Which type of gas is not allowed to be stored outdoors according to safety regulations?

The correct choice highlights that medical air is not permitted for outdoor storage under safety regulations. This restriction is primarily due to the specific composition and use of medical air within healthcare settings. Medical air, which is a mixture of nitrogen and oxygen, is essential for various medical procedures and equipment. However, storing it outdoors presents potential risks related to contamination and environmental factors that could affect its purity and usability.

Moreover, gases that are stored outdoors must adhere to strict safety guidelines to prevent leaks, spills, or other hazardous occurrences. Medical air systems are designed for indoor use where the environments can be controlled and monitored, ensuring that the air remains free of pollutants and appropriate for patient care.

On the other hand, oxygen, carbon dioxide, and nitrous oxide have been deemed safe for outdoor storage under specific conditions and regulations, as long as safety measures are established to mitigate associated risks like flammability or pressurization issues.