Paper money is notoriously nasty stuff. After being in use for a while, crinkly bills pick up residue from cocaine, fecal matter and live flu virus. Mostly, though, they are coated in a fine layer of "human sebum"—the "oily waxy substance the body produces," Popular Science reports, which accumulates on the bills and turns yellow.
After three to 15 years—depending on how quickly it gets dirty—paper bills are pulled out of circulation by the central banks and about 150 billion new bills are printed to replace them, PopSci says. That whole process costs about $10 billion.
To end this cycle of waste, researchers looked for a way to clean the sebum off the bills. A simple bath with supercritical carbon dioxide—which is used in other industrail cleaning processes—was enough to free the bills from biological filth as well as motor oil. The bills emerged from the tub nearly good as new, with all security features and details intact, sans dirt. Cleaning money, the team concluded, could save money.