In just two years, President Trump has unleashed a regulatory rollback, lobbied for and cheered on by industry, with little parallel in the past half-century. Mr. Trump enthusiastically promotes the changes as creating jobs, freeing business from the shackles of government and helping the economy grow.

The trade-offs, while often out of public view, are real — frighteningly so, for some people — imperiling progress in cleaning up the air we breathe and the water we drink, and in some cases upending the very relationship with the environment around us.

Since Mr. Trump took office, his approach on the environment has been to neutralize the most rigorous Obama-era restrictions, nearly 80 of which have been blocked, delayed or targeted for repeal, according to an analysis of data by The New York Times.

With this running start, Mr. Trump is already on track to leave an indelible mark on the American landscape, even with a decline in some major pollutants from the ever-shrinking coal industry. While Washington has been consumed by scandals surrounding the president’s top officials on environmental policy — both the administrator of the Environmental Protection Agency and the Interior secretary have been driven from his cabinet — Mr. Trump’s vision is taking root in places as diverse as rural California, urban Texas, West Virginian coal country and North Dakota’s energy corridor.

While the Obama administration sought to tackle pollution problems in all four states and nationally, Mr. Trump’s regulatory ambitions extend beyond Republican distaste for what they considered unilateral overreach by his Democratic predecessor; pursuing them in full force, Mr. Trump would shift the debate about the environment sharply in the direction of industry interests, further unraveling what had been, before the Obama administration, a loose bipartisan consensus dating in part to the Nixon administration.

In the words of Walter DeVille, who lives on the Fort Berthold Indian Reservation in North Dakota, “This is our reality now.”

Dismissing Science

BAKERSFIELD, Calif. — The spring air was cool. There was the slightest breeze. The smell floated into the cabbage field about six weeks after the newly installed Trump administration brushed aside scientifically established health concerns and overturned a planned ban of one of the world’s most potent pesticides.

It was the early morning of Cinco de Mayo — May 5, 2017 — but there was no day off on this holiday. In three groups, 48 farm workers, most of them women, were scattered around a field in the southern part of California’s vast and flat Central Valley. Some did the back-aching work of bending over and using a knife to chop the heads from the plants. They passed them up to packers standing on a flatbed trailer behind a tractor.

Vicenta Rivera, now 49, was one of the first to feel it — a pesticide drift, the agriculture industry calls it, in this case of chlorpyrifos, one of the most powerful and toxic pesticides in widespread use, that had been sprayed on a nearby grove of 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 nearby cars to avoid the toxic air. Others kept picking and packing, squinting and covering their faces and trying not to breathe. They were afraid of the repercussions of walking away. They needed the money. Women coughed. Some vomited.

Bricmary Lopez fainted. A 37-year-old mother of three, she remembers the smell, the dizziness, the overwhelming feeling of nausea. Other workers thought she was faking it, trying to be funny, when she hit the ground and started convulsing.

“We were teasing each other, saying, ‘Ha, you don’t want to work,’” said Lucia Martinez Polido, 57.

Reality bit. Soon, nearly everyone felt the burning sensation and queasiness. Some fainted. Those not immediately incapacitated helped the others. They put a pillow under Ms. Lopez’s head and stood over her, waving their arms over her face, trying to offer fresh air.

Fire trucks and ambulances came. They stayed out on Copus Road, a couple of hundred yards beyond the cabbage field and adjacent almond groves, because the rescuers did not want to get exposed.

Ms. Lopez remembers being ushered behind curtains, a makeshift room assembled for roadside decontamination. She remembers being naked in a temporary shower, and the ambulance ride to a Bakersfield hospital, about 20 miles away.

She said she was given medicine and released. No longer working the fields and riddled with health problems, Ms. Lopez has been searching for answers.

“All I want to know,” she said at her home recently, “is am I going to be O.K.?”

Trump Wins, and E.P.A. Quashes Chlorpyrifos Ban

Had Donald J. Trump not won the presidency in 2016, millions of pounds of chlorpyrifos most likely would not have been applied to American crops over the past 21 months. It would not have sickened substantial numbers of farm workers, or risked what the Environmental Protection Agency’s own studies suggest could be continued long-term health problems for others exposed to the chemical at low levels.

Widespread concerns about chlorpyrifos led to its removal for nearly all residential uses in 2000. Environmental groups kept pushing, and two filed a petition with the E.P.A. in 2007 to ban it entirely on food crops. The E.P.A. eventually agreed in 2015, 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 assessment was released, Mr. Trump won the election. DowDuPont, the leading maker of the pesticide, donated $1 million to his inauguration. One of the early acts of the man Mr. Trump appointed to head the E.P.A., Scott Pruitt, was to quash the chlorpyrifos ban on March 29, 2017.

