What must the minimum oxygen content of the air delivered to working places be?

The minimum oxygen content of the air delivered to working places must be 19.5%. This standard is crucial for ensuring the safety and health of workers in mining environments, as insufficient oxygen levels can lead to serious health issues, including hypoxia—a condition caused by low oxygen levels in the bloodstream.

Maintaining a minimum of 19.5% oxygen ensures that there is enough breathable air for miners to perform their tasks effectively and safely. The value is set just above the threshold where deficiency effects may begin to occur. Lower thresholds, such as 18.0% or 19.0%, may not adequately protect workers' health and are not compliant with established safety regulations. A standard of 20.0% is typically representative of normal atmospheric oxygen levels but is not necessary to maintain in active working conditions where 19.5% is considered sufficient for safety.

In summary, the correct answer emphasizes the balance needed to ensure miners have oxygen-rich air while adhering to safety protocols designed to protect their well-being.