A crop-dusting plane from Palmer Aviation Services sprays a field of corn north of Manito, Ill., in July 2011. The approval of genetically engineered crops that are resistant to 2,4-D would lead to increased use of the toxic herbicide. Ron Johnson/Journal Star/AP

While my sister-in-law put the finishing touches on Thanksgiving dinner, I listened to her friend recount the losing battle her husband, a Vietnam veteran, fought with lung cancer. She explained her husband’s illness was caused by his wartime exposure to the toxic defoliant Agent Orange, produced primarily by two companies, Dow Chemical and Monsanto. Named for the colored band on its transport tanks, Agent Orange was a cocktail of chemicals, including an herbicide called 2,4-D. Shortly after the spraying — conducted to deprive guerrilla fighters of cover and a food supply — started in 1962, reports began to emerge of serious health effects, from birth defects to other illnesses. To this day, the U.S. Department of Veteran Affairs offers an Agent Orange registry health exam for the possible long-term problems caused by exposure, and more than 40,000 veterans have submitted disability claims. The Red Cross estimates that 1 million Vietnamese were affected, including third-generation children born with severe birth defects.

In January the U.S. Department of Agriculture opened a public comment period on the environmental and health impacts of a new suite of crops engineered to be resistant to 2,4-D. These corn and soybean plants, produced by Dow AgroSciences, a subsidiary of Dow Chemical, would be the first developed to be resistant to the herbicide.

According to experts, the introduction of these new crops could cause 2,4-D use to jump, big time. Chuck Benbrook, a pesticide policy expert with the Center for Sustaining Agriculture and Natural Resources at Washington State University, has estimated that if it’s approved, the engineered corn could cause applications of 2,4-D to jump 20-fold by 2019.

That’s particularly concerning because experts have long shown that 2,4-D causes serious harm to humans, especially when used over vast swaths of farmland and lawns. Largely because of such concern, the Natural Resources Defense Council (NRDC) petitioned the Environmental Protection Agency to revoke the chemical’s approval, first granted in 1948.

NRDC researchers and other critics of 2,4-D point to studies showing the chemical is a neurotoxin and that exposure to it can cause hormone disruption, certain forms of cancer and genetic mutations. The chemical has also been linked to lowered sperm counts, liver disease and Parkinson’s disease as well as adverse effects on reproductive and immune systems. What’s further worrisome is that 2,4-D is known to drift, affecting areas near farms, including streams, rivers and wildlife.

In April 2012 the EPA rejected the NRDC’s petition, stating that the group did not prove that the chemical was unsafe in the manner it is used. Despite the EPA’s actions, public health advocates have maintained that there are serious human health impacts, based on compelling evidence from peer-reviewed studies around the world. A University of Minnesota study found a greater frequency of genetic mutations in pesticide applicators who had higher rates of 2,4-D in their urine. A National Cancer Institute study found farmers exposed to 2,4-D upward of 20 days a year had a higher risk of developing non-Hodgkin’s lymphoma than nonfarmers did, by a factor of six. The EPA’s fact sheet notes that the chemical has shown toxic effects on the thyroid and gonads and expresses concern about potential “endocrine-disrupting effects.”

With all these risks, why are chemical companies like Dow and Monsanto formulating seeds to be resistant to this decades-old chemical with a terrible health track record? The USDA said these new crops are intended to “help address the problem of weeds that have developed resistance to other herbicides.”