The Environmental Protection Agency on Monday said it would soon propose tougher restrictions on pollution from heavy-duty trucks, an uncharacteristic move to tighten existing standards from an administration that has prided itself on a series of regulatory rollbacks.

Heavy-duty vehicles are the largest mobile source of nitrogen oxide, a pollutant linked to heart and lung disease. They also tend to remain in service far longer than other vehicles.

Still, some environmental advocates said they feared that the EPA’s proposal could preempt even more stringent rules being considered in California.

At an announcement in rural Virginia, EPA Administrator Andrew Wheeler said the agency would soon move forward with its “Cleaner Trucks Initiative,” aimed at further curbing highway emissions of nitrogen oxide and other pollutants from heavy-duty engines. Nitrogen oxides, or NOx, have been linked to a number of respiratory problems, particularly asthma, and can react with sunlight and other chemicals to form smog.

The EPA had said in November 2018 that it intended to update the standards for heavy-duty trucks, which were last revised in 2001. On Monday, Wheeler said the agency would issue a formal proposal in early 2020 after a public comment period.

“Through this initiative, we will modernize heavy-duty truck engines, improving their efficiency and reducing their emissions, which will lead to a healthier environment,” Wheeler said in the Monday announcement.

Overall, the nation’s NOx emissions dropped more than 40 percent between 2007 and 2017 due to decreasing emissions from various sources, including power plants and industry, according to the EPA.

In 2016, 20 state and local air regulators, backed by public health groups, petitioned the agency to revamp its regulations of NOx, citing adverse effects on health and air quality. The agency agreed at the time that changes were necessary, “particularly in areas of the country with elevated levels of air pollution.”

Obama administration officials said they planned to work with states such as California, the industry and others to update a national set of standards.

Monday’s announcement received instant praise from the trucking industry, members of which stood alongside Wheeler at his announcement.

“A strong new national standard has the potential to create significant investment in American jobs and manufacturing, cost-effectively reduce harmful emissions in the nation’s most populated areas in a timely fashion, and help deploy American-developed advanced control technologies here and around the world,” Chris Miller, executive director of the Advanced Engine Systems Institute, said in a statement that accompanied Monday’s EPA announcement.

California regulators have already begun to develop tougher nitrogen oxide standards of their own for the trucking industry, testing vehicles and undertaking other technical work as the state seeks to put new restrictions in place. State officials have said reductions in NOx pollution are necessary to combat smog and to meet air-quality standards.

California is considering new standards that would cut emissions 90 percent compared with today’s diesel engines, Stanley Young, a spokesman for the state’s air regulator, said in an email.

While the EPA and California ultimately could harmonize their new rules, the possibility also exists that the federal proposal could undermine the more stringent standards proposed by California. The state and the Trump administration, after all, have remained at a standoff over fuel-efficiency standards for the nation’s cars, pickup trucks and SUVs.

“I am very suspicious about what is going on here,” said Margo Oge, who directed the EPA’s Office of Transportation and Air Quality from 1994 to 2012. She said she worries that the agency might ultimately try to aid the trucking industry by proposing a weaker national standard than California’s plans.

Other clean-air advocates said they hope the federal government will follow through on its vow to put tougher rules on the books to rein a major contributor to air pollution around the country.

“I think it’s a good thing that EPA is doing this,” said Paul Billings, senior vice president for advocacy at the American Lung Association.

He said a strong national standard is essential, given that heavy-duty trucks routinely cross state lines on their travels. But Billings said he was waiting to see the details of the plan.

“That’s where the proof will be,” Billings said. “Is this truly a ‘cleaner trucks’ initiative, or is it something else?”