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The Critical Oxygen Index test is widely used as a measure of the flammability of many substances. This paper reviews attempts that have been made to compare Oxygen Index values with the results from other fire tests. Varying degrees of correlation are apparent, depending on the properties being measured in the respective tests.

Measured Critical Oxygen Index values at 40°C for Opal polymethylmethacrylate (Perspex) sheet have been found to be remarkably dependent on the gas velocity within the column of the Hot Flammability Test Apparatus. Investigation of the atmosphere present in the column has indicated that the effect is almost solely attributable to varying quantities of ambient air entering the column as the flow rate is altered. A modification to the apparatus is proposed that successfully prevents this entrainment of air at normal testing flow rates.

The use of Critical Oxygen Index testing columns in the horizontal configuration is investigated. Gas analysis results from within columns of simple design indicate that they are unsuitable for burning studies because of entrainment of the external atmosphere and recirculation of combustion products. A new design of column and holder is proposed that eliminates these effects and provides a possible means of studying small-scale horizontal combustion in different oxygen/nitrogen mixtures.

The Critical Oxygen Index test has been used as a measure of the flammability of a wide range of substances, including plastics, textiles and liquids, since its introduction in 1966. The index, however, is only appropriate to the configuration and conditions used in the test, and extrapolation in order to predict the flammability of materials in a large-scale fire is not valid. The index is dependent on a number of sample and test parameters and these are reviewed; the ASTM test standard, the reproducibility of the results, and certain theoretical aspects are also discussed.

The dependence of the Critical Oxygen Index values of some thermoplastic materials on sample sixe is reported. The extent of melting of this class of material is shown to be important in determining the effect of sample size on the index.