Since taking office, Mr. Trump has consistently sided with powerful economic constituencies in setting policy toward the air we breathe, the water we drink and the presence of chemicals in our communities.

In the process, he has frequently rejected or given short shrift to science, an instinct that has played out most visibly in his disdain for efforts to curb global warming but has also permeated federal policy in other ways. Mr. Trump has expressed skepticism about the safety and effectiveness of vaccines. His administration supported rolling back safeguards for workers exposed to some toxic substances. And in rolling back nearly 80 environmental regulations, he has regularly played down findings that bolstered the need for the rules in the first place.

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.

The effects of such large-scale decision making are felt locally. And in the case of chlorpyrifos, there may be no place where the tension between science and the administration’s policy inclinations plays out more than in California’s Kern County, the vast crosshatched expanse roughly the size of New Jersey that surrounds Bakersfield.

California is the nation’s leading state for agriculture production. Kern County is a primary hub, with more than $7 billion in agricultural commodities in 2017, led by almonds, citrus and grapes, among dozens of crops.

It also leads the state in the use of chlorpyrifos. And its toll may extend well beyond farm workers. Neighborhoods and schools are carved out of old orchards and groves. They may be separated from crops by mere feet, maybe a road. Even in the middle of the small towns, nobody is far from the fields or the nearly 29 million pounds of pesticides — including 200,000 pounds of chlorpyrifos — spread on them in 2016, just within Kern County.

“For the federal government to not just ignore, but throw away, 10 years of science showing that chlorpyrifos is damaging, we feel that at the local level,” said Valerie Gorospe, a community organizer in Delano, near Bakersfield, whose mother was a longtime advocate for farm workers and pesticide overhaul.

Weighing the Risk When Exposed

Chlorpyrifos is part of the same chemical family as sarin nerve gas. An estimated six million pounds are spread each year across dozens of agricultural crops nationwide, including alfalfa, almonds, citrus, corn, cotton and grapes. It is useful as a broad-spectrum pesticide, farmers and agricultural experts say, because it kills virtually every kind of insect.

The chemical’s effect on humans is the subject of some debate but its toxicity is not in doubt. Acute poisonings, from things like spills or drift, can result in respiratory distress, vomiting, convulsions, unconsciousness and death.

There is also broad concern over the impact of low-level exposure, including in drinking water and on the fruits and vegetables we eat. Chlorpyrifos is deemed particularly dangerous to young women in farming communities, as studies have found it in the blood of pregnant women and their babies. It has been linked to neurodevelopmental problems such as reduced birth sizes and weights, lower I.Q.s, 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 degree of the health risk often point, paradoxically, to the problem that the chemical is too toxic to be tested on humans, leaving scientists to rely on epidemiological studies of people who might have been exposed to it in their normal environment over long periods.

An increasing number of the studies show correlations to neurological problems in children, among other issues. But are their problems the result of chlorpyrifos?

Dow says no, and other people associated with the agriculture industry, in an echo of the argument against aggressive action to confront a warming planet, say not enough is known yet to come to a firm conclusion.

“Our attitude is not that chlorpyrifos is good or bad,” said Gabriele Ludwig, the director for sustainability and environmental affairs for the California Almond Board. “We don’t know. We feel it hasn’t gone through the proper process.”

Hundreds in California have been acutely sickened by chlorpyrifos in the last 15 years, mostly farm workers and mostly by drift. The incidences have slowed since 2015, when state regulators added restrictions to chlorpyrifos use, requiring licensing and training.

But as chlorpyrifos use has continued, so have the accidents — at least a half-dozen reported episodes of drift in California, including the one in May 2017 that sent Ms. Lopez to the hospital and sickened 36 other cabbage pickers.

More recently, in July, 10 workers were sickened in Solano County, between Sacramento and San Francisco. They were working in a sunflower field when chlorpyrifos apparently drifted from sprayers in an adjacent almond orchard.

The vast majority of incidents, involving chlorpyrifos or any other pesticide, go unreported. Most farm workers in California are undocumented immigrants. They worry about everything from missing much-needed work to reprisal from bosses and deportation from the authorities.

“There is no incentive to report these things,” said Eriberto Fernandez, the research and policy coordinator for the United Farm Workers Foundation in Bakersfield, whose parents have worked the fields for decades.

Most farm workers, like most other people in Kern County, do not distinguish chlorpyrifos among hundreds of pesticides used in agriculture. They may smell different, or cause different reactions, but few, if any, can name them. That was certainly true during the drift incident in May 2017. No one knew it was chlorpyrifos that was sickening the workers, because most had never heard of it until the Trump administration halted the proposed ban.

“A pesticide is a pesticide,” said Lucia Martinez Polido, one of the women picking cabbage that day.

