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When Mr. Rogers stepped through his front door, he would hang up his coat, slip off his shoes, and trade them for a cardigan and slippers. But chances are that not everyone in Mr. Rogers’ neighborhood took off their shoes the moment they walked inside. A lot of people happily tromp through the house wearing sneakers or stilettos or sensible flats.

As with many things in this world, though, Mr. Rogers may have had it right. There’s some evidence that wearing shoes indoors is not the healthiest choice, as it tracks in all sorts of bacteria, fungi, and plain old dirt that are best left outdoors. This is not exactly a new discovery. Back in 2008, microbiologists at the University of Arizona, studied how many and which kinds of bacteria linger on the bottom of shoes. They gave ten participants new shoes and then spent the next two weeks studying what ended up on the bottom of them. The results were, well, gross.

While the study was admittedly small, the researchers quickly determined that it was extremely common to things like mold, pollen, fecal matter, and coliform bacteria, including E. coli and C. Diff., on the bottoms of shoes. “If you wear shoes for more than a month, 93 percent will have fecal bacteria on the bottom of them,” Dr. Charles Gerba, who ran the study, told Today. The results were so marked that the professor in charge, that Gerba reportedly didn’t even want to put his feet on his desk afterwards.

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Where does all that nastiness come from? Well, think about everywhere your shoes go—public bathrooms, taxis, buses, movie theater floors, and sidewalks that dogs may have used as a restroom. Every sticky floor and stained carpet and strange puddle that you walk through deposits things on the soles of your shoes, which are then carried into your living room and kitchen and bedroom if you wear shoes indoors. If you have a child who is crawling, that is all passed along to them as well. “Essentially, when you wear your shoes in a house, you are bringing in everything you stepped in during the day,” Jonathan Sexton, a laboratory manager at the University of Arizona told The Wall Street Journal.

That said, as The New York Times points out, while wearing shoes inside may increase your exposure to dirt, germs, and other microorganisms, bopping around the kitchen in your Skechers probably won’t make you sick. If you’re a healthy adult or child (as in, someone with a fully functioning immune system) the chances of getting a serious illness like salmonella or meningitis from something you dragged in on your shoe is fairly unlikely. According to one expert the Times spoke to, the spread of germs through wearing shoes indoors was “more of a gross reaction than a health threat.” Another expert argued that a little dirt is actually good for you and even better for kids, because it can stimulate the immune system and make people healthier in the long run.

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