concept

Limiting oxygen index

The lowest oxygen fraction in a flowing gas mixture at which a material will still sustain candle-like downward flaming. It is the standard way of saying how far an atmosphere sits from not supporting fire at all: the oxygen volume fraction in the stream passing the sample is lowered until the flame no longer creeps downward on its own. Untreated wood and cotton sit around 20-21.5%, which means Earth air at 20.9% oxygen clears the bar only barely - drop a few percentage points and most dry vegetation stops carrying flame. The index gives the threshold; airflow and fuel thickness decide what happens right at it.