In 2008, researchers tracked new shoes worn by 10 participants for two weeks and found that coliform bacteria like E. coli were extremely common on the outside of the shoes. E. coli is known to cause intestinal and urinary tract infections as well as meningitis, among other illnesses.
"Our study also indicated that bacteria can be tracked by shoes over a long distance into your home or personal space," Gerba said in a statement.
(The study was not published in a peer-reviewed journal, involved a limited number of participants and was supported by the shoe company Rockport, which was testing machine-washable shoes.)
Gerba said in an interview this month that the study's findings had made him change even some of his own behaviours: "It kept me from putting my feet on my desk."
Contaminated shoes are unlikely to make you sick
It's possible to transmit germs from your footwear if you touch your shoes and then your face or mouth, for instance, or if you eat food that's been dropped on the floor.
But in the hierarchy of potential health hazards at home, bacteria-caked shoes rank comparatively low, according to Donald W. Schaffner, a food microbiologist at Rutgers University in New Jersey.
He said there are more important considerations. Is anyone in the house sick? Are there frogs, turtles or snakes nearby, which can carry salmonella? Is food being stored and prepared properly?
Sponges, which retain water and food particles, are a "cesspool" of bacteria, said Dr. Aaron E. Carroll, professor of paediatrics at the Indiana University School of Medicine in Indianapolis.
Outside the home, there are objects and surfaces that are frequently touched but seldom, if ever, washed, such as money, ATM buttons and gas station pump handles, he said, adding, "Focusing on people's shoes feels like focusing on the wrong vector."
Overall, experts emphasised that washing your hands with soap and water remained the most important health practice.
Lisa A. Cuchara, professor of biomedical sciences at Quinnipiac University in Hamden, Connecticut, said that faecal bacteria were certainly transferred from your shoes to your floor at home but that "for most healthy adults, this level of contamination is more of a gross reaction than a health threat."
Putting the threat in perspective, she noted that the floor in a public restroom has around 2 million bacteria per square inch. A toilet seat, on the other hand, has an average of about 50 million per square inch.
"Think about that the next time you place your purse or knapsack on the bathroom floor and then bring it home and put it on the kitchen table or counter," she said.
Consider what the dog drags in
If you are concerned about what two-legged residents track in, then what about your dogs?
"We don't wash the dog's paws every time he comes in the house, and I don't want to think about where he's been walking," said Carroll, who has a Cavalier King Charles spaniel named Loki.
Andrea Kaufmann of Cape May Court House, New Jersey, said she changed out of her shoes into slippers to keep dirt off the floors, but added that she has two Labrador retrievers.
"I could sweep and vacuum three times a day and still have dirt on the floors from the dogs," she said. "They can't take their shoes off."
Dirt can be healthy. Really.
Considering the benefits of modern-day sanitation, vaccinations and health care, the likelihood of getting sick from our shoes is "infinitesimally small as to almost be unwarranted," said Jack A. Gilbert, a professor in the department of paediatrics and Scripps Institution of Oceanography at the University of California, San Diego.
Gilbert, an author of the book Dirt Is Good, said there were theories suggesting that bringing elements of the outdoors indoors could help stimulate autoimmune systems, particularly in children.
In the first year of life, physical interaction with a dog can reduce a child's likelihood of developing asthma by 13 per cent, while interactions in a barn or farm can reduce it by 50 per cent, he said.
Emily Ledgerwood, an assistant professor of biological and environmental sciences at Le Moyne College in Syracuse, said her 3-year-old daughter had recently helped her crack eggs to make breakfast. When they were done, Ledgerwood made sure they both washed their hands to prevent any possible cross-contamination with salmonella.
Later, her daughter helped weed the garden and pick vegetables. Though she had been working in dirt, Ledgerwood let her have lunch without first washing her hands.
"When we find out about all the microbes in our environment, we can get a bit squeamish, but we're not getting sick all the time," she said.
When should you take off your shoes?
It's best to take your shoes off if you have young children crawling on floors or people in the home who have allergies, because pollen can be transferred to floors, especially to carpets.
"In cases where your immune system is compromised — people who have cancer, have undergone an organ transplant, have an infection — then there is much more of a reason to take your shoes off when you come home," Cuchara said.
If the person you are visiting prefers that you take your shoes off, it's sound etiquette to abide by their wishes, said April Masini, who writes about relationships and etiquette for her website, Ask April.
"Even if you don't see shoes at the entrance, you can always ask if your host would like you to take off your shoes upon entering," she said.
It is also a common practice observed in Asian and Middle Eastern countries, said Benjamin Hiramatsu Ireland, an assistant professor of modern language studies at Texas Christian University in Fort Worth.
"Removing one's shoes upon entering a home stems from the respectful observance of religious practices that have been integrated within the cultural fabric and expected 'to-dos' of each of these countries and, of course, for reasons pertaining to hygiene," he said.
Written by: Christopher Mele
Photographs by: Caleb Kenna
© 2019 THE NEW YORK TIMES
