Which of the following indicates a safe level of oxygen?

A safe level of oxygen in the air is recognized as being between 19.5% and 23.5%. This range indicates that the oxygen concentration is adequate for human respiration while also allowing for a safety margin that prevents potential hypoxia (oxygen deficiency) or hyperoxia (excess oxygen). Oxygen levels below 19.5% could lead to breathing difficulties and other health hazards, as they indicate an atmosphere that is oxygen-deficient, posing risks for individuals involved in rescue operations. Levels above 23.5% can also present safety concerns, as they increase the risk of fire hazards due to the heightened reactivity of higher oxygen concentrations. Therefore, the range defined between 19.5% and 23.5% ensures a stable and safe environment for responders and individuals in situations requiring rescue operations.