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The autoignition temperature or kindling point of a substance is the lowest temperature in which it spontaneously ignites in a normal atmosphere without an external source of ignition, such as a flame or spark. This temperature is required to supply the activation energy needed for combustion. The temperature at which a chemical ignites decreases as the pressure or oxygen concentration increases. It is usually applied to a combustible fuel mixture.

• The ignition temperature of a substance is the least temperature at which the substance starts combustion.
• Substances which spontaneously ignite in a normal atmosphere at naturally ambient temperatures are termed pyrophoric.

Autoignition temperatures of liquid chemicals are typically measured using a 500-millilitre (18 imp fl oz; 17 US fl oz) flask placed in a temperature-controlled oven in accordance with the procedure described in ASTM E659.[1]

When measured for plastics, autoignition temperature can be also measured under elevated pressure and at 100% oxygen concentration. The resulting value is used as a predictor of viability for high-oxygen service. The main testing standard for this is ASTM G72.[2]

## Autoignition equation

The time ${\displaystyle t_{\text{ig}}}$ it takes for a material to reach its autoignition temperature ${\displaystyle T_{\text{ig}}}$ when exposed to a heat flux ${\displaystyle q''}$ is given by the following equation:[3]