Most coal miners that get trapped underground during a collapse normally die of CO2 poisoning. Their lungs don't have clean air to breathe, CO2 will build up in their blood and they go to sleep. Eventually they will die without proper intervention.
My Mother-In-Law has CO2 build up in her blood and carbon monoxide in her lungs. She is a medical miracle, why she hasn't been less alert we may never know. I guess God will decide when to call her home.
Her respiratory problems started 10 years ago with bronchitis that was reoccurring frequently. She was treated with antibiotics several times a year. Four years ago she was put on oxygen but she didn't need it all the time.
Three years ago she had a heart attack. It was so bad the hospital was surprised she even made it there. The hospital attempted to do a catheterization but found an aneurysm on top of her heart and life flighted her to a larger hospital better equipped to deal with the aneurysm. The larger hospital tried twice before they were successful. She developed pneumonia during this time and during her checkup a dark spot was seen on her lung. We now know that spot is cancer. Her lung function is very limited and only partially does the job it's supposed to thus the CO2.
She made her choice to decline further intervention such as being put on a ventilator. That is her right to do so, I would do the same thing. Know your rights and let your family know what you want. It's your body and life. Put it in writing so there's no confusion when the time comes.