Well before that dollar bill in your pocket retires at age five, it will have touched hundreds, perhaps even thousands, of people’s hands. From one perspective, that’s amazing. Money is the one single object that strangers readily share. From another, that’s a lot of dirty hands!
And as it turns out, our currency can harbor and spread some nasty bacteria—though the good news for most people reading this is that the U.S. dollar and the euro, and their mixture of fibers like cotton and stabilizers like gelatin, both perform pretty well . . . especially compared to their more plastic-based counterparts, such as the Romanian leu, which is quite literally the dirtiest money known to humankind.
The research, published in Antimicrobial Resistance and Infection Control, just won an Ig Nobel (basically, the Razzies of good science for gross findings) and was led by Andreas Voss, a professor at Radboud University who was the first professor of infection prevention in the Netherlands. It began as what he dubs “a fun project” with his son Timothy, who is also an author on the paper.
Researchers incubated various bacteria, like the dangerous MRSA and E. coli, and using a dropper, seeded them onto all of the aforementioned bank notes, along with those from Croatia (the kuna), Canada (the Canadian dollar), and India (the rupee). The notes were sterilized with UV light, squirted with bacteria, and then allowed to dry before being tested for bacteria over the next 3, 6, and 24 hours. Then, in a few trials with some less-dangerous bacteria, people were actually instructed to rub these bank notes between their hands for 30 seconds, to see if anything rubbed off.
After only three hours, many bank notes retained their bacteria—but by 24 hours, most swabbed as clean. The big exception was the leu, a polymer-based bill with the image printed on top of it, kind of like the label on a jar of peanut butter. (Since Australia first introduced polymer currency in the 1990s, it’s become increasingly popular for both its expense, durability, and difficulty to counterfeit.) The leu continued to exhibit growth of all bacteria after six hours, and some remaining MRSA even after a day.
As Voss confirms over an email interview, the culprit is the polymer, which for whatever reason, seems to be a pretty welcoming home to bacteria. Its smooth surface also transmits bacteria back to humans pretty well, too. Whereas rubbing an infected euro led to no transmission in testing, and rubbing an infected dollar led to measurable but limited “single colony” transmission, the leu passed along multiple colonies of each bacteria to those who handled it.
Notably, Voss has no idea why the euro performed better than the dollar. He also tells us that he didn’t bother testing coins, because metals like silver and copper are known for their antimicrobial properties (copper is used in some medical equipment for just this reason). And he points out that digital payments, like Apple or Google Pay, would be the ideal currency for preventing pathogen transmission.
But all in all, the findings don’t bother him as an infection specialist, and he doesn’t think they should worry you. “[It’s] not an issue. [There’s] no danger for public,” he writes. Though after handling money, he suggests that you “wash your hands before contact with your mucous membranes.”