Rodent Studies Show Chlorpyrifos Harm - Evidence Overlooked

A new scientific article provides a critical analysis of Dow AgroSciences’ (Dow Chemical) unpublished laboratory rodent studies conducted in 1998 and 1999 that U.S. Environmental Protection Agency (EPA) and European Union (EU) Agencies relied upon to evaluate the brain-damaging effects of the insecticide chlorpyrifos.

The unpublished Dow studies report no harm from chlorpyrifos exposures, even when test rodents were exposed to chlorpyrifos prenatally, and at high exposures. The Dow study results are in conflict with non-industry studies published in reputable scientific journals that identify adverse effects on the brain development and function of prenatally-exposed laboratory rodents, and in children from residential uses of chlorpyrifos that are now banned (see review in PlosMedicine 2018).

The article by the trans-Atlantic scientific team of Drs. Axel Mie, Christina Ruden, and Philippe Grandjean is titled, “Safety of Safety Evaluation of Pesticides: developmental neurotoxicity of chlorpyrifos and chlorpyrifos-methyl”. The article provides a detailed and documented scientific analysis of the Dow Chemical data, showing what appears to be scientific misconduct – missing or unreported data, misinterpretations of evidence, and misstating observations of harm.

"We have looked at the study design and raw data from the manufacturer-funded study and found several weak points," says Axel Mie, assistant professor at Karolinska Institutet's Department of Clinical Research and Education at the Stockholm South General (Söder) Hospital. "For instance, we observed a clear effect on the height of the cerebellum in young rats that were exposed to the substance while still at the fetal stage, even at the lowest tested dose. This was reported neither in the study's summary nor in its conclusion."

Moreover, internal EPA documents show that its scientists and technical experts identified many of these problems in its written reviews, which appeared to go unheeded by EPA regulatory decision-makers (see Makris 2000 internal EPA memo), who I can only presume must have been under tremendous pressure from Dow Chemical.

Meanwhile, the unknowing public continues to eat poisoned fruits and vegetables.

The most egregious examples of scientific malfeasance that the trans-Atlantic team identified in the Dow studies are as follows:

Positive control fail should be an automatic study fail - Dow authors failed to detect any neurobehavioral effects from the study positive control test chemical, lead nitrate, even though lead is a well-known neurotoxicant. If the study can’t detect lead poisoning (or if the authors can’t recognize lead poisoning), then it (or they) can’t be relied upon to recognize chlorpyrifos poisoning. Positive control groups are included in studies to make sure the study works, that it can detect what it is designed to detect. If it fails to detect the effects of the positive control – lead poisoning from lead treatment – then absent some really impressive explaining the study should be tossed.

Scrambled brain – significant or not? The Dow study data indicated alterations in the structure and size of particular regions of the brains of prenatally exposed rodents (morphometric effects), at all treatment doses.  Dow pathologists dismissed these effects, but EPA scientists disagreed, instead noting that the effects should be considered statistically and biologically significant, and related to chlorpyrifos treatment (see Makris EPA memo, p. 9-11).

The missing data - EPA scientists also expressed concern that, “no brain measurements were presented in the data for the low- and mid-dose male [rodents] or for low-dose females at [post-natal day 66]; it is presumed that there were not measured”, and requested that these data be submitted to EPA for further examination. (see Makris EPA memo, p. 11). The trans-Atlantic team found that ” 40 out of 80 data points are missing in the [chlorpyrifos methyl] report, with an explanation provided only for 40% of the missing data (females on PND 72, unspecified “technical error at trimming”. Yipes! Failure to report on data is a serious breach of scientific protocol; failing to report on the data showing adverse effects will bias the study outcome in favor of Dow Chemical – something Dow surely must have considered.

The trans-Atlantic team concludes, “Our observations thus suggest that conclusions in test reports submitted by the producer may be misleading” and that, “The difference between raw data and conclusions in the test reports indicates a potential existence of bias that would require regulatory attention and possible resolution.” I agree.

This very important published analysis comes while the Trump Administration continues to fight a court-ordered ban on chlorpyrifos. In response to a lawsuit from NRDC and other farmworker and environmental health groups, a federal appeals court ruled in August that the EPA must ban the pesticide due to “scientific evidence that its residue on food causes neurodevelopmental damage to children.” Based on the analysis of EPA’s own scientists, the EPA originally proposed to cancel the use of chlorpyrifos on food crops in 2014 and again in 2016. Unfortunately, the Trump EPA political appointees reversed course promptly after President Trump took office (more info here).

About the Authors

Jennifer Sass

Senior Scientist, Federal Toxics, Health and Food, Healthy People & Thriving Communities Program

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