Detroit’s air quality slightly worsened Thursday as firefighters continue to contain a large mulch fire on the city’s west side.

Crews are still on the scene at the Detroit Mulch Company on Prairie Street as of Thursday morning, said James Harris, the Detroit Fire Department’s chief of community relations.

Firefighters are battling the blaze until it is fully extinguished, he said. Investigators have not yet determined what caused the fire.

The air quality in Detroit was on the higher end of moderate safety levels Thursday, according to AirNow, an air quality data site reporting from the officials U.S. Air Quality Index.

Officials report a particulate matter, PM-2.5 of 97, bordering close to the 100 to 150 range for air qualities considered unhealthy for sensitive groups.

The particulate matter measurement is an upward shift from a reported PM-2.5 of 66 on Tuesday at noon, hours before the DFD first received calls about the flames at the mulch site.

The Detroit Mulch Company is closed for business due to the “unexpected fire,” according to a recorded answering message on the company’s listed phone.

Co-owners Casey and Brooke Oberg plan to reopen the store as soon as it is safe to do so, according to a company statement issued Thursday.

“We’re deeply saddened by the fire at our mulch yard and by any inconvenience experienced by local residents—including our customers and employees,” they said in the statement. “In light of this event, we eagerly continue to work with local officials to ensure safety precautions and, to the extent possible, prevent such incidents in the future.”

Large stacks of mulch caught fire Monday, engulfing the immediate area in a cloud of smoke blooming from the local tree disposal and mulch production business.

The Detroit Mulch Company is a local, family-owned business whose majority owner is a Detroit native, according to its release.

The Obergs founded the company to recycle trees in Detroit and remain “deeply invested in the city’s success,” according to the release.