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When life spread to land, the air filled with new species. The winds scattered mats of terrestrial microbes, and then plants and fungi began releasing spores into the wind. Later, some plants evolved flowers that released pollen grains. Their airborne journeys became part of the recipe for their enormous evolutionary success. The greening land also lured animals ashore. To get their oxygen, they adapted to breathing air rather than pumping water through gills. And then some animals—insects first, then birds and bats—evolved to move through the air, with leaping legs and flapping wings.

The animals became, in turn, hosts to another kind of airborne life: pathogens that floated from one host to another. Hantaviruses, for instance, infect rodents and then escape in their urine and saliva. On the ground they can survive in dried dust. Days later, a breeze can pick up hantaviruses and carry them into the nose of another rodent visiting the same spot, causing a fresh infection.

Other pathogens turned the lungs of air-breathing animals into both a home and a launching pad. They get drawn into a host with an inhaled breath. Animals often react to a respiratory infection with an onslaught of immune cells and inflammation. This attack leaves an extra supply of mucus in the airway. To clear it out, the animals will use their lungs to deliver a powerful cough or sneeze. The contaminated mucus droplets can then strike other animals or contaminate the ground. But even regular breathing can be enough to spread some microbes. When an animal exhales air, the outgoing flow pulls droplets off the moist walls of the lungs, like a breeze passing over the ocean. Those droplets can evaporate down to droplet nuclei and float away.

Airborne diseases also took advantage of the social lives of animals. As some species evolved to live in close groups—in nests, burrows, flocks, and herds—they made it easier for a cloud of exhaled pathogens to infect a new host. It's likely that airborne diseases have fared best among animals that live together. If a solitary creature breathes out microbe-laden droplets, they may fail to reach another member of its species before they fall to the ground or get damaged by sunlight. In a herd or a den, a sick animal can release clouds of pathogens that have better odds of getting inhaled by another nearby host.

Measles, the most infectious pathogen ever found, belongs to a family of viruses that typically infect grazing mammals. Some infect seals. While seals spend much of their lives out at sea, they also haul out onto beaches where, huddling together in groups, they mate, raise their young, and breathe viruses on one another. Dolphins get their own form of measles too. While they never come ashore, they still have lungs and breathe through blowholes—a legacy of their terrestrial ancestors, which lived on land 50 million years ago. Swimming in pods, they surface together to exhale blasts of air and suck in new ones. The measles virus takes an airborne hop before the dolphins dive underwater again.

When aerobiology emerged in the 1930s, it generated great excitement, but within a few years it faltered. In World War II, the United States and other countries recruited aerobiologists to make biological weapons. And when the war ended, the aerobiologists kept on growing pathogens to wipe out cities and starve nations. A shroud of secrecy fell across much of aerobiology. Even today, the science is not entirely free of it.

In those postwar years, some aerobiologists tried to persuade public health officials to take the threat of airborne infection seriously. They largely failed. Infectious disease experts who led the fight against outbreaks and prepared for the emergence of new diseases mostly ignored the aerobiologists, even when it meant accepting some basic mistakes about the physics of air.

The Covid-19 pandemic finally rattled that consensus. In so doing, it provided an opportunity to rethink our history with the air. The Covid-19 pandemic was not a fluke. It belongs to a deep history of airborne life, one that has adapted with astonishing efficiency to our species's rapid rise—from the dawn of agriculture ten thousand years ago to the rise of cities, to the Industrial Revolution, and now to the twenty-first century's megacities and decimated wilderness. SARS-CoV-2 is only one species in an airborne habitat that we largely ignore, but would do well to understand.

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The Skagit Valley Chorale singers finished their concert with the Sanctus—"Heaven and earth are full of thy glory"—and then took their bows. As the audience cheered, I checked my monitor. The level of carbon dioxide had reached 903 parts per million. In our communion, we had altered the air.

After the applause ended, Grace and I filed out into the high-ceilinged lobby. We congratulated Burdick and the Backlunds, passed by the fifth graders swarming their teacher as if she were Taylor Swift, pushed open the outer doors, and walked into the night air. It was the same air we had just breathed inside McIntyre Hall, the same seamless blanket of gases. The only difference now was there was no ceiling to hold it down. I could exhale all the carbon dioxide I wanted, but my CO2 monitor would not budge. A few miles overhead, the moisture in the atmosphere formed clouds that blocked our view of the stars. Above the clouds lay the stratosphere, a huge realm of thin gases that rises to about thirty miles over our heads, where it gives way to the mesosphere and the exosphere, the edges of the Earth's air where meteors glint as they die. Scientists now refer to the life that teems in this space as the aerobiome.