What is a key characteristic of compressed oxygen gas as defined by USP standards?

Compressed oxygen gas, as defined by the United States Pharmacopeia (USP) standards, must meet specific purity requirements for human use. This characteristic is crucial because medical oxygen is intended for patient treatment and must be free from contaminants that could be harmful to health. The USP sets stringent standards to ensure that the oxygen provided for medical purposes meets the necessary quality and safety for effective therapy. This ensures that patients receive a product that will deliver the therapeutic benefits without introducing any risk of adverse effects due to impurities.

In the context of the other choices, while non-medical applications exist for gases, the primary focus of this question is on medical oxygen's purity standards specific to human health. Interchangeability with other gases is not characteristic of medical oxygen, as each gas has distinct usages and properties that are not interchangeable in medical contexts. Additionally, while the density of gases is a relevant aspect in some discussions, it is not a defining characteristic outlined by USP standards for medical oxygen. The emphasis remains on the necessity for purity in applications where oxygen is used for medical treatment.