A Japanese steel giant has pledged billions in new investment in Pennsylvania’s Monongahela River Valley, raising hopes that it would clean up pollution from its steel mills in the region. But documents show that its plans to ramp up production will worsen local air quality.

As it runs north to Pittsburgh from its headwaters in West Virginia, the roughly 130-mile-long Monongahela River carves a winding path through a steep-walled basin dotted with small towns and forested hillsides. But the prettiness of this place, known to locals as the Mon Valley, belies a long history of pollution and a drawn-out battle for clean air.

In 1875, Andrew Carnegie opened his first steel mill along the river in Braddock, near Pittsburgh, launching more than a century of heavy industry. By 1901, Carnegie’s company had merged with others to become U.S. Steel.…