This
https://www.right2die.org/
Basically if you take sodalime it absorbs CO2. Your body will eat the O2, and with that eating the CO2. Since N2 will basically be the primary gas after that, your body will treat N2 as O2. Because it isn't O2 your body will shut down and die. So it's in theory a death that is painless. And due to the lack of O2, you pass out quickly.
It's like an exit bag method, but far easier to get your hands on the right stuff and it is easier to deal with. Like with an exit bag if you get the pressures right, you might blow your lung out. Where with this, it in theory is hard to get wrong and in theory it is painless.