Final answer:
A medical gas system whose failure can cause minor injury to patients is defined as Category I.
Step-by-step explanation:
A medical gas system whose failure can cause minor injury to patients is defined as Category I according to the NFPA 99 standard. Category I involves failure of important equipment or systems that may cause injury but are not likely to cause life-threatening situations. Examples of Category I failures include improper adjustment or maintenance of ventilators or medical gas equipment.