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The Chlorpyrifos Controversy

Chlorpyrifos is an “organophosphate“ insecticide that kills insects by inhibiting the action of acetylcholinesterase, an enzyme that normally degrades acetylcholine, a “neurotransmitter” that is critical to the functioning of the nervous system.

“Science not silence,” screamed the signs carried by many of the marchers in a number of American cities last Earth Day. They were protesting what is being seen as an increasingly callous approach to science by President Trump and his administration.  An often-highlighted example is the Environmental Protection Agency’s (EPA) reversing a recommendation made by the Agency during the Obama era to ban the widely used pesticide, chlorpyrifos.  The decision was made by newly appointed EPA Administrator Scott Pruitt, a non-scientist, who has also rejected the scientific consensus on climate change and as Oklahoma Attorney General sued the EPA over a dozen times in an effort to fight regulations concerning issues such as mercury pollution and methane emissions.

Chlorpyrifos is an “organophosphate“ insecticide that kills insects by inhibiting the action of acetylcholinesterase, an enzyme that normally degrades acetylcholine, a “neurotransmitter” that is critical to the functioning of the nervous system. When this enzyme is inhibited, acetylcholine builds up and insects die from overstimulation of their nervous system. Since the human nervous system also depends on acetylcholine, concerns about chlorpyrifos’ neurotoxicity had to be addressed before the insecticide was introduced in 1965 by the Dow Chemical Company. Studies at the time showed that when used as indicated, the amount of chlorpyrifos that people were likely to encounter was too small to have any effect. These studies, however, were mostly carried out with animals and examined acute, or short-term effects.

Chlorpyrifos’ popularity increased when DDT was banned in the early 1970s because of neurotoxic effects and it quickly became one of the most common pesticides used in household insect control as well as in commercial farming. Since it is cheap and effective, it is commonly sprayed on cotton, corn, almonds and fruit trees, raising questions in many people’s minds about the presence of residues. The acceptable daily intake (ADI) of chlorpyrifos has been determined to be 0.3 micrograms per kg of body weight. Surveys have shown that in the general population consumption from residues is about 0.009 micrograms per kg, in other words about 30 times less than the ADI. There can also be a small contribution from water supplies in some areas due to agricultural run-off.

Since the chemical has now been used for half a century, researchers have had a chance to study not only acute exposure in humans, but also exposure to small doses over a long period and have come up with some disturbing findings. Household use was linked with neurotoxicity in the 1990s, resulting in a chlorpyrifos ban for this purpose in 2001.  The ban had an interesting spinoff, making possible a comparison of possible toxic effects before the ban and after it, when exposure was greatly reduced.  A research team at Columbia University, led by Virginia Rauh, found that children born before the ban who were exposed to the household insecticide in infancy were many fold more likely to develop intellectual and attentional disorders including than those born after the ban. Follow-up studies have shown across the board long-term impairments in working memory, IQ, lower body weight and possibly ADHD as well as Autism. That is worrisome but the exposure from household use was significantly greater than what would be encountered with residues on food.

As far as acute toxicity goes, there have been instances of chlorpyrifos poisoning in countries where the household ban is less strict. In 2011, Sarah Carter, a 23 year old New Zealander, died after staying in a Thai hotel room that had been sprayed with the pesticide to kill a bed-bug infestation. An investigation by the television program 60 Minutes revealed high concentrations of the chemical in her room, something the Thai government apparently tried to cover up to protect a lucrative tourist industry.

As is often the case with toxicological studies, controversy abounds. The Dow Chemical Company conducted extensive research on chlorpyrifos and claims that its 1989 study found no statistical difference in neurological impairment, symptoms, or illness between workers who had been exposed to organophosphates and those who had no contact with the chemical. However, in 2008, a dozen Stanford neuroscience researchers produced an in-depth review of all of Dow’s data, including the 1989 study, and identified “incredible amounts” of elementary science mistakes, including arbitrarily rejecting results, failing to pursue near-significant findings, and various logical errors.

With growing uncertainty about the safety of chlorpyrifos, environmentalist groups set out to lobby the EPA for a total ban of the chemical. After years of pleading, in November 2016, under the Obama administration, the EPA finally produced a ground breaking report agreeing that that there was enough evidence of the pesticide’s negative effects to go ahead with a ban. Then came the presidential election and the overturning of the proposed ban with Scott Pruitt citing the need to use “sound science in decision-making” and stating that there is not enough certainty in the researchers’ conclusions to go forward with the ban. Environmental groups, on the other hand, maintain that the research is “iron-clad” and that chlorpyrifos should be banned, especially given that safer alternatives are available. The “safer alternatives” question is debatable, especially with almonds where no other insecticide is as effective. One possible alternative with some crops is the introduction of a gene that codes for the production of the Bacillus thuringiensis toxin but activist groups generally oppose genetic modification, curiously even when it provides a safer alternative to chlorpyrifos.

Since taking office, President Trump has emphasized reducing regulations, which of course is welcomed by industry, including Dow Chemical. Farmers also claim that limiting chlorpyrifos will hamper production, increasing the cost of produce. Currently, the evidence indicates that exposure from being in the vicinity of a spraying operation could be a problem especially during pregnancy, but that chlorpyrifos residues on food are unlikely to be worrisome. For those consumers who wish to limit exposure from food, there is always the organic alternative. Unfortunately, science often leaves us with an array of “ifs,” “buts” and “maybes.” The only certainty about chlorpyrifos is that there will be more studies claiming potential harm and others that dismiss concerns.

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