Standing in an enclosed space with a running combustion engine is, in fact, "arguing for standing in an airtight coffin" and equivalent to breathing "only our own exhalations".
If you have any doubts, the crew of Apollo 13 will be happy to give you a seminar about the dangers of carbon dioxide and by extension carbon monoxide produced by combustion engines.
If you have any doubts, the crew of Apollo 13 will be happy to give you a seminar about the dangers of carbon dioxide and by extension carbon monoxide produced by combustion engines.