We need oxygen to live, yet there's always too much of a good thing. Pure oxygen can be deadly. Oxygen is like alcohol in an alternate universe where we're all alcoholics. (Would also make a good movie. Again, Steven Spielberg, hit me up.)
Personally, I believe oxygen is killing us and it just takes several decades to take full effect on humans, 10-15 on dogs, 15-20 on cats, 7 on rabbits, 150 on tortoises, etc. What if tortoises have an immunity to oxygen? That would be heckin cool. This image is eerie.
This is kind of opposite to the question, but it's a cool article. It could potentially make a good thriller movie if it were possible to capture 21% of the atmosphere. Steven Spielberg where you at?
Models of past eras show that oxygen can influence global temperature and humidity as its concentration changes. What if increasing oxygen levels would cause more climate change?