It's true that we breathe a lot of molecules which have passed into the lungs of Leonardo, but also into those of Adolf Hitler or all the human beings who have frequented this planet. The calculation that leads to this conclusion is not very complicated -- but I'll do without it here, lack of formatting tools (high-powered mathematical-chemical formulas).
In all cases, we can nevertheless show, that we've all even breathed five of the molecules exhaled by Leonardo during his last breath!
Obviously, we assume that the mixture of Leonardo's molecules with that of the atmosphere is good (in five hundred years, there has been time to do so), that Leonardo did not re-inspire some of the expired molecules, and that there was no loss to the atmosphere due to combustion, fixation by plants, etc. Either way, it wouldn't make much difference to the end result.
Better yet, not only do we breathe the air it has exhaled, but it's likely that we swallow, in each glass of water we drink, a little of the liquid it has released. Urine or sputum, you have a choice.
The law of conservation of matter means that atoms are infinitely recycled in the universe. However, if you consider the time during which the dinosaurs lived, it's almost certain that each of your breaths has already been partly breathed by a dinosaur, and that each apple that you eat contains atoms that have in the past made up an animal or a man. Obviously, this may pose some problems for vegetarians.
On the other hand, this question should make homeopathy followers think. In any glass of water there's an assortment of active homeopathic molecules of all the diseases we may have. And it's free.