The Nazis developed organophosphates during World War II as nerve gas agents. (Sarin gas is in this family of chemicals.) After the war, the chemical companies adapted the organophosphates to be used as pesticides, primarily as insecticides.

Chlorpyrifos is an organophosphate pesticide first registered as an insecticide in the U.S. for both agricultural and residential uses, before Silent Spring and adoption of environmental and health standards in U.S. laws governing pesticide use.

EPA orders DowElanco to pay $876,000, the largest fine up to that time, for violating a federal law requiring it to report human health problems from chlorpyrifos.

Dow stops home uses of chlorpyrifos after EPA finds unacceptable risks to children who crawl on treated carpets or hug their pets after a flea bomb. Termiticide uses are also phased out.

EPA re-registers chlorpyrifos and the other organophosphates, purporting to bring them into compliance with health and environmental standards put in place after they were initially registered for use in the United States. EPA allowed risks of poisonings to workers to continue, ignored pesticide drift, and dismissed the growing evidence that prenatal exposures damage children’s brains.

Air monitoring detects chlorpyrifos at levels that exceed what EPA considered safe for children. California Air Resources Board monitoring finds chlorpyrifos at elementary schools and other sites near orange fields in Tulare County, California, at unsafe levels.

On behalf of United Farm Workers and other farmworker advocates, Earthjustice and Farmworker Justice file a lawsuit challenging EPA’s re-registration of chlorpyrifos despite the harm to workers and from toxic drift.

Pesticide Action Network and Natural Resources Defense Council file petition seeking a ban on chlorpyrifos based on evidence of brain damage from prenatal exposures and toxic drift.

On behalf of farmworkers and health advocates, Earthjustice files a petition asking EPA to protect children from pesticide drift.

Centers for Children’s Environmental Health and Disease Prevention Research at Columbia, Berkeley, and Mt. Sinai study children exposed to CPR in utero and find statistically significant neurodevelopmental harm including reduced IQ, delayed development, loss of working memory, and attention deficit disorders. A 2012 study found chlorpyrifos exposure led to changes in the physical structure of the developing brain.

EPA documents toxic drift from chlorpyrifos in its preliminary risk assessment, and EPA acknowledges its legal obligation to protect children from pesticide drift.

EPA reaches an agreement with the chlorpyrifos registrants to put buffer zones around schools, day cares, homes, playfields, and other places occupied by people. The buffer zones vary in size from 10 feet for groundboom applications, 10–50 for airblast applications depending on the amount applied, and 10–100 for aerial spraying depending on the amount applied and the droplet size. In setting the buffer zones, EPA ignored direct drift onto people and inhalation exposures from groundboom and airblast spraying.

EPA releases its revised human health risk assessment:

(1) acknowledging the extensive body of peer-reviewed science correlating chlorpyrifos exposure with brain damage to children and that the brain damage occurred at exposures far below EPA’s regulatory endpoint based on acute pesticide poisoning risks;

(2) finding acute poisoning risks of concern to workers from over 200 activities, including mixing and loading various pesticide formulations, airblast, aerial, and groundboom spraying, and re-entering fields after spraying to perform tasks like thinning, irrigating, and hand harvesting.

EPA represented that it was going to negotiate with the registrants to agree to mitigation or stopping activities that expose workers to excessive poisoning risks. By June 2015, those negotiations had stalled.

Declaring it “necessary to end the EPA’s cycle of incomplete responses, missed deadlines, and unreasonable delay,” the 9th Circuit Court of Appeals orders EPA to act on the 2007 petition to ban chlorpyrifos by Halloween.

EPA proposes to revoke all food tolerances based on drinking water contamination, but it holds open the possibility that it might be able to allow some uses to continue. EPA takes no action to stop nonfood uses or to protect workers from unacceptable risks. Publication date was 11/06/2015.

More than 80,000 people submit comments on the proposal, urging EPA to ban all uses of chlorpyrifos, not just on food crops, and to start proceedings to stop uses that harm workers. Some of the comments submitted during public comment periods on chlorpyrifos:

The 9th Circuit Court of Appeals gives EPA a deadline of March 31, 2017, to take final action on the 2007 petition to ban chlorpyrifos and its proposed revocation of food tolerances.

On behalf of United Farm Workers, California Rural Legal Assistance Foundation, Farmworker Association of Florida, GreenLatinos, Labor Council for Latin American Advancement, League of United Latin American Citizens, Learning Disabilities Association of America, Migrant Clinicians Network, National Hispanic Medical Association, and Pineros y Campesinos Unidos del Noroeste, Earthjustice and Farmworker Justice petition EPA to immediately suspend all chlorpyrifos uses that pose unacceptable risks to workers, and to cancel all uses of chlorpyrifos.

EPA releases a revised human health risk assessment that uses neurodevelopmental effects as its regulatory endpoint. The new risk assessment found that:

• All food exposures exceed safe levels; children 1–2 years of age are exposed to 140 times the “safe” levels
• There is no safe level of chlorpyrifos in drinking water
• Toxic spray drift reached distances of 300 feet or more from the field’s edge
• All workers who mix and apply chlorpyrifos are exposed to unsafe levels of the pesticide even with maximum personal protective equipment and engineering controls
• Field workers are allowed to re-enter fields within 1–5 days after pesticide spraying, but unsafe exposures continue on average 18 days after applications

Public interest groups submit technical comments on EPA Proposal To Revoke Chlorpyrifos Tolerances.

