Confined spaces can be buildings too....argon leak asphyxiates two

On January 30, 2023, at approximately 5:30 p.m., argon gas was released at the Propulsion Systems facility in Utah. Exposure to the argon gas fatally injured two employees, who asphyxiated.
The company treated carbon fiber blocks at the facility. As part of the treatment, a specialized vessel (autoclave) containing carbon fiber blocks was pressurized with argon gas. On the day of the incident, two employees were working to locate argon leaks and troubleshoot the vessel’s pressure control system. During this work, the autoclave did not contain carbon fiber blocks. Throughout the workday, the vessel was pressurized with argon gas several times, but the task was not completed by the end of the employees’ shift. The employees changed out of their work clothes and removed their personal oxygen monitors. However, before meeting with the oncoming shift workers, the two employees returned to the basement. The crew arriving for the next shift found their two coworkers unconscious in the basement’s stairway. Emergency responders transported the two employees to the hospital, where they were declared deceased.
The company’s investigation found that argon had leaked through the seals of the pressurized vessel while the basement’s ventilation system was off, which allowed the basement (about 4,500 cubic feet) to begin to fill with argon and create an oxygen-deficient atmosphere. The company’s investigation did not determine why the two employees returned to the basement at the end of their shift, however. After the incident, the company classified the basement as a confined space and installed an oxygen monitoring system. The company also implemented safeguards to prevent entry into the basement when the oxygen concentration is unsafe, the exhaust ventilation fan is off, or the vessel is pressurized.
Probable Cause
Based on the company's investigation, the CSB determined that the probable cause of the incident was the release of argon gas from the autoclave into a confined area. The two workers were fatally injured when they entered this oxygen-deficient environment. The lack of effective engineering controls, such as forced air ventilation and continual oxygen monitoring, contributed to the severity of the incident.

 Source;CSB.gov