Limiting Oxygen Index (LOI)
Minimum concentration of oxygen (in the air surrounding the sample) that will support flaming combustion. Only measured for polymeric and plant-based materials.
Oxygen concentration in ambient air is 21%, and materials with an oxygen index above 21% will exhibit some degree of self-extinguishing behaviour. Some sources recommend 27% as a safer threshold.
Oxygen index is a specific case of an oxidant index, which is the concentration of a given oxidant (oxygen, nitrous oxide, fluorine, etc.), expressed as percent volume within a dilutant (nitrogen, carbon dioxide, helium, etc.) that will support continuing burning. Each combination of parameters gets a separate value for oxidant index.