What is the proper concentration range of oxygen before entering a confined space?

The proper concentration range of oxygen before entering a confined space is 19.5 percent to 23.5 percent. This range is critical for ensuring the safety of individuals entering such environments.

Levels below 19.5 percent indicate an oxygen-deficient atmosphere, which can pose serious health risks, including unconsciousness or asphyxiation. Conversely, concentrations exceeding 23.5 percent are considered enriched oxygen atmospheres, which significantly increase the risk of fire and explosion due to the heightened flammability of materials in such conditions.

Maintaining oxygen levels within this specified range is therefore essential for the safety and health of personnel, ensuring they can perform their work without undue risk from oxygen deficiency or excessive richness. The other choices do not provide a safe range, either by including risk of deficiency or danger from enrichment.