The "respirator" worn by the perpetrator is a Scott self contained breathing apparatus, which is most commonly used in firefighting. This device does not carry high concentration oxygen. It carries regular air. Putting it on Danny is pointless, as there is no more oxygen in the SCBA air than there is in room air.
The quarantine area in which the Five-O crew is housed is a patient care area, as evidenced by the heart monitoring equipment. Every such area in a hospital would have pre-piped oxygen, which would be much more effective than giving Danny compressed air.