More than 200 chemical plants across the country will be required to curb the toxic pollutants they release into the air under a regulation announced by the Biden administration on Tuesday.

The regulation is aimed at reducing the risk of cancer for people living near industrial sites. This is the first time in nearly two decades that the government has tightened limits on pollution from chemical plants.

The new rule, from the Environmental Protection Agency, specifically targets ethylene oxide, which is used to sterilize medical devices, and chloroprene, which is used to make rubber in footwear.

The E.P.A. has classified the two chemicals as likely carcinogens. They are considered a top health concern in an area of Louisiana so dense with petrochemical and refinery plants that it is known as Cancer Alley.

Most of the facilities affected by the rule are in Texas, Louisiana and elsewhere along the Gulf Coast as well as in the Ohio River Valley and West Virginia. Communities in proximity to the plants are often disproportionately Black or Latino and have elevated rates of cancer, respiratory problems and premature deaths.

Michael S. Regan, the administrator of the E.P.A., traveled last year to St. John the Baptist Parish in Louisiana, the heart of Cancer Alley, to announce his agency’s intention to limit pollution from the plants.

In a telephone call with reporters on Monday, Mr. Regan recalled that he had been struck by the concentration of chemical plants and by the way they had affected families for decades. “I saw firsthand how the multigenerational and widespread effects of pollution were affecting the health of the local community,” Mr. Regan said.

He said that the rule would cut toxic pollutants by 6,200 tons annually and reduce emissions of ethylene oxide and chloroprene by 80 percent.

Under the rule, chemical manufacturers must monitor vents and storage tanks for ethylene oxide and chloroprene emissions and plug any leaks.

Plants will also be required to reduce emissions of four other toxic chemicals: benzene, which is used in motor fuels as well as oils and paints; 1,3-butadiene, which is used to make synthetic rubber and plastics; and ethylene dichloride and vinyl chloride, both of which are used to make a variety of plastics and vinyl products.

One year after monitoring begins, facilities will be required to submit quarterly data to the E.P.A. The data will be made public so that communities can understand any risks they face.

Patrice Simms, vice president for litigation for healthy communities at Earthjustice, an environmental group, said it was impossible to overstate the importance of the new regulation to families that live next to large polluting facilities.

“In a very real sense this is about life and death,” he said.

Mr. Regan has made it a priority to address the environmental hazards facing communities that surround industrial sites, but his efforts have been met with significant roadblocks.

In 2022, in response to complaints from Louisiana residents, the E.P.A. began an investigation into whether the state had violated civil rights laws by permitting scores of industrial facilities to operate in and around St. John the Baptist Parish, a predominantly Black community. Title VI of the Civil Rights Act allows the E.P.A. to investigate whether state programs that receive federal money are discriminating on the basis of race, color or national origin.

But Louisiana sued the E.P.A., arguing that the federal government could enforce the Civil Rights Act only in cases in which state policies were explicitly discriminatory. The E.P.A. ended the investigation last year, but the state continued its legal challenge. In January, the U.S. District Court for the Western District of Louisiana ruled in the state’s favor.

The new chemical rule is widely viewed as part of the E.P.A.’s effort to find ways to police polluting plants despite the setback. On Monday, Mr. Regan insisted that the rule was not related to the civil rights case.

“As administrator, what I’ve pledged to do is use every single tool in our toolbox to do whatever we can to protect these frontline communities,” he said.

Last month the E.P.A. finalized separate standards that require plants that sterilize medical equipment and other facilities that use ethylene oxide to install pollution controls to reduce their emissions.

Republicans and industry groups said that the rule announced on Tuesday was onerous, and they questioned the E.P.A.’s scientific assessment of the chemicals.

“E.P.A. should not move forward with this rule-making based on the current record because there remains significant scientific uncertainty,” the U.S. Chamber of Commerce wrote in a letter to the agency.

One company that will be affected by the new rule is Denka Performance Elastomer, a synthetics manufacturer in Laplace, La. Air monitoring near the plant has consistently shown chloroprene levels as high as 15 times the recommended concentration deemed safe over a lifetime of exposure, according the E.P.A. Saying the company’s plant posed an “imminent and substantial endangerment to public health and welfare,” the agency sued Denka last year, seeking to compel it to reduce its emissions of chloroprene.

The company said that concentrations of the chemical were well below what would constitute a public health emergency. It also said that it had cut its chemical emissions significantly since 2015.

In a statement, Denka called the new rule “draconian.” It said that the requirements would force the company to “idle its operations at tremendous expense and risk to its hundreds of dedicated employees.”

The company said that it intended to challenge the rule in court.