SOUTH DEERFIELD — Interstate 91 southbound was closed from the South Deerfield weigh station to the Greenfield rotary for about six hours Thursday morning after a tanker truck began to release medical-grade oxygen.

Massachusetts State Police spokesperson David Procopio said troopers and members of the State Police’s truck team responded to the area near the weigh station shortly after midnight and identified the white vapor coming from a valve on the stopped tanker truck, which had a flat tire. The driver was not injured.

The Deerfield Fire District and a hazardous materials specialist responded, as did the state Department of Environmental Protection. According to Procopio, troopers set up a safety perimeter and shut down the portion of Interstate 91 southbound.

The tanker was towed into the weigh station, according to Procopio. Emergency responders determined it was safe to allow the oxygen to leak out of the tank until it was empty. The roadway was reopened shortly after 6 a.m.

Reach Domenic Poli at: dpoli@recorder.com or 413-772-0261, ext. 262.