Doesn't have to be air tight, just needs to restrict the flow of oxygen enough to be less than what the body needs. Also, causing most of the air being inhaled to be the same air he exhaled, resulting in a feedback loop as he starts to hyperventilate due to oxygen deprivation.

Doesn't have to be air tight, just needs to restrict the flow of oxygen enough to be less than what the body needs. Also, causing most of the air being inhaled to be the same air he exhaled, causing a feedback loop as he starts to hyperventilate due to oxygen deprivation.