Millions of viruses daily circle around the Earth's atmosphere as well as get deposited on the planet's surface, a team of international scientists have discovered.

The study has for the first time quantified the viruses being swept up from the Earth's surface into the free troposphere, the layer of atmosphere beyond Earth's weather systems but below the stratosphere where jet airplanes fly. These viruses can also be carried thousands of kilometres there before being deposited back onto the Earth's surface.

"Every day, more than 800 million viruses are deposited per square metre above the planetary boundary layer -- that's 25 viruses for each person in Canada," said Curtis Suttle, virologist at the University of British Columbia in Canada. The researchers also found the majority of the viruses carried signatures indicating they had been swept up into the air from sea spray.

This may explain why scientists, from 20 years ago, began finding genetically similar viruses occurring in very different environments around the globe. "This preponderance of long-residence viruses travelling 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," Suttle said, in a paper detailed in the ISME Journal.

Further, the viruses tend to hitch rides on smaller, lighter, organic particles suspended in air and gas, meaning they can stay aloft in the atmosphere longer. "Bacteria and viruses are typically deposited back to Earth via rain events and Saharan dust intrusions. However, the rain was less efficient removing viruses from the atmosphere," said Isabel Reche, microbial ecologist from the University of Granada in Spain.

Using platform sites high in Spain's Sierra Nevada Mountains, the researchers found billions of viruses and tens of millions of bacteria are being deposited per square metre per day. The deposition rates for viruses were nine to 461 times greater than the rates for bacteria, the study showed.