What is the approximate oxygen concentration level at which a human's respiratory rate begins to increase?

The human body typically requires a certain level of oxygen concentration in the air to function properly. Normal atmospheric air contains about 21% oxygen. When the oxygen concentration drops to around 18%, the body senses this decrease and may begin to respond by increasing the respiratory rate in an effort to obtain more oxygen. This physiological response helps to maintain adequate oxygen levels in the blood and supports vital functions of the body.

At lower oxygen concentrations, such as 15% or 10%, the body's responses become more pronounced and may lead to significant discomfort or hypoxia, but it is the threshold of approximately 18% that primarily triggers the initial increase in breathing rate. This is crucial for firefighters and first responders to understand, as they may encounter situations where atmospheric oxygen levels are compromised.