Air pollution doesn't stop at state borders, and a new study by two Lehigh University researchers shows how policies in one state can affect another.

The study found the health of fetuses improved after the 2014 shutdown of the coal-fired portion of the Portland Generating Station, a power plant on the Pennsylvania side of the Delaware River that sent pollution wafting into New Jersey.

The Northampton County plant stopped using its coal-fired boilers in June 2014, lehighvalleylive.com reported at the time, and has since switched over to natural gas and diesel oil. Impacts from the plant's emissions have been seen in Warren County, directly across the Delaware, as well as Morris, Hunterdon and Sussex counties.

The study compared the plant's emissions 18 months before the shutdown -- when sulfur dioxide emissions were at 2,596.648 tons per month -- with the 18 months after, when the same emissions were nearly zero. They also looked at births in ZIP codes downwind within 60 miles of the plant.

The Lehigh University researchers found that the coal shutdown reduced the likelihood of low birth weight by 15 percent and preterm birth by 28 percent.

The report by the Bethlehem university's economics department chair Shin-Yi Chou and associate economics professor Muzhe Yang was published online Friday in the Journal of Environmental Economics and Management. It will also appear in an upcoming print edition of the journal, Lehigh said in a news release.

This is not their first study examining the Portland station. In April, they released a similar report that examined the effects of the plant's emissions on fetuses -- that one did not look at the changes after the shutdown.

Lehighvalleylive.com previously reported that NRG Energy, which owns the plant, halted use of the station's coal-fired portions in June 2014.

In the most recent study, the researchers note that the coal-firing operation ceased because the federal Environmental Protection Agency overruled state regulations. In a news release, they called it "a case where cross-border air pollution had not been effectively dealt with by decentralized, state-level policymaking."

Federal regulation, they say in the report's summary, can make states accountable for the effects of pollution that drifts outside its borders.

Steve Novak may be reached at snovak@lehighvalleylive.com. Follow him on Twitter @type2supernovak and Facebook. Find lehighvalleylive.com on Facebook.