UK: During September 2020, National Geographic magazine posted an online story titled, “Wallabies exposed to common weed killer have reproductive abnormalities,” based on a study in the journal Reproduction, Fertility and Development.
In a detailed review of the underlying study, Syngenta posits that the findings were predicated on flawed scientific methodology, disregard for real-life circumstances and uses for atrazine, as well as impossible dosing scenarios.
Syngenta contacted National Geographic in order to clarify these issues. However, despite an attempt by the publication to incorporate Syngenta’s viewpoint into its online article, there remains a significant and crucial misunderstanding of the flawed nature of the study on which the story is based.
Implausible exposure level cited
The authors of the study claim to have calculated the concentration of atrazine in treated drinking water as 450 parts per million (ppm), or 450 mg/L. As the solubility limit of atrazine in water is 33 mg/L at 22 °C (or 33 parts per million), it is impossible that animals in the wild could be exposed to 450 ppm. It is therefore also highly unlikely that a dissolved concentration of 450 ppm atrazine in water (13.6 times greater than solubility) was achieved from the formulated product used (Gesaprim® Granules 900 WG), which was also not analytically verified by the study authors.
The authors also state that the wallabies were administered atrazine via drinking water at a concentration of 450 ppm (450 mg/L) to achieve an approximate dose of 10 mg/kg body weight (BW)/day. Further, this concentration was deemed to be environmentally relevant because it was nearly eight times the concentration of the highest recorded daily concentration from a forestry buffer strip run-off study. However, that particular value was recorded as 53 mg/L or 53 ppm and there are several problems with these concentrations, the foremost being that they are far higher than the solubility limit for atrazine in water which is 33 ppm (33 mg/L).
It is physically impossible to have water samples containing atrazine concentrations of 58 ppm (environmentally) let alone 450 ppm (drinking water). By the authors’ own admission, 450 mg/L is still almost an order of magnitude higher than the 53 mg/L reported concentration in a forestry buffer (worst case, although still highly improbable depending on the ambient temperature of the water) and more than an order of magnitude greater than the solubility of atrazine in water at 22 °C. It is indeed a stretch to claim that this is an environmentally relevant exposure.
Finally, the U.S. has the most extensive historical database of atrazine water monitoring data in the world for both ambient (ecological) and drinking water sources. It does not contain any sample concentrations remotely approaching any of the values used in this study. For example, the highest single-day sample ever recorded in the Atrazine Ecological Monitoring Program (AEMP) in a small intermittent stream was 334 ppb (.334 ppm; 3 orders of magnitude lower than the nominal test concentration of 450 ppm).
Analytically confirming exposure levels key
Syngenta also notes that the authors of the study in question did not measure the concentration of the mixture (i.e., analytically confirm exposures) after dissolving the solution ‒ a standard step that would have provided the information necessary to understand what was being measured. Confirming exposure concentrations analytically is considered a fundamental tenet of most quality refereed journals.
Other factors that were not accounted for in the study include the:
Diet high in genital-anomaly-associated phytoestrogens (clover and alfalfa);
Potential impact of other chemical ingredients in the water fed to the wallabies that the control group did not receive; and
Other technical and interpretative limitations that were outlined by Syngenta for National Geographic – but were not addressed.
In summary, Syngenta does not dispute that using high doses of a substance is a core principle of toxicological study in order to characterize the level of exposure required to elicit a toxicological response. The rationale for highlighting the doses employed – and corresponding research on the measured presence of atrazine in the aquatic environment – is to underscore the need to contextualize the usefulness of trying to measure the effects at doses that are completely implausible relative to environmental exposures.