What is the threshold for an atmosphere to be considered oxygen-rich?

An atmosphere is considered oxygen-rich when the concentration of oxygen exceeds the normal atmospheric level, which is approximately 21% at sea level. The threshold for oxygen-rich environments is set above this baseline because higher levels of oxygen can significantly affect combustion processes and respiratory functions.

At concentrations above 21%, the likelihood of fire becoming more intense increases, as oxygen acts as a crucial element for combustion. This can create hazardous situations, making it vital for fire behavior and safety considerations to recognize when a space is oxygen rich.

While levels above 18% suggest a heightened presence of oxygen, it is the surpassing of the 21% mark that most directly aligns with the term "oxygen-rich," leading to changes in how fires might behave and increase in flammability risks.