A simple calculation shows it's likely that you have breathed in the air breathed out by anyone from Marilyn Monroe to Genghis Khan.
When wandering around a public space and looking at the incredibly diverse and vibrant population that fills it with life and culture, usually the last thing you want to think about is breathing in something that was once inside one of the grubby, germ-filled weirdos around you. Sadly, not only are you doing that, you are also breathing in a few atoms which have wormed their way through the phlegmy respiratory system of almost every human in history. Don't want to believe it? Too bad. Believe it you shall. It's been mathematically proven.
Let's start with the weight of earth's atmosphere: 5 x 10^21 grams.
A mole of air consists of around 6 x 10^23 molecules and 28.8 grams, which means that there are about 1.04 x 10^44 molecules of air in earth's atmosphere.
That's a lot of atoms to choose from. Surely there's enough molecules of atmosphere to provide fresh air to both you and, say, Edgar Allan Poe.
As it turns out, humans cycle through a lot of air in our lives.
One mole (6 x 10^23 molecules) of a gas, occupies around 25.4 liters, and humans take one liter breaths. This means that each breath uses about 2.4 x 10^22 molecules.
Twenty-five breaths per minute, times 60 minutes per hour, times 24 hours per day, times 365 days per year, in say, a 30 year lifespan, means a person breathes 9.46 x 10^30 molecules of air, even during olden times when people died of tiger bites and infected splinters, and getting crushed by the newly-invented wheel.
That shrinks earth's atmosphere of 1.04 x 10^44 molecules considerably. It means that one in every 1.1 x 10^13 molecules will have passed through any specific person's lungs. And since, as we've established, every breath you take contains 2.4 x 10^22 molecules, you have been breathing some very, very historic air.
There are complications. Time has to pass in order to circulate the air from one side of the globe to the other. And many of us do our breathing inside enclosed spaces, meaning we get a lot of overlap, re-breathing our own, overused molecules. Still, chances are that during your lifetime, you will breathe in some air breathed by almost every human who came before you.