If pesticides are not floating in the air, they are smeared on their fingers and skin from nearly everything they touch. Shiny citrus leaves are dulled by a white film. Pruning grape vines at the end of the season, for example, can free a chemical odor. Kicking up dust releases it from the soil.

It is why many farm workers cover their skin from head to toe, wrapping bandannas around their noses and mouths, even in the teeth of triple-digit summer days. They bring snacks to the fields that they do not have to touch with their hands. They carry their own bottles of eye wash.

But concern over chlorpyrifos extends well beyond farm workers and acute poisonings.

Each spring, the dozens of schools and licensed child-care centers in Kern County within a quarter mile of an active crop receive a list of the restricted-use pesticides that the growers plan to use, sometime between July 1 and June 30. Some of the lists contain more than 50 different chemicals. Chlorpyrifos is often one of them.

If the pesticides are known to drift, like those spread by crop dusters, they cannot be used within a quarter mile of schools, this year’s notifications state. If they are less susceptible to drift, “such as most applications using a tractor,” the buffers shrinks to 25 feet. Monday through Friday, they cannot be sprayed from 6 a.m. to 6 p.m.

Precautions at the Day Care Center

Among those notified is Sunset Child Development Center, a day care south of Bakersfield. It is part of the Arvin Migrant Center, commonly called a “labor camp,” with rows of housing for itinerant farm workers.

The day care is next to grape vineyards. The children there range from 6 months to 5 years old. Because most of the parents are farm workers themselves, they drop off their children as early as 4:30 a.m. — within the allowable time slot for spraying pesticides like chlorpyrifos.

One parent of a 3-year-old girl is Byanka Santoyo, 28, a single mother from Arvin. She is a daughter of farm workers, and spent time in the fields herself.

Ms. Santoyo works as a community organizer for the Center on Race, Poverty and the Environment. It was the area’s notoriously polluted air, sometimes the worst in the nation, that led her to activism. It was the drift episodes experienced by her parents over the years, and now the concern over her daughter’s health, that pulled her toward fighting unsafe use of pesticides.

“We all live surrounded by ag,” Ms. Santoyo said. “We just want it to be safe.”

The day care center is required to allow the children outside for parts of the day. There are sun shades to protect from the heat, but nothing to protect from whatever pesticides might be drifting in the air. Playground equipment is scrubbed several times a week because it often becomes covered in a sticky film that sometimes works its way indoors. Ms. Santoyo says her daughter’s clothes get tacky, and the residue is visible when she wears something like black leggings.

Chlorpyrifos is not the most-used pesticide, but it is the most contentious. In California in 2016, according to the state’s Department of Pesticide Regulation, 902,575 pounds of chlorpyrifos was used in more than 12,000 applications across 640,000 acres on more than 60 different crops.

The fight to ban chlorpyrifos in California has several fronts, fought by organizations that include the United Farm Workers, the Natural Resources Defense Council and Californians for Pesticide Reform. They are ramping up pressure on state regulators and politicians as uncertainty over the federal ban persists.

This August, a three-judge panel from the United States Court of Appeals for the Ninth Circuit, ordered the E.P.A. to ban it. In its ruling, the panel wrote, “There was no justification for the E.P.A.’s decision in its 2017 order to maintain a tolerance for chlorpyrifos in the face of scientific evidence that its residue on food causes neurodevelopmental damage to children.”

The ruling has been appealed, and in the meantime California is weighing tighter regulations of its own. While lawyers and lobbyists fight, the pesticide, sold under a variety of brand names like Lorsban and Vulcan and employed by farmers nationwide, remains in use.

What Happens, a Farmer Asks, If It Is Banned?

Dennis Johnston is a fourth-generation farmer in Exeter, east of Bakersfield. He remembers using chlorpyrifos on the citrus groves since he left college in 1980.

“We use it for katydid, which takes a bite out of the fruit when it’s in the flower form,” Mr. Johnston said, referring to a pest that looks like a grasshopper. “It’s a little bite when they do it, but it grows and becomes a big chunk when it’s ripe and makes it unmarketable.”

Chlorpyrifos, to Mr. Johnston and countless other farmers, is a go-to pesticide when others do not work. But Mr. Johnston, who said he has never had a drift incident, is using it less and less. He simply would like to have it available.

The cloudy future of chlorpyrifos has left farmers like Mr. Johnston unsure how to proceed. Will the appeals court force the E.P.A. to ban it? Will California 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,” Mr. Johnston said. “Will it kill us if we can’t use it? Probably not, not in my business. 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.”

That concern echoes across the agricultural landscape, in Kern County and beyond.

