Experts debate use of epidemiology study to decide chlorpyrifos' fate

WASHINGTON, April 27, 2016 - The end of chlorpyrifos on farms? Not yet. An epidemiological study that linked chlorpyrifos exposure to neurodevelopmental effects in children may not be enough to support EPA’s proposed revocation of tolerances for the widely used organophosphate insecticide. But EPA’s eventual decision could have far-reaching consequences on many other crop protection products.

After a three-day meeting that included presentations from EPA and comments from industry, environmental and farmworker representatives, the members of a Scientific Advisory Panel (SAP) convened by the agency said they were concerned about the use of one study in particular to determine the regulatory fate of chlorpyrifos. The chemical, originally developed by Dow Chemical, is sold under the trade names of Lorsban and Dursban.

Chlorpyrifos has been used in agriculture since the mid-1970s and is now applied on a wide variety of crops, including corn, soybeans, fruit and nut trees, Brussels sprouts, cranberries, broccoli and cauliflower. Usage has fallen since 1994, when about 12.5 million pounds were applied in the U.S., to about 7 million pounds in 2013, according to the U.S. Geological Survey.

The panel now has 90 days to present EPA with its report and does not have to reach a consensus, but it sounded as if its conclusions will not differ greatly from previous SAPs, which said epidemiological data alone were not enough to set a safe level for chlorpyrifos exposure. Epidemiology is the study of patterns, causes, and effects of disease conditions in defined populations.

Three epidemiology studies were included in EPA’s analysis, but based on recommendations from a 2012 SAP panel, the agency took a closer look at results from one conducted by the Columbia Center for Children’s Environmental Health (CCCEH), which used cord blood data in pregnant women to estimate exposures to the fetus. The CCCEH study found IQ and memory deficits in seven-year-olds who had been exposed to chlorpyrifos in the womb.

Cholinesterase inhibition refers to the long-accepted method of measuring the effects of organophosphates, which act on the nervous systems of insects “by interfering with, or ‘inhibiting’ cholinesterase,” according to Extoxnet, the Extension Toxicology Network. “While the effects of cholinesterase inhibiting products are intended for insect pests, these chemicals can also be poisonous, or toxic, to humans in some situations.”

Cholinesterase is “one of many important enzymes needed for the proper functioning of the nervous systems of humans, other vertebrates, and insects,” the web site explains.

Summarizing results of three children's environmental health studies, including the Columbia study, EPA said in its proposal to revoke chlorpyrifos tolerances that the authors “consistently identified associations with neurodevelopmental outcomes in relation to (organophosphate) exposure. There is evidence of delays in mental development in infants (24-36 months), attention problems and autism spectrum disorder in early childhood, and intelligence decrements in school age children who were exposed to chlorpyrifos or OPs during gestation.”

“I have great respect for the discipline of epidemiology,” said SAP member Alvin V. Terry Jr., an associate vice president for Basic Science Research at Augusta University in Augusta, Georgia. “But I don’t believe epidemiology alone should drive a decision of this magnitude.” He said epidemiological and animal data should be looked at in combination in order to derive a POD. “What does the preponderance of the evidence suggest?” he asked.

“I fully think that additional information is required, and I encourage you (EPA) to try to come up with experimental approaches,” said James McManaman, professor and chief of the Basic Reproductive Sciences section at the University of Colorado-Denver’s Department of Obstetrics & Gynecology, Physiology and Biophysics.

And Sharon Sagiv, assistant adjunct professor of epidemiology at Berkeley’s School of Public Health, said the use of epidemiological studies can have significant value for risk assessments.

“Animal studies can take us just so far. We’re not rats.”

However, the heavy reliance on the Columbia study unsettled her. “I think that using one study… does set a bad precedent,” she said.

Representatives from Dow AgroSciences, Syngenta and CropLife America all spoke against the use of epidemiological studies in general — and the Columbia study in particular — to support tolerance revocation. “Any causal link of chlorpyrifos with working memory and Full Scale IQ within the CCCEH or other studies has not been established,” Dow said in comments to EPA.

The industry representatives also expressed concern that they had not been able to analyze the raw data from the Columbia study, which have not been available for review. In addition, they raised questions about “confounding factors” – other risk factors such as lead, for example, or even the method of childbirth.

There is also the issue of replicating the Columbia study, which some on the panel said would not be possible because chlorpyrifos was voluntarily pulled for residential use in 2000.

But panel member Stella Koutros, an investigator in the Division of Cancer Epidemiology and Genetics at the National Cancer Institute, said, “It’s shocking to me” that some of the panelists could not accept Columbia’s conclusions, and defended the use of the cord blood data.

Jennifer Sass, speaking for Natural Resources Defense Council, Farmworker Justice and other groups, supported the use of the Columbia study, saying that “the critical effect for risk assessment is neurodevelopmental outcomes in children because these represent the most sensitive endpoints, not 10 percent (acetylcholinesterase) inhibition.”

In comments submitted to EPA, the groups said EPA had “performed a robust analysis which reveals risks of concern for women of child-bearing age and children from food, drinking water and occupational exposures.” Not only should EPA revoke food tolerances for chlorpyrifos, the agency should cancel all registrations, they said.

USDA weighed in at the meeting, as well. “The shift EPA is suggesting – from an established point of departure based on acetylcholinesterase inhibition to a new point of departure based on the Columbia University epidemiological study – is momentous and cannot be understated,” said Sheryl Kunickis, director of USDA’s Office of Pest Management Policy.

EPA’s decision about how to regulate chlorpyrifos “will reach far beyond this one active ingredient, and will affect not only how other organophosphates are regulated, but many other broad classes of pesticides as well,” Kunickis said.

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