TIME: US studies point to bacteria being carried into homes on shoes
Studies in the United States indicate that shoe soles can carry bacteria and chemical contaminants from the street into homes. As TIME reports, in homes where shoes are not removed at the entrance, the most contaminated area is usually the first four steps from the door.
Bacteria on soles
In a 2008 study, Charles Gerba, a professor of virology at the University of Arizona, analyzed 26 pairs of shoes worn outside the home over three months. Between 3,600 and 8 million colony-forming units of bacteria were found on the inner and outer surfaces of the shoes. Fecal bacteria, including E. coli, were found on about 80% of soles.
Researchers at the University of Houston reported that Clostridioides difficile, an intestinal bacterium that can cause diarrhea, was detected on 40% of the shoