One issue here is that the molecules we breath in and out stop existing because they change and are broken apart. In simple terms cellular respiration involves

One issue here is that the molecules we breath in and out stop existing because they change and are broken apart. In simple terms cellular respiration involves Sugar molecules (C6H12O6) reacting with oxygen molecules (O2) to produce Carbon Dioxide (CO2) and Water (H20). Now a common misconception folks have from school is that the oxygen “turns into” carbon dioxide. This isn’t true. What actually happens is that the Hydrogen atoms from the sugar molecule eventually combine with the Oxygen molecule to produce water. The water is generated from hydrogen being accepted by the oxygen molecule. The carbon dioxide is what remains of the sugar molecules once the hydrogen has been removed. Of course there are many steps in this process but this is broadly how cells generate energy. As you can see this process leads to the creation of molecules and the destruction of others. Now let’s move away from the cell and towards the lungs more broadly. During the act of breathing we breath in many molecules. Most of what we breath in is Nitrogen molecules which are inert and don’t do anything. We also breath many other molecules including oxygen, water, etc. Now not all the oxygen molecules we breath in enter the blood stream. Some will be expired out. When we breath out we also breath out those newly created carbon dioxide molecules I mentioned. (It actually gets even more complicated because some of those carbon dioxide molecules get converted into other molecules that get removed through the urine). Furthermore some of those water molecules that are created by our cells are also in the breath that we expire. It is important to keep the lungs moist. The breath we inspire is more humid than what we inspire. But it gets even more complicated. Because some of those water molecules being expired are created by our cells but others enter the body from the water we drink. Some of the water molecules our cells create leave in the urine or through our skin or through feces and some of those water molecules that leave from those areas are also those that enter the body from the water we drink. So Caesar’s last breath happened thousands of years ago right? And we have countless animals and countless plants in all that time creating molecules and destroying them. Given all these animals and plants I would say that it stands to reason that these molecules either don’t exist or may no longer be in gaseous form. And remember the nitrogen molecules I mentioned earlier? They are inert in our lungs but not inert in other parts of nature. Anyway the point I’m making is that this question is more complicated because it’s doubtful a large chunk of Caesar’s Last breath’s molecules even exist anymore.