system
Given the following post, respond as if you're commenting on Reddit:
human
"You breath in a single atom of Julius Caesars final breath every breath you take." How far can this be extrapolated?

It is said that you breath in a single atom of Julius Caesars final breath every breath you take. 

Given that there's nothing special about either Ceaser or which breath it was, can this be extrapolated to say we breath in an atom from every breath that any person who ever existed took? (Or maybe from before a certain date of death - I doubt I breath any atoms from the most recent breath of someone on the other side of the world to me, right?)

How far can this go? I presume it becomes unreasonable to say that if some oxygen breathing creatures existed on a different planet in a different solar system that any of their atoms would have made it to here?
gpt
The extent of it depends on a thorough mixing of atoms, and is therefore limited by diffusion. Working out diffusion on a global scale is not easy at all, so it would become very difficult to pinpoint exactly where in the world at each point in history this holds for. 

In terms of the question of other oxygen breathing species: If they required oxygen to breath as we do, It would have to mean that their planet had a sufficient mass to stop oxygen being able to exit the atmosphere, as we go here on earth. Therefore, the amount of oxygen being released by such a planet would be more or less nil. Added to this that the chance of even a single atom being able to travel from another solar system to ours make it so improbable that it is basically impossibile, even when you are playing with the number of atoms in a single atmosphere.

I got bored and decided to prove the idea of us breathing in Caesar's last breath, for anyone interested

Working with a breath being a full lung capacity (to maximise the amount of atoms Julius Caesar breathed) That gives us 6L of air to work with. Wolfram Aplha gives this as 7.65g. The mass of the Earth's atmosphere is usually given as 5.1*10^18 kg, so 5.1^10^21 g. Converting this to numbers of atoms gives 1.59*10^23 atoms in 1.06*10^44 atoms, so one atom in every 1.5*10^21 was part of Caesar's last breath. When we breath in, we take in about 500mL of air, or 1.33*10^22 atoms. That means that on average, we breath in 10 atoms that were a component of Julius Caesar's last lung capacity.