For a discussion of safety issues relevant to all
batteries refer to Section 3.8
Cell jars and covers are almost always made from plastics. To prevent the spread of fire resulting from accidents, many users require that a flame-retardant plastic be used. Flame retardancy is specified either as Limiting Oxygen Index (LOI) of 25 percent minimum per ASTM D2863 or a UL94 V-0 rating.
Cells are sometimes equipped with a flash arrester that will prevent a spark occurring outside the cell from igniting the hydrogen-oxygen mixture inside the cell.