The incredibly diverse mix of molecules and particles that we call air has a history stretching back to 4.5 billion years ago, when space rocks and planetesimals bombarded the still-forming Earth. As those objects collided, they released a thick soup of carbon dioxide and nitrogen gases. Later, other kinds of meteorites and comets, vaporizing on impact, brought water to our planet, along with additional carbon and nitrogen. Volcanic eruptions then began pumping steam and still more gas out of the young planet’s interior. As the eons passed, most of the carbon dioxide was absorbed into carbonate rocks, and Earth’s atmosphere, which started out 10 to 20 times as thick as it is today, gradually thinned.

Even then, the air was far from the oxygen-rich, life-sustaining stuff we enjoy today. About 2.7 billion years ago, photosynthetic algae in the oceans started making their mark, taking in carbon dioxide as fuel and sending the by-product—oxygen—skyward. The sun’s ultraviolet rays split some of these oxygen molecules, and the freed elemental oxygen combined with normal oxygen to create ozone. Over time, the new molecules piled up and formed the ozone layer, a natural sunscreen floating a few tens of miles above the ground. Shielded from harmful UV rays, which would otherwise eat away at DNA, plant life thrived. More plants meant more photosynthesis and still more oxygen in the atmosphere. Eventually, the atmosphere contained enough oxygen and high-altitude ozone to sustain air-breathing life as we now know it.

As Earth’s land and seas became more crowded, so, too, did the air. The atmosphere consists primarily of 78 percent nitrogen, 21 percent oxygen, a highly variable smattering of water vapor, and trace amounts of argon, carbon dioxide, and other gases. Plants, animals, and especially humans add their own distinctive chemical imprints on top of that bulk composition. There are the usual suspects—excess carbon dioxide, carbon monoxide, sulfur dioxide, nitrogen oxides, soot, and lead produced by human activity. There is also a long list of other pollutants: Cars, boats, wood-­burning fireplaces, and other sources add more than 250,000 tons of cancer-­causing formaldehyde each year. Home cooking and heating contribute to the 375,000 tons of benzene, another carcinogen, that Americans release into the air each year. Agricultural operations in the United States alone emit more than 2 million tons of ammonia. Even water vapor can contribute to pollution. Additional moisture near coal-burning power plants accelerates the conversion of sulfur dioxide fumes into sulfuric acid, increasing acid rain in the area.

“We’re breathing in this soup of stuff that is not the dominant part of our atmosphere but present in sizable quantities,” says environmental engineering professor William Nazaroff of the University of California at Berkeley. “There’s a lot we understand, and there’s an enormous amount we don’t understand about this wonderful and complex system.”

Anything released into the atmosphere spreads quickly because air is constantly on the move. The sun’s warming radiation drives circulation from the equator to the subtropics. Temperature and pressure variations power winds and storms, from minor eddies to angry funnel clouds. On a global scale, the heating of atmospheric molecules causes the lower atmosphere, or troposphere, to expand and stretch higher during the day; it then settles back down as it cools at night. At the smallest level, air molecules continuously jostle against one another at hundreds of miles per hour, ricocheting like billiard balls. In a matter of weeks, the molecules in any given volume of air will circumnavigate the globe. After a few years, the original molecules will be evenly distributed throughout the lower atmosphere.

To put a human face on this: When Julius Caesar exhaled his last breath, he bequeathed approximately 10²² molecules to the atmosphere around him. Air leaving the lungs is 78 percent nitrogen, 16 percent oxygen, and 4 percent carbon dioxide—depleted in oxygen and enriched more than a hundredfold in carbon dioxide compared with what went in, due to human metabolism. Some of the carbon dioxide probably got trapped and digested by plants in a nearby garden, but the vast majority of the exhaled molecules began to fan out over an ever-widening area. Within a decade, that breath had dispersed completely around Earth, and most of it is still in circulation. Odds are, at least one of the molecules that Caesar (or Mozart or Martin Luther King, for that matter) relinquished when he died is flowing into your lungs as you read these lines.