When you pay for food, do you wash your hands before sitting down to eat? If not, you may want to start, because a growing body of research suggests that cash is filthy. Paper money can harbor thousands of microbes from every environment it touches—whether that’s someone’s fingers, a waiter’s apron, a vending machine or the dank area under someone’s mattress.
In a 2017 study published in the journal PLOS ONE, researchers swabbed $1 bills from a bank in New York City to see what was living on paper currency. They found hundreds of species of microorganisms. The most abundant were ones that cause acne, as well as plenty of harmless skin bacteria. They also identified vaginal bacteria, microbes from mouths, DNA from pets and viruses.
Cash is also often streaked with drugs. In a study of 10 one-dollar bills from cities across the country, nearly 80% of them had traces of cocaine.
All of that may sound unsavory, but it's hardly surprising, given how cash gets around. Bills get traded constantly, and depending on the denomination, they can stay in circulation for five to 15 years. “A lot of people aren’t washing their hands, and they’re at a restaurant and money is going back and forth," says Susan Whittier, a microbiologist at New York-Presbyterian and Columbia University Medical Center. "You don’t know who’s touched it." Other research has shown that some bank notes and coins contain pathogens like Escherichia coli (E. coli), salmonella and staphylococcus aureus, which can lead to serious illness.
The presence of these microbes won’t necessarily make you sick, however. “Certain subtypes of organisms are better or worse at infecting people,” says Emily Martin, assistant professor of epidemiology at the University of Michigan School of Public Health. “Organisms also grow better in certain specific environments. Just being on a surface doesn’t give it everything it needs.”
U.S. currency is a pretty plush place for germs to land. It's 75% cotton and 25% linen, which offers a soft environment into which microbes can settle. Yet cash doesn’t typically have the right temperature or moisture conditions to allow microbes to grow and proliferate. Its porous surface actually helps it hold on to most of the germs it’s carrying, so not many microbes wipe off on your hands—meaning money is not very good at transmitting diseases.
Even if some microbes do come off on your hands, they’re unlikely to hurt you there, explains Martin. "You don’t want to introduce bacteria into areas of the body that have less protection," she says, "but our skin is a really good protector." (Just don't lick your money, she advises.)
Some research has shown that plastic polymer bank notes, like those used in Australia and Canada, are cleaner than American bills. Scientists have also explored ways to clean currency using carbon dioxide and high temperatures to kill microbes, but experts say it’s generally not worth it for you to worry about personally cleaning your cash.
“Be aware that every surface you touch has stuff on it: money, the subway pole, the ATM,” Whittier says. “You just have to wash your hands a couple times a day.”