Why is nitrogen not suitable for breathing?

The suitability of a gas for breathing is primarily determined by its composition and the needs of human physiology. Nitrogen, which makes up about 78% of the Earth's atmosphere, does not support human respiration because it does not contain oxygen. While nitrogen is an inert gas and non-toxic, our bodies require oxygen to sustain metabolic processes and produce energy. Without oxygen, humans cannot perform essential functions like cellular respiration, leading to suffocation.

Other aspects of nitrogen, such as its temperature or density, do not inherently affect its ability to support breathing. While nitrogen is often at a low temperature in certain conditions and can be denser than air, these properties do not prevent it from being present in the atmosphere or historically used in various applications. The absence of oxygen makes nitrogen unsuitable for breathing purposes, reiterating that the primary factor in gas suitability for respiration is the presence of oxygen.