The U.S. Chemical Safety and Hazard Investigation Board released an update on the agency’s investigations into incidents involving the release of hazardous nitrogen oxides gas at two facilities owned by the Austin Powder company in Ohio and Tennessee.
The incidents occurred Nov. 24, 2024, at the U.S. Nitrogen facility in Midway, Tennessee, and June 11, 2025, at the Red Diamond explosives manufacturing facility in McArthur, Ohio. Nitrogen oxides are highly hazardous chemicals capable of causing serious respiratory injuries and environmental harm.
On June 11, 2025, more than 3,900 pounds of NOx gas was released from the Red Diamond explosives manufacturing facility. The release occurred through an emergency pressure relief valve and a process vent associated with the facility’s nitric acid storage and recovery operations.
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A large, yellow-reddish-brownish-colored plume was seen coming from the facility. The visible emissions led to the evacuation of residents in the nearby town of Zaleski and prompted the Federal Aviation Administration to restrict airspace for a 30-mile radius around the facility. The release lasted for more than three hours.
CSB Investigators found that temperatures in a nitric acid storage tank increased dramatically after cooling systems had been shut down for an extended period. During normal operation, the tank’s bulk liquid temperature is below 50℉. With the chilled water system off, the temperature steadily increased during the week preceding the incident.
By the morning of June 10, 2025, approximately 24 hours before the release began, the storage tank temperature had increased to more than 80°F. On the morning of the incident, the temperature increased to over 150°F, more than 100°F above normal.
Shortly after 7:54 a.m. on the morning of the incident, maintenance personnel discovered that an emergency pressure relief valve on the tank was repeatedly opening and closing approximately every 30 to 60 seconds, discharging NOx gas. By 8:19 a.m., the excess nitric acid tank’s emergency pressure relief valve was open continuously and remained open until approximately 10:19 a.m.
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The U.S. Nitrogen facility in Tennessee produces nitric acid that is used at Austin Powder’s Red Diamond explosives manufacturing facility in Ohio, in addition to other substances. On November 24, 2024, two NOx releases occurred during multiple attempts to start the nitric acid unit at the U.S. Nitrogen facility after the unit had been shut down for several days for maintenance.
The releases, which occurred at approximately 6:47 a.m. and 8:42 a.m., resulted in the release of more than 900 pounds of NOx gas. The unit was shut down after visible emissions were observed. Like the release in Ohio, a large yellow-reddish-brownish-colored plume of NOx gas was emitted.
The Tennessee Department of Environment and Conservation has issued a Notice of Violation and an order to U.S. Nitrogen, seeking a civil penalty and asserting that the company failed to follow its standard operating procedure for starting up the nitric acid plant.
CSB Chairperson Steve Owens said, “These incidents underscore the serious hazards that can occur with nitric acid processes. We are concerned that hazardous nitrogen oxides gas was released at two Austin Powder facilities in the span of less than seven months.”
The CSB’s investigation into the incidents is ongoing. The CSB is reviewing equipment performance, operating procedures, safeguards, alarms, and emergency response actions. Final findings and safety recommendations will be issued in the CSB’s final investigation report.
The CSB is an independent, nonregulatory federal agency charged with investigating incidents and hazards that result, or may result, in the catastrophic release of extremely hazardous substances. The agency’s core mission activities include conducting incident investigations; formulating preventive or mitigative recommendations based on investigation findings and advocating for their implementation; issuing reports containing the findings, conclusions, and recommendations arising from incident investigations; and conducting studies on chemical hazards.
The agency’s board members are appointed by the President subject to Senate confirmation. The Board does not issue citations or fines but makes safety recommendations to companies, industry organizations, labor groups, and regulatory agencies such as OSHA and EPA.
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