Submarine anecdote: we'd bump up oxygen levels a little bit to make the crew more alert. Oxygen had a max spec that we worried about because high oxygen could c

Submarine anecdote: we'd bump up oxygen levels a little bit to make the crew more alert. Oxygen had a max spec that we worried about because high oxygen could cause fires. Anecdotally a submarine crew is too exhausted to feel effects from CO2 when it's in spec. If atmospheric controls were down and CO2 shot up, headaches were normal. Far from a controlled experimental environment.