A combustion needs three components to occur: combustible material, oxygen and heat. If one of these components is lacking, then the combustion cannot occur. In nature, there are no incombustible materials; however, any product can be flame-retardant to a certain degree depending on its LOI value. Additionally, every material needs oxygen to keep burning, and the amount of oxygen required differs with each material. The amount of oxygen a material needs to burn is called the Limiting Oxygen Index (LOI). The higher the value, the lower the flammability of the material.
As an example: the LOI value of cotton is 18 percent. The oxygen content of the atmosphere is 21 percent. Because there is more oxygen in an atmospheric environment (21 percent) than the LOI value needed to burn cotton (18 percent), any cotton fabric will burn out once it starts to burn. In other words, cotton will have more oxygen than it needs in an atmospheric environment.