Food safety laws require EPA to revoke food residue tolerances after making the determination that there are no safe food uses of a pesticide. Because EPA’s November 2016 risk assessment found that there are no safe food uses of chlorpyrifos, tolerance revocation must necessarily follow. Therefore, the farmworker and health advocate groups withdrew their September 2016 Chlorpyrifos Suspension Petition as tolerance revocation would end most uses of chlorpyrifos that harm workers.

Two days before a court ordered deadline, the EPA refuses to ban chlorpyrifos, despite the overwhelming evidence that the pesticide harms children, workers and the environment.

Earthjustice—representing Pesticide Action Network and the Natural Resources Defense Council—asked the court to order the EPA to act based on the agency's own scientific conclusions, which, under the law, would require EPA to ban chlorpyrifos. Read the legal document.

EPA opposes the April 5 motion. Read the legal document.

A dozen health, labor and civil rights organizations, represented by Earthjustice filed an administrative appeal to the EPA, urging the federal government to ban chlorpyrifos. The attorney generals of New York, California, Washington, Massachusetts, Maine, Maryland and Vermont filed their own appeal calling for a ban also Monday. It is now up to EPA Administrator Scott Pruitt to decide the appeal.

The Associated Press reveals that EPA Administrator Scott Pruitt met briefly with the chief executive of Dow Chemical—the largest producer of chlorpyrifos in the United States—before reversing EPA's push to ban the pesticide.

The 9th Circuit Court of Appeals declined to order EPA to make a new decision on banning chlorpyrifos, leaving the validity of the March EPA order to the administrative appeal and related lawsuit.

Senators Tom Udall (D-NM), Kirsten Gillibrand (D-NY), Cory Booker (D-NJ), Richard Blumenthal (D-CT), Kamala Harris (D-CA) , Dick Durbin (D-Ill.), Ben Cardin (D-MD) and Edward J. Markey (D-MA) unveiled a first-of-its-kind bill—The Protect Children, Farmers & Farmworkers from Nerve Agent Pesticides Act, S. 1624—that would ban chlorpyrifos. Take action to tell your senators you support S. 1624.

The EPA releases a series of documents in response to a FOIA request submitted by Earthjustice for communications between the agency and Dow, as well as certain trade associations. Dow Chemical is the largest producer of chlorpyrifos in the United States. Read the documents.

The court grants a motion to expedite the case and denied EPA’s motion to dismiss Earthjustice’s petition to review the Pruitt order on chlorpyrifos. Several states who have also called for a chlorpyrifos ban were granted permission to intervene in the case. Earthjustice’s opening brief is due on January 23.

A National Marine Fisheries Service (NMFS) biological opinion finds that chlorpyrifos, malathion, and diazinon — all organophosphate pesticides — harm salmon and their habitat to the point that their survival and recovery are at risk. NMFS crafted the report to comply with a 2014 court deadline for the agency to determine whether these pesticides threatened to salmon with extinction. The biological opinion offers three options for protective measures including buffer zones, spray reduction technologies and pesticide stewardship programs. Read more about this case and the report.

Final arguments challenging EPA's refusal to ban chlorpyrifos took place in the 9th Circuit Court of Appeals in Seattle, Washington. It was the last hearing where health and labor groups represented by Earthjustice — as well as states — presented their arguments for a ban. A decision could happen in weeks or months.

The 9th Circuit Court of Appeals rules that EPA finalize its proposed ban on chlorpyrifos within 60 days, based on undisputed findings that the pesticide is unsafe for public health, and particularly harmful to children and farmworkers. “The Court ended EPA's shameful actions that have exposed children and farmworkers to this poison for decades,” said Earthjustice attorney Marisa Ordonia. “Finally, our fields, fruits, and vegetables will be chlorpyrifos-free.”

The U.S. EPA asks the 9th Circuit Court of Appeals to re-hear the case. Filing the request has the effect of postponing the effectiveness of the court order. “Trump’s EPA is delaying the inevitable and putting people in harm’s way. By keeping this unsafe pesticide in our food and drinking water, EPA is violating the law,” said Patti Goldman, managing attorney at Earthjustice. “Every day we go without a ban, children and farmworkers are needlessly eating, drinking and breathing this dreadful pesticide.”

Independent scientists publish in the journal Environmental Health a review of Dow-funded chlorpyrifos studies, finding flaws in design, execution, and statistical analysis. Dow frequently points to the findings of animal studies it has funded to counter the growing body of scientific evidence demonstrating harm to children’s brains from low level exposure to chlorpyrifos.

The 9th Circuit Court of Appeals grants U.S. EPA's request to rehear the case in which the court decided that the agency must ban chlorpyrifos within 60 days.

The 9th Circuit Court of Appeals holds oral arguments in the rehearing.

The 9th Circuit Court of Appeals issues its ruling, ordering the U.S. Environmental Protection Agency to decide by mid-July whether to ban chlorpyrifos, the organophosphate pesticide linked to neurodevelopmental damage in children.