Rapid heart rate and rapid breathing. Nitrox reduces the decompression obligation of the diver. It can make nitrogen narcosis worse. Carbon monoxide is toxic due to CoHb production and the effect on the cytochrome A3 system. Breathe nitrox and limit dives to 165 fsw.
More nitrogen is generated through metabolism.
Helium is used to prevent decompression sickness in divers. Limit the duration of oxygen breathing to 20 minutes, followed by 5 minute air breathing periods. There is an excess of nitrogen in the surrounding water. All of the above. Sorry, no guessing allowed.