> Anyone know the medical term for this.

I'm pretty sure it's asphyxiation if you die as a result of carbon monoxide or too little oxygen. I found this little bit on the cause of death for mustard gas:
Death may also result due to pulmonary oedema, Adult respiratory distress syndrome, airway obstruction, arrythmias and cardiac arrest.

So If you don't suffocate I'd guess the cause of death would be related to the type of poison.

So who are you going to kill? Can I watch? SNUFF!