Nitrogen asphyxiation is commonly used to kill stray animals. No deadly chemicals to endanger people working in the death chamber, mortuary or funeral home. Easily administered. Relatively painless.
Respiratory distress is caused by the inability to expel carbon dioxide. Carbon dioxide asphyxiation is extremely painful. A convict undergoing nitrogen asphyxiation becomes light headed before passing out, and eventually dying. It is similar to what a person experiences in a decompression event without oxygen. I have experienced rapid decompression and gradual decompression in Air Force training (as a contractor - one week course) and it was not at all unpleasant. They managed not to kill us.
If it’s good enough for voluntary euthanasia it’s good enough for executions.