Oxygen index is defined as minimum concentrationof oxygen, expressed as volume percent in a mixture of oxygen and nitrogen thatjust support flaming combustion of plastic materials under equilibriumcondition of candle like burning. The equilibrium is established by therelation between the net lost to the surroundings as measured by either time ofburning or length of specimen burned. This point is approached from both sidesof the critical oxygen concentration in order to establish the oxygen index