A fire broke out late Wednesday morning at Westlake Chemical Corporation’s vinyls plant inside a refrigeration unit that stores a nonhazardous, but reactive organic chemical used in making common household plastic PVC.

The blaze, which lasted about an hour and caused no injuries, sent out a dark plume of smoke from the polyvinyl chloride facility along La. 30 in Ascension Parish, company and emergency officials said.

Plant manager Jim Best said the plant’s emergency response team extinguished the fire with a jet of water about 12:20 p.m.

Though Best said the fire did not directly involve any of the plant’s three production units, a PVC unit nearest the storage facility where the fire happened was in the process of being shut down midday Wednesday as a precaution.

Greg Langley, spokesman for the state Department of Environmental Quality, said community air monitoring did not detect any emissions into the atmosphere that were at levels harmful to human health.

He said DEQ officials received the all clear at 1:40 p.m.

The Westlake plant had a more significant blaze in March 2012, when the plant’s vinyl chloride monomer unit caught fire and forced state highways and a 34-mile section of the Mississippi River to close.

The U.S. Occupational Safety and Health Administration issued citations and fines for that blaze, which also had no injuries.

Best said Wednesday’s fire was unrelated to the VCM unit involved in the 2012 fire and was in a different section of the plant than that unit.

State Police Lt. J.B. Slaton said the State Police’s hazardous materials unit responded to the fire and is investigating its cause.

OSHA, the nation’s workplace safety regulator, also has opened an investigation, said Juan Rodriguez, OSHA spokesman.

Best, who said Westlake is probing the fire’s cause as well, said the blaze burned liquid organic peroxide, which is used as an additive to initiate the production of PVC. He said the peroxide must be kept at cold temperatures to keep it from chemically reacting.

“We don’t know what happened to cause the cooling to fail, or whatever, but that will be part of the investigation as to why that started on fire,” he said.

Best said emergency plant personnel continued to spray water on the peroxide to keep it cool even after the fire was put out. He said when the peroxide starts reacting, it releases heat in an exothermic reaction.

Best said the plant had notified all law enforcement and other state and local agencies and also called 911, when the fire broke out at 11:15 a.m.

Plant employees first noticed the fire, Best said.

Best said local firemen came to the plant, but the fire was under control when they arrived.

Rick Webre, director of the Ascension Parish Office of Homeland Security and Emergency Preparedness, said the fire produced a lot of smoke.

He said Wednesday morning that employees at the plant in Geismar had been directed to shelter in place and nearby plants had been asked to implement their own protective measures.

However, Best said later that, because of the nonhazardous nature of the material that burned, no order was issued to shelter in place.

In September 2012, OSHA cited Westlake for the VCM unit fire from the previous March, giving the company seven violations, including six serious ones, and $67,000 in fines, the agency website says.

When the VCM column caught fire, a plume of potentially toxic VCM, chlorine and hydrochloric acid, as well as other chemicals, went into the air.

In October 2012, OSHA and Westlake reached an informal settlement, reducing the number of violations to six and the the total fines to $38,300. OSHA’s website says the case is not closed but is still “pending abatement of violations.”