🫁 4 — Oxygen

What is the primary oxidizing agent in most fires?

Oxygen.

Approximately what percentage of oxygen is normally present in air?

About 21 percent.

How is energy release in a fire related to oxygen availability?

Energy release is directly proportional to the amount of oxygen available for combustion.

What limits most compartment fires?

Oxygen availability rather than the amount of fuel available.

At normal ambient temperature, how low can oxygen concentration be while some materials can still ignite and burn?

As low as approximately 15 percent.

What happens as oxygen concentration becomes limited?

Flaming combustion diminishes and combustion may continue in a nonflaming or smoldering mode.

Can smoldering combustion continue at very low oxygen concentrations?

Yes.

What is an oxygen-enriched atmosphere?

An atmosphere containing more oxygen than normal.

How does increased oxygen concentration affect combustion?

Materials can burn more intensely and may ignite more readily.

What can happen to some petroleum-based materials in oxygen-enriched atmospheres?

They may autoignite.

What can happen to materials that normally do not burn at normal oxygen levels?

They may burn in an oxygen-enriched atmosphere.

At approximately what oxygen concentration can Nomex burn vigorously according to the outline?

Approximately 31 percent oxygen.

Where might firefighters encounter oxygen-enriched atmospheres?

Hospitals, healthcare facilities, some industrial occupancies, and homes where occupants use equipment containing pure oxygen.

What must happen after fuel becomes gaseous for combustion to occur?

The fuel must mix with air or another oxidizer in the proper ratio.

What is the flammable or explosive range?

The range of fuel-vapor concentrations in air that can support combustion.

What does LEL stand for?

Lower Explosive Limit or Lower Flammable Limit.

What is the LEL?

The minimum concentration of fuel vapor in air that supports combustion.

What happens below the LEL?

The mixture is too lean to burn.

What does UEL stand for?

Upper Explosive Limit or Upper Flammable Limit.

What is the UEL?

The concentration above which combustion cannot take place.

What happens above the UEL?

The mixture is too rich to burn.

What is the ideal concentration within the flammable range?

The concentration containing the proper amount of fuel and oxygen for combustion.

Can temperature and pressure change the flammable range?

Yes. Variations in temperature and pressure can cause the flammable range to vary considerably.