What is the starting level of oxygen in the atmosphere?

The starting level of oxygen in the atmosphere is 21 percent, which is essential for various combustion processes and affects fire behavior. This level allows for a sufficient amount of oxygen necessary for fires to ignite and sustain combustion. Oxygen is a key element in the fire triangle, which comprises heat, fuel, and oxygen; without the appropriate level of oxygen, fire cannot thrive.

The values of other choices may seem plausible, but they either represent an incorrect understanding of atmospheric composition or exceed the actual concentration of oxygen found in the Earth’s atmosphere. Knowing that the typical oxygen level is about 21 percent helps fire investigation technicians assess scenarios involving combustion and understand how altering oxygen levels can impact fire behavior, fire suppression, and fire dynamics in various environments.