Remember when Heath Ledger’s Joker burned all that money in The Dark Knight? Pretty crazy, right? Actually, maybe not. If the Joker was aware of how much bacteria exists on your average piece of currency, you might think he performing an act of heroism, not super-villainy. A new study suggests cash is really gross. Here’s what’s going on.
Money is touched by hundreds if not thousands of hands in the course of its lifetime, which means it’s pretty gross. This is something people grasp intuitively, but a Dutch professor of infection prevention and his young son put some real scientific data behind the impulse you have to wash your hands after handling bills. Fast Company reports that Andreas Voss and his son Timothy began their work on this question as a “fun project.” They exposed a variety of global currencies to UV light, sterilizing them before dropping samples of dangerous bacteria like MRSA and E. coli on them. They then tested the notes for bacteria after 3, 6, and 24 hours.
They also ran a separate test, with safer bacteria, in which people rubbed these notes between their hands for half a minute to see if anything rubbed off.
Here’s what the Vosses found. At three hours, many banknotes were still teeming with bacteria, by a full day later the vast majority were clean. The Romanian Leu, however, is polymer-based. The makes it expensive, durable, and more difficult to counterfeit, but it was also the bill that remained covered in bacteria even after a day.
The smooth polymer also proved to be a fine surface for transmitting bacteria. Multiple bacterial colonies were left on the hands of those who rubbed it, compared to a single colony for a U.S. dollar and no colonies for a Euro.
So while circulation can make paper money gross, American currency is relatively resistant to bacterial growth and circulation. And Voss—the professor not the kid—says you’re probably OK even if you handle money on a regular basis. He does, however, suggest that you “wash your hands before contact with your mucous membranes,” which seem like pretty solid advice even if you’re living a cash-free lifestyle. And that’s no joke.