Abstract: At 10:00 p.m. on September 5, 2017, five employees were working near a ventilated natural gas fuel fryer system. All of the employees began experiencing headaches, nausea, weakness, and other symptoms. Approximately forty percent of the burner units for the natural gas fuel production fryer system had been replaced with the same burner units the day before. The ventilation system was not rebalanced after the burner unit replacements. The ventilations control system indicated the ventilation system was working correctly, but it was not keeping up with the carbon monoxide generated that was leaking from the fryer burner section into the employee work area. It was determined that the system needed additional fresh air to be used by the new burner units. A reported carbon monoxide reading of 313 ppm was taken in the area by responding fire fighters. All five employees were hospitalized overnight with elevated carboxyhemoglobin levels from exposure to carbon monoxide.