Yes, it’s annoying when someone asks you to take off your shoes before you’re allowed to step inside their home. But it’s also annoying (and painful and embarrassing) to contract some form of explosive diarrhea, which is something that can happen when germs from the street are tracked all over a kitchen or bedroom floor.
In fact, a study conducted by Charles Gerba, a microbiologist at the University of Arizona, found more than 421,000 units of bacteria on the outside of the average pair of shoes. What’s worse, it found that the transfer rate from a contaminated place to a previously uncontaminated place — e.g., that dive bar bathroom to your bathroom — was 90 to 99 percent. In other words, what was on the floor of that restroom is now almost definitely in yours.
But why is the transfer rate so high? Are germs really that good at hitchhiking? “Surfaces that are softer with more texture [like the soles of your shoes] are more likely to retain materials from the surfaces they come in contact with compared to harder, smooth surfaced materials,” says Dave Westenberg, associate professor of biological sciences at Missouri University of Science and Technology. “I also imagine shoes are more likely to transfer microbes because not only do they contact surfaces, but we apply pressure and other motions that help to pick up material from the surfaces we walk on.”
So what kind of microbes are you dragging into your home? Let’s take a tiny, microbial tour.
Better known as E. coli, which thrives in fresh animal feces and is literally everywhere. Sure, you avoided stepping in that big pile of dog shit on your way home, but that doesn’t mean other people (or animals) did. If Gerba’s study proved anything, it’s that on a microscopic level, poop bacteria — mainly E. coli — is all over the place. “We walk through things like bird droppings, dog waste and germs on public restroom floors, all of which are sources for E. coli,” Gerba notes in the study. And as you know from the last time you ate some bad sushi, E. coli causes intestinal and urinary tract infections, diarrhea and, in extreme cases, meningitis.
Klebsiella pneumoniae is a bacterium that normally lives inside the human intestine, where it won’t cause disease. If the bacteria gets itself into other parts of your body, however, it is a “superbug” that can be a source for lung and bloodstream infections, as well as meningitis. That said, it’s considered a “nosocomial” infection, meaning you’re most likely to get this on your shoes in a hospital or clinical setting rather than in your everyday wanderings. According to Gerba’s study, if you walked through a hospital or clinical setting where this might have been lingering on the floor, you have a 90 percent chance of tracking it all the way back into your home.
Another bacteria closely related to E. coli but much less prevalent, S. ficaria is mostly found around fig trees, or as with Klebsiella pneumoniae, hospital settings, where it can be the cause of infections in wounds and the respiratory tract. As comparatively rare as this bacteria is, it was still found on the shoes of 10 percent of the participants in Gerba’s study over the course of two weeks.
A University of Houston study found a higher percentage of C. diff on the bottom of shoes (25 of 63 tested, or 39.7 percent) than on bathroom/toilet surfaces (33.3 percent), house floor dust (33.3 percent) and other surface swabs (18.9 percent). This pathogen is typically found only in hospitals, but Westenberg thinks you’re probably fine if you do get it on your shoes. “If you have a healthy immune system and a healthy microbiome you’re at very little risk,” he says. “C. diff is mainly a concern for people who’ve been on extensive antibiotic therapy so their microbiome is out of balance, so C. diff is able to colonize and cause complications.” If you’re an older adult, however — or otherwise have a weakened immune system — C. diff can lead to life-threatening inflammation of the colon.
The only non-bacterium on this list, 2,4-Dichlorophenoxyacetic acid is one of the most widely-used herbicides in the world, used to kill those pesky broadleaf weeds that sprout up all over your lawn. Since it kills these weeds and leaves grass unharmed, users will coat their lawns in the chemical, where it could rub off on your shoes either directly or through water runoff. Though cleared by the EPA, it’s listed as a possible carcinogen by the World Health Organization, since there’s data to suggest that this particular acid is linked to causing non-Hodgkin’s lymphoma (it was also one of the ingredients of the infamous Agent Orange). Once tracked indoors on the bottom of a shoe, 2,4-D can stay in your carpet for up to a year.
So what to do about this hive of disease living on your shoes? According to the University of Arizona study mentioned earlier, simply washing your shoes with detergent will kill up to 99 percent of the bacteria. And even if you don’t, Westenberg concludes that assuming you’re a healthy individual, the majority of bacteria you pick up are of the more harmless variety. “Your shoes can track in all sorts of microbes and many have scary-sounding names, but as long as you’re healthy, you shouldn’t be concerned,” he says. “The biggest concern is if we come in contact with surfaces that have a high risk of disease-causing organisms — things that are clearly contaminated such as rotting food, vomit from someone who’s sick, etc.”
At the end of the day, when you’re in your own home, it’s up to you if you want to take the chance of not having stepped in a biohazard. But really — if someone else asks you to remove your footwear off in their home, just do the polite thing and take your damn shoes off. After all, they’re the ones who are gonna have to end up dealing with your microbes.