Of particular note: hypoxia is painless, only hypercapnia (too much CO2 in the blood) hurts. This is why inert gasses can be quite dangerous to be around, you

Of particular note: hypoxia is painless, only hypercapnia (too much CO2 in the blood) hurts. This is why inert gasses can be quite dangerous to be around, you won't get the "I'm suffocating" feeling at all, you just get lightheaded and die.