This part of ISO 4589 specifies methods for determining the minimum concentration of oxygen, in a mixture with nitrogen, that will support combustion of small vertical test specimens under specified test conditions over a range of temperatures typically between 25 °C and 150 °C. The results are defined as oxygen index values at the test temperature, which is typical of the practical temperature that a plastic material may experience in an overheated service situation.

Methods are provided for testing materials that are self-supporting at the test temperature in the form of vertical bars or sheet up to 10,5 mm thick. These methods are suitable for solid, laminated or cellular materials characterized by an apparent density greater than 100 kg/m³. The methods may also be applicable to some cellular materials having an apparent density of less than 100 kg/m³. A method is provided for testing flexible sheet or film materials while supported vertically.

This part of ISO 4589 also includes a method (see annex A) for determining the temperature at which the oxygen index of small vertical test specimens in air is 20,9 under specified test conditions. The result is defined as the flammability temperature (FT) and the method is limited to the determination of results less than 400 °C. The method is not applicable to materials having an oxygen index of < 20,9 when measured at 23 °C.

Results obtained in accordance with this part of ISO 4589 should not be used to describe or appraise the fire hazard presented by a particular material or shape under actual fire conditions, unless used as one element of a fire risk assessment which takes into account all of the factors which are pertinent to the assessment of the fire hazard of a particular application for the material.