The limiting oxygen index (LOI) is the minimum concentration of oxygen, expressed as a percentage, that will support combustion of a polymer. It is measured by passing a mixture of oxygen and nitrogen over a burning specimen, and reducing the oxygen level until a critical level is reached.
LOI values for different plastics are determined by standardized tests, such as the ISO 4589 and ASTM D2863.
References
Limiting oxygen index Wikipedia(Text) CC BY-SA