Fresh air might not be quite as fresh as we think. Viruses and bacteria get swept up into the atmosphere in enormous amounts, and now a new study has quantified that amount, finding that untold billions of microorganisms are raining down across the Earth every day.

For decades, genetically identical viruses have been found in very distant parts of the world. Scientists determined that the bugs were hitchhiking on airborne particles that are swept up into the atmosphere and carried long distances before being deposited back to the surface.

"Roughly 20 years ago we began finding genetically similar viruses occurring in very different environments around the globe," says Curtis Suttle, senior author on the new study. "This preponderance of long-residence viruses traveling the atmosphere likely explains why — it's quite conceivable to have a virus swept up into the atmosphere on one continent and deposited on another."

The mechanism may have been known, but it wasn't clear just how many viruses and bacteria were using this method to get around. Conducted by scientists at the University of British Columbia, University of Granada and San Diego State University, the new study set about quantifying that number.

To make any long-distance journeys, microorganisms would need to get above the whims of Earth's weather systems, which means climbing to an altitude of 2,500 to 3,000 m (8,200 to 9,800 ft). So, the researchers set up their work station high in the Sierra Nevada mountains in Spain, which can reach those elevations.

There, they found that over 800 million viruses and tens of millions of bacteria are deposited per square meter per day, from that planetary boundary layer of the atmosphere. In some cases, the rate that viruses rain down is up to 461 times higher than that of bacteria. The researchers say this is likely because viruses can cling onto smaller particles than bacteria, meaning they can stay airborne longer and travel further. Most of the viruses also showed signs of having been swept into the air via sea spray.

"Bacteria and viruses are typically deposited back to Earth via rain events and Saharan dust intrusions," says Isabel Reche, an author of the study. "However, the rain was less efficient removing viruses from the atmosphere."

The research was published in the International Society for Microbial Ecology Journal.