What is the approximate percentage of oxygen in pure air at sea level?

The approximate percentage of oxygen in pure air at sea level is about 20.94%. This value is significant because it reflects the standard composition of Earth's atmosphere, which is essential for various processes, including respiration in living organisms and combustion. Understanding this percentage is crucial for various fields, including mining, where oxygen levels can affect both safety and operational procedures.

The other answers reflect either percentages that are slightly lower or higher than the established standard, which could lead to misconceptions about atmospheric composition. For example, values like 19.5% could indicate a more hazardous environment or reduce the efficiency of combustion processes, while percentages above 21% suggest an enriched oxygen environment that could pose safety risks in confined spaces. In mining situations, being aware of the actual oxygen content helps ensure a safe working atmosphere and supports effective emergency response plans.