Yes- because they used CO2. CO2 forms carbonic acid in the blood stream which upsets the acid-base homeostasis. Our brains detect that imbalance which is what triggers the breathing reflex. The more CO2 you give them- the higher the level of carbonic acid- and the stronger the reaction. In other words- it's not lack of oxygen that causes us to want to breathe- it's too much CO2.
If you fill a room with only Nitrogen, or just place a non-rebreather mask on their face and connected to a Nitrogen tank- they will simply fall unconscious and die within a couple of minutes. There is no pain, and no panic.
It is, in fact, a painless and quick death.