By that I was just using an example when I mentioned plants. And your thinking of Carbon Monoxide not Carbon Dioxide. Carbon Monoxide is created by fossil fuel burning when there is not enough oxygen when you burn the fuel. That is what people mean when they refer to Carbon Dioxide poisoning. Carbon Dioxide just prevents you from breathing. Holding your breath makes you run out of oxygen, and when you do you start producing Carbon Monoxide. I was agreeing with you when you were talking about the balancing features. And Mars and Venus do have 3% nitrogen in their atmosphere, so it is breathable, if you can breath nitrogen. Unfortunately, Venus has lots of Carbon Monoxide (and acids) and Mars' atmosphere has too low of a density to be breathed. But still, this seems pretty unrealistic: You change the entire human bio-chemistry down to the DNA level, just so you don't have to bring Oxygen tanks with you? That kind of makes you a different organism in my opinion. And it is chemically impossible to breath any other atmospheric bass other than nitrogen. You breath Oxygen by reacting the Oxygen molecule with the Carbon atom to release energy. CO2 has no obtainable chemical potential in the human body, unless you have a very reactive chemical in your lungs, which would destroy them. Nitrogen is a largely inert gas, so it is impossible to breath, unless you have a very reactive compound, of which there is precious few. I was not criticising your ideas, just pointing out how unrealistic this idea is. If it can ever be done, it would be a near end game event likely involving genetic modification. What isn't unlikely though, is some machine that turns CO2 into Oxygen and Carbon, and you could then breath the Oxygen, so you are technically 'breathing' the CO2