“That is a real dance that goes on out in the grove,” said Jim Cranney, the president of the California Citrus Quality Council. “You can have, at some point, the need to use a pesticide, but in other cases, you can allow the insects already in a grove to get rid of the harmful insects. When we use a pesticide that is harmful to the beneficial insects it can cause harmful insects to go haywire.”

In November, in the absence of federal guidelines over chlorpyrifos, California’s Department of Pesticide Regulation announced tighter, though voluntary, restrictions, effective Jan. 1. The state agency wants to declare the chemical a “toxic air contaminant,” which would mean much stricter rules on chlorpyrifos use. But the process could take two years, so the agency made recommended changes in the interim.

That was little relief to people like those in the Coalition Advocating for Pesticide Safety.

In November, in Lindsay, 60 miles north of Bakersfield, the group opened a storefront office. Staffed by volunteers, it is seen as a safe place for farm workers and concerned residents — most of them Latino, many of them undocumented — to ask questions without worry over repercussions from labor contractors or government officials.

On the night of its opening, about 50 people stood in the dark parking lot. There were balloons and a ribbon to be cut. Jill Stein, the Green Party presidential candidate who entered politics partly because of concerns over pesticides, gave a short speech, pausing as each line was translated to Spanish.

She spoke the language of organophosphates, with a deep knowledge of chlorpyrifos — its regulatory history and the risks increasingly attached to it. (She is an author of a 2000 report for Physicians for Social Responsibility called “In Harm’s Way: Toxic Threats to Child Development.”)

“If it’s too dangerous to be used in the home, why isn’t it too dangerous for farm workers?” Ms. Stein said later, as people mingled inside and ate bowls of pozole.

Unable, or Afraid, to Return to the Fields

Lucia Montero came to the United States from Mexico at age 15 and immediately began working the fields. Now 33, she was a foreman in that cabbage patch on May 5, 2017, overseeing one of the three crews, when the invisible but toxic cloud of chlorpyrifos arrived.

At first, she thought she smelled grease or burned oil. Believing it was the tractor, she turned off the engine.

Then came the burning, the itching, the nausea. Ms. Montero called the supervisor on her phone, who told her to get her crew out of the field. By then, several were vomiting. Two or three fainted, she said.

An investigation by the Kern County agricultural commissioner’s office was complicated by the fact that four separate pesticide applications took place that morning within 1.5 miles of the cabbage field.

Ultimately, investigators found that a company called Sun Pacific applied chlorpyrifos at seven tangerine (a common name for a type of mandarin) sites half a mile to 1.5 miles from the cabbage field. Sun Pacific, a large grower behind the Cuties brand of mandarins, was fined $30,250.

Another company nearby had sprayed grapes with a sulfur compound found to have drifted into the cabbage patch, too. That company was fined $20,000.

The investigation said that 37 workers reported illness. Almost all declined medical treatment. In interviews, several said they were afraid of the repercussions, like missed paychecks or being blacklisted by contractors.

Ms. Montero has not been back since that day. She said she cannot fully shake recurring headaches, dizziness and nausea. Still living in Bakersfield, she has taken courses to become a barber.

This month, Ms. Montero learned she was pregnant. She no longer worries just for herself.

“I am scared,” she said. “I just hope I have a healthy baby.”

Bricmary Lopez, the only one of the workers in the field that day to be taken to a hospital, has not returned to the fields either.

She fainted many times after she was sickened by the drift incident, three or four times a week at first, she said. Her asthma has spiked, her skin gets blotchy, and her red-blood cell count has dipped.

She filed a workers’ compensation claim with the packing company she was working for, but action has been slow. Most farm workers, advocates said, never take it this far.

But Ms. Lopez, who arrived from Colombia in 2015 and has filed for asylum, does not fear the repercussions.

“I shouldn’t be worried about calling attention to this,” Ms. Lopez said in her living room. Her daughters, ages 15, 18 and 21, stood at her shoulder, her fiancé sat at her side. “It was not my fault it happened.”

THE REGULATION Chlorpyrifos, developed as a nerve agent, has been in use as a broad-spectrum pesticide since 1965 despite growing evidence of its harmful effects, especially in children and pregnant women. Most residential uses ended in 2001. The E.P.A. moved to ban it for agricultural purposes, with a March 2017 deadline to act.

THE ROLLBACK On March 29, 2017, the new E.P.A. administrator, Scott Pruitt, canceled the proposed ban on chlorpyrifos. The reason: not enough science. “By reversing the previous administration’s steps to ban one of the most widely used pesticides in the world, we are returning to using sound science in decision-making — rather than predetermined results,” Mr. Pruitt said at the time.

THE CONSEQUENCES The use of chlorpyrifos, an estimated six million pounds a year, continues across the country. Only Hawaii has banned its use.