In what could prove a significant move for communities facing air pollution, the Environmental Protection Agency proposed on Thursday that chemical plants nationwide measure certain hazardous compounds that cross beyond their property lines and reduce them when they are too high.

The proposed rules would reduce cancer risk and other exposure for communities that live close to harmful emitters, the EPA said. The data would be made public and the results would force companies to fix problems that increase emissions.

"This is probably the most significant rule I'm experiencing in my 30 years of working in cancer alley," said Beverly Wright executive director of the Deep South Center for Environmental Justice and member of the White House Environmental Justice Advisory Council. She referred to an area dense with petrochemical development along the Gulf coast.

In the past, Wright said, even when emissions caused harm, residents weren't able to sue and reduce the threat.

The proposed measure is also intended to address short-term emissions spikes when plants start up, shut down and malfunction. If the proposal is finalized, it would impact roughly 200 chemical plants, the agency said.

Fence line monitoring has long been a priority of the environmental justice movement and a number of refinery communities have won it in recent years. This measure would extend some of those changes nationwide.

EPA Administrator Michael Regan announced the plan in St. John the Baptist Parish, Louisiana, home to the Denka chemical plant, which makes synthetic rubber and emits chloroprene, listed as a carcinogen in California. Denka is less than a half mile from an elementary school and has been targeted by federal officials for allegedly increasing the cancer risk for the nearby, majority-Black community.

"For generations, our most vulnerable communities have unjustly borne the burden of breathing unsafe, polluted air," Regan said.

The changes also focus on manufacturers of ethylene oxide, which is commonly used in medical sterilization plants. Long-term exposure to that chemcial can increase the risk of lymphoma and breast cancer. The agency plans to issue proposed regulations for medical sterilization plants in the near future.

According to the agency, the proposal would slash ethylene oxide emissions nationwide by about two-thirds and chloroprene by three-quarters from 2020 levels. Emissions that worsen smog would be reduced as well.

The American Chemistry Council said industry emissions have declined over the last decade. It is concerned about the EPA's proposal for reducing ethylene oxide, and says it is based on a faulty EPA risk assessment.

"Overly conservative regulations on ethylene oxide could threaten access to products ranging from electric vehicle batteries to sterilized medical equipment," said council spokesman Tom Flanagin, adding that the EPA may be rushing its work on significant regulations.

The Biden administration has prioritized fighting cancer and environmental enforcement in communities overburdened by pollution.

Federal officials sued Denka in February, demanding it cut its emissions.

A spokesperson for Denka said it is waiting to review the proposed language before commenting. Data show the plant has drastically reduced its emissions over time and it already conducts fence line monitoring, but the EPA said the plant remains dangerous to those who live nearby.

The agency says it conducted a new community risk assessment in preparing this proposal.

"There are aspects of this rule that communities have been fighting for for decades," said Deena Tumeh, an attorney with the environmental group Earthjustice.

The federal government lawsuit against Denka was part of a series of efforts by the Biden administration to target pollution in the country's biggest petrochemical corridor.

Last year, the EPA said it had evidence that Black residents face an increased cancer risk from the chemical plant and state officials were allowing pollution to remain too high. The agency's letter was part of an investigation under the Civil Rights Act of 1964, which says anyone who received federal funds cannot discriminate based on race or national origin.

Regan visited the parish in 2021 on a five-day trip from Mississippi to Texas to highlight low-income and mostly minority communities harmed by industrial pollution.

"This is a day to celebrate," Wright said.

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