(Reuters) – Six workers were treated for respiratory problems on Tuesday after nitric acid leaked from a package at a United Parcel Service distribution facility in southern New Hampshire, a fire official said.

The facility, in Nashua, about 45 miles northwest of Boston, was evacuated following reports of a chemical leak just after 7 a.m. local time, said Nashua Fire Rescue Deputy Chief Karl Gerhard.

Chemical fumes from a package forced temporary evacuation of the facility, UPS said in a written statement. By late morning employees were allowed to return to the facility.

Six workers who reported minor respiratory symptoms went to two area hospitals for treatment, Gerhard said.

“Nitric acid leaked from a package onto an elevated conveyer belt system they use to sort packages,” Gerhard said. He added that a contractor was on site cleaning up the spilled product, which leaked through machinery. Operations would likely be affected for “the better part of the day,” he said.

UPS said it was cooperating with the Nashua Fire Department. The U.S. Occupational Safety and Health Administration, which monitors safety in the workplace, did not immediately respond to a request for comment.