At what percent of oxygen will a person start to breathe faster and deeper?

A person typically starts to breathe faster and deeper when the oxygen concentration in the environment falls below normal levels, which is around 21% in the atmosphere. At approximately 19% oxygen, the body's respiratory drive begins to respond by increasing both the rate and depth of breathing to compensate for the reduced oxygen availability. This physiological response occurs due to the body’s need to maintain adequate oxygen levels for cellular functions.

At lower oxygen percentages, such as 18% or 17%, the body's oxygen deficiency becomes more pronounced, leading to even greater respiratory efforts. However, to answer the original question regarding when this faster and deeper breathing begins, it is primarily at the 19% threshold that noticeable changes in breathing patterns occur. Therefore, while responses may escalate at lower levels, the significant trigger point is typically around 19%.