It's pretty much guaranteed. As long as you're able to acquire the space and materials needed to reach the proper ppm. The higher the ppm the faster it will render you unconscious. I have 1st hand experience with this so anything anyone else tells you will be their opinion. This is not an opinion. I was on a crew and our job was to replace sewer main. We were on a horrible job that ended up needing alot of safety after a tragic 1st day. Without going into major detail for a boring read. The first laborer entered the test hole to drop a video system, within 15 seconds he was just kind of leaning against the manhole wall, 10 seconds later he was unconscious. The 2nd laborer jumped in to help him out and passed out during his struggle to lift him off the ground in the mud and muck that had accumulated over the years in the drainage system. The tail laborer just finished suiting up in full tyvex and pro-mask to prepare for pressure washing because the township wanted to tie into existing manholes with new pipe. He literally almost broke his neck on the casting jumping in. At this time I myself and another laborer masked up to help. We managed to get both out. Because of my facial hair my mask didn't form a tight seal and I felt a light burning in my eyes and a sensation like I wanted to faint within about 3 minutes, luckily my mask bought me time. 1 laborer died by the time he was out of the hole. The other was alive but unable to resuscitate due to contamination. He died before the ambulance arrived. There was no pain, no gasping for air, no suffering. It was so fast and so undetected we thought they were messing around at first. It was a horrible day that I'll never forget. That is why this was my first chosen method of CTB. I have exactly what I need to carry out the process flawlessly and I know it is very peaceful. You breathe, you fall asleep and you die. That's it.
But here's the catch. The people who discover you and will want to help you, may do the same. This is why I decided to use my abundance of toilet bowl cleaner to clean my toilets and gallon of lime sulfur for my plants instead. I've chosen the tourniquet method in place of h2s.
Hope this helps.
