Off-gassing occurs when electrolyte is vaporized and pressure builds inside the battery. When sufficient pressure is generated, vapor will be released through built-in vent ports.
The specific gases produced can vary based on the electrolyte formulation and battery temperature. Some typical gases include carbon dioxide, carbon monoxide, ethane, ethylene, hydrogen and methane. You should expect an evolving mixture of flammable and toxic gases to be released.
As off-gassing begins, you can still take immediate preventive action to control the situation. The off-gassing phase is considered the best time to act, since rapid, specialized gas detection can provide 2 to 30 minutes of early warning.