The LOI (limiting oxygen index) 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. 

The most logical is the Limiting oxygen index (LOI): it is the oxygen concentration in % required to maintain steady burning.  

Fresh air has about 21% oxygen in it.  A polymer with an oxygen index lower than this will burn freely in air; 

one with an oxygen index that is larger will extinguish itself unless a flame is played onto it – then it burns.  Thus a high oxygen index means resistance to self-sustained burning. 

Flame retardant treatments is to increase the LOI rating, after THPC FR treatment, cotton fabric material reach LOI 30.3%