PITTSBURGH—The air above this city is raising alarms once again.

People who live in Allegheny County, which surrounds Pittsburgh, have lodged more than 2,300 air-quality complaints this year. For most of January, county officials urged residents in 22 municipalities in the Mon Valley nearby to limit outdoor activities out of concern for high levels of sulfur dioxide, and U.S. Steel has begun offering free health screenings.

The latest concerns come after a fire late last year knocked out pollution controls at a nearby U.S. Steel plant. Since then, the Clairton Plant and two other facilities in the Mon Valley south of the city have been emitting high levels of sulfur dioxide. One day in January, the three emitted a total of 74,000 pounds—35 times the amount emitted on a single day before the fire (2,100 pounds), according to data the company recently reported to the county.

Mark Dixon, 44 years old, a filmmaker in Pittsburgh, has printed out more than 11,000 complaints from an app known as Smell Pittsburgh on a 200-foot-long scroll of paper and reads them at Board of Health meetings.

“There has been an explosion of new public concern about this issue since the fire,” he said.