At what oxygen level do visible flames start to diminish in a fire?

Visible flames in a fire begin to diminish when oxygen levels drop to around 16%. This is because combustion requires sufficient oxygen to sustain the flame; when the oxygen concentration falls below this level, the intensity of the flame is affected. Fire requires a minimum oxygen concentration of approximately 16% in the atmosphere to maintain a visible flame. Below this threshold, fire can still occur, but it will tend to burn less vigorously and may produce more smoke and heat instead of a clear flame.

At levels below this, such as 10% or even slightly above 16%, flames may struggle to remain visible due to insufficient oxygen to support the necessary chemical reactions for combustion. On the other hand, a concentration of 20% or higher is considered ideal for maintaining robust flames, as it is closer to the normal atmospheric oxygen level, which is about 21%.