If you’re one of the billions of people worldwide to use mass public transit regularly, you’re sharing a lot more than a commute with your fellow passengers, suggests a new study published Tuesday in Cell Reports. You’re also sharing and swapping the teeming microbes that call our bodies home.

Researchers in Hong Kong—home to a public transit system that services 5 million commuters every day—recruited volunteers for an unique experiment. Over the course of several days, volunteers were asked to ride one of eight subway lines on the Hong Kong Mass Transit Railway system during the morning and evening rush hour. Before they boarded, they washed their hands, and once on board, they made ample use of the handrails. After they spent 30 minutes on the train, they exited and had their palms swabbed by researchers.