Cuz it's not for CO2. It's for CO. That's less dense than O2. Methane, CH4, also floats, but they add an extra gas in your natural gas so it smells like rotten eggs. It's probably mercaptan, H2S or ,SO2, which is more dense.

Also meaning, if you smell rotten eggs, you have a gas leak and the air in your house is potentially explosive. Get out and call the emergency crews. It's either natural gas or sewage gas.

Also, if it smells like fish, it could be propane or butane if you have tanks of those around. Some RVs use propane as a coolant instead of Freon.

Violence = Extinction. We do have a tendency to kill each other, and our many various descriptions of death are merely ways to either blame or justify the killing.

It's been scientifically proven why all the (more peaceful,btw) Neanderthals are missing. We humans murdered the ones we couldn't breed.