Louis Pasteur opened a glass flask on Montanvert Glacier in the French Alps in 1860 and collected some air. A few days later the bottom of that flask was teeming with goo—proof to Pasteur and his colleagues that there was something in the air, something invisible but quite real. Today we understand what that invisible stuff is—microbes aloft in our atmosphere—but despite the more than 150 years that have passed since Pasteur's experiment, scientists are just beginning to understand how microorganisms in the air affect life on earth.

Recently scientists captured more than 2,100 species of microbes traversing the Pacific Ocean from Asia to North America on huge plumes of air in the upper troposphere—up to 12 miles above the surface of the earth. A good fraction of them were bacteria, which can mean trouble for human health. In Africa, in a region known as the meningitis belt, dust storms carry the bacterium Neisseria meningitidis (pictured above), which infects around 200,000 people there annually. Yet for most people in most places, the microbes in the air are totally harmless, says David Smith, a microbiologist at the nasa Kennedy Space Center and lead author on the work that found the 2,100 traveling microbes. “You don't need to be worried,” says Smith, whose findings were published online last December in the journal Applied and Environmental Microbiology. “This has been happening naturally, always.”

Beyond health, microbes in the atmosphere might also be important for climate. “We're interested in whether they can contribute appreciably to the concentrations of cloud nuclei,” says Susannah Burrows, an atmospheric scientist at Pacific Northwest National Laboratory in Richland, Wash. Bacteria can clump together, forming the seed around which clouds form and thus providing a key component of our atmosphere, she notes.

Other researchers wonder exactly how microbes behave while aloft and if they can reproduce as they travel. “We have several indications that microbes in the air are alive and active” and not just hitching a ride, says Paraskevi Polymenakou, an atmospheric microbiologist at the Hellenic Center for Marine Research in Greece.

For Dale Griffin, a microbiologist at the U.S. Geological Survey, the questions push beyond the atmosphere. “No matter how high we look, we seem to be able to find life,” he says. Smith wonders not just how high that life goes but how it survives at such heights. “As a student in biology, I felt like everything had already been investigated,” he says. “The atmosphere allows the opportunity to characterize a place where nobody has looked for life.