Pennsylvania will slash its summertime air emissions of nitrogen oxides from power plants and large industries by nearly 90,000 tons by 2003 under recently approved regulations.

The action, approved in late August by the state’s Independent Regulatory Review Commission, means that the state will have reduced NOx emissions for industries and power plants 75 percent from 1990 levels.

“This is a major step forward to help Commonwealth residents breathe easier,” said Department of Environmental Protection Secretary Jim Seif.

NOx is a major component of ground level ozone — the main component of smog — which causes a variety of human health problems. Because the reductions are aimed at improving ground-level ozone, they are only required during the summer months when that is a problem.

Pennsylvania’s action was praised by environmentalists. “Pennsylvanians can breathe easier thanks to the IRRC’s approval of the new clean air rule,” said Charles McPhedran, senior attorney for the advocacy group PennFuture. “Power plants will finally be required to do their fair share in the effort to eliminate ozone smog pollution.”

The action should also benefit the Bay. About a quarter of all the nitrogen that enters the Chesapeake is thought to result from air pollution, mainly stemming from NOx emissions from power plants, cars, industries and other activities that burn fossil fuels.

Pennsylvania’s action was taken to comply with a 1998 EPA order that set strict new limits on NOx emissions for 18 states as part of an effort to reduce levels of summertime smog that plague many East Coast cities.

Virginia and Maryland were also affected by the order, but the reductions for Pennsylvania — which had the largest NOx emissions to begin with — were by far the greatest.

Measured from 1997 levels, Pennsylvania had to reduce summertime NOx emissions 25 percent, from 345,201 tons to 257,592 tons. Maryland’s reductions were 21 percent, from 103,476 tons to 81,898 tons. Virginia’s reductions were 15 percent, from 210,784 tons to 180,195 tons.

The states must submit emission reduction plans to the EPA by the end of October.

Virginia had joined several other Southern and Midwestern states in challenging the EPA’s order, but a U.S. Appeals Court ruled against them this summer. Seif called on those states to drop any further legal challenges and begin cleaning up the air.

“Pennsylvania has reduced its own NOx pollution by two thirds since 1995,” he said. “It’s time for states in the South and Midwest to stop their legal wrangling and make a commitment to cut their emissions. One-third of Pennsylvania’s NOx emission and ground-level comes from those states, which prevents Pennsylvania from meeting federal standards for ozone.”

In all, the EPA ordered states to make summertime reductions totaling more than 1 million tons, measured from 1997 emissions. It was the agency’s first attempt to curb emissions from upwind states to improve downwind air quality, primarily along the East Coast.

That also benefits the Bay because the majority of the nitrogen deposition on the Bay and its watershed originates outside its 64,000-square-mile drainage basin.

Although the action was aimed at improving air quality for public health, a 1998 EPA report estimated the action, when fully implemented in 2003, would keep about 8 million pounds of nitrogen a year from entering the Bay.

NOx is generated by the burning of fossil fuels. While efforts to comply with the EPA’s 1998 action mainly affect power plants and industries, the EPA in the past year has issued new rules that eventually could dramatically reduce NOx emissions from cars, sport utility vehicles, diesel trucks and other pollution sources.