Approximately what percentage of the air is necessary for human survival in the form of oxygen?

The percentage of oxygen in Earth's atmosphere that is essential for human survival is approximately 21%. This concentration is crucial because it is the optimal level that allows for efficient respiratory function and metabolic processes in humans. When oxygen levels fall significantly below this percentage, individuals can experience hypoxia, which can lead to various health issues, including confusion, unconsciousness, and, in extreme cases, death.

Oxygen is used in the process of cellular respiration, where it plays a vital role in converting glucose into energy necessary for all bodily functions. While very small percentages of oxygen are sufficient to sustain basic life forms, the standard atmospheric concentration at sea level has evolved to approximately 21%, making it the ideal model for supporting human life.

The other options, such as 15%, 30%, and 42%, each represent levels that would either be insufficient (in the case of 15%) or excessively high and potentially harmful (as seen in 30% and 42%). High concentrations of oxygen can lead to toxicity and other adverse effects, reinforcing why 21% is the established necessary level for human survival.