At what oxygen percentage volume will a person start to breathe heavily?

A person typically begins to experience noticeable increases in respiratory rate, or starts to breathe heavily, when the oxygen percentage in the air drops below normal levels. The normal oxygen concentration in the atmosphere is around 21%. When oxygen levels decline to about 19%, which corresponds to roughly 1% below normal, individuals may begin to feel effects such as shortness of breath and may start to breathe more heavily as their bodies instinctively attempt to intake more oxygen.

At 17% oxygen, the effects would become significantly more pronounced, leading to additional physiological responses, but it is around 19% where heavier breathing becomes noticeable for many people, marking the threshold where the body signals that it needs more oxygen to maintain normal function. Thus, the choice indicating 19% is aligned with physiological responses observed in individuals under lower oxygen conditions.