The smell floated into the cabbage field about six weeks after the newly installed Trump administration overturned a planned ban of one of the world’s most potent and toxic pesticides.

It was early on Cinco de Mayo — May 5, 2017 — but there was no day off on this holiday. In three groups, 48 farmworkers were scattered around a field in California’s Central Valley. Vicenta Rivera, 49, was one of the first to feel it — a pesticide drift, the agriculture industry calls it, in this case of chlorpyrifos, that had been sprayed on mandarin oranges. There was a strong odor, a taste in the back of the throat, numb lips, itchy skin and watery eyes. A headache set in quickly.

Some workers scurried to cars to avoid the toxic air. Others kept picking and packing, afraid of the repercussions of walking away. Women coughed. Some vomited.

Soon, nearly everyone felt the burning sensation and queasiness. Firetrucks and ambulances came.

Lopez remembers being ushered behind curtains — a makeshift room for roadside decontamination — given medicine and released. No longer working the fields and riddled with health problems, Lopez said, “All I want to know is am I going to be OK?”

Had Donald Trump not won the presidency, millions of pounds of chlorpyrifos most likely would not have been applied to U.S. crops over the past 21 months. The administration’s choice not to curb the use of chlorpyrifos is a case study in how ideological and special-interest considerations outweighed decades of evidence about the potential harm associated with its use.

Widespread concerns about chlorpyrifos led to its removal for nearly all residential uses in 2000. Pushed by environmental groups, the EPA agreed in 2015 to ban it on food crops. It released its revised human health risk assessment in November 2016 and was ordered by a court to “take final action” by the end of March 2017.

Days after the health risk assessment was released, Trump won the election. DowDuPont, the leading maker of the pesticide, donated $1 million to his inauguration. Trump’s EPA chief Scott Pruitt quashed the chlorpyrifos ban on March 29, 2017.

Chlorpyrifos is part of the same chemical family as sarin nerve gas. Its effect on humans is the subject of some debate but its toxicity is not in doubt. Acute poisonings can result in respiratory distress, vomiting, convulsions, unconsciousness and death.

There also is concern over the impact of low-level exposure, including in drinking water and on the fruits and vegetables we eat. It has been linked to neurodevelopmental problems in babies, such lower IQs, attention deficit problems and disorders on the autism spectrum — symptoms found with higher frequency in farming communities, studies suggest.

Those in the agriculture and chemical industries who say there is still doubt about the health risk often point, paradoxically, to the problem that the chemical is too toxic to be tested on humans. “Our attitude is not that chlorpyrifos is good or bad,” said Gabriele Ludwig, director for sustainability and environmental affairs for the California Almond Board. “We don’t know.”

Dennis Johnston is a fourth-generation farmer in Exeter. For him, chlorpyrifos is a go-to pesticide when others do not work. He is using it less and less but simply would like to have it available.

Farmers are unsure how to proceed. Will the appeals court force the EPA to ban chlorpyrifos? Will the state step in? And what if any of that happens while this season’s crop is growing? “We’re cautious about it, as most farmers are,” Johnston said. “Will it kill us if we can’t use it? Probably not. Can we make something else work? Yes, but it’s what happens when you lose it. You have to use something in a different manner, or more of it. It’s kind of a daisy chain sometimes.”