Years ago Jonathan Miller had a TV show about the body. To close one of the episodes, he put on a breathing mask that was connected to a set of bellows, forcing him to rebreath his own air. Naturally after a few minutes he had to tear the mask off to get some fresh air. He then inserted a canister full of carbon dioxide absorbing chemicals in the tube going to the bellows and tried again. He was repeatedly signing his name while saying how he felt. You could see his signature getting increasingly haphazard while he was saying that he felt fine. He was saying that he was feeling ok when he collapsed. They then rolled the credits and you saw people rush in with oxygen to revive him.

It is easy to forget that our senses will not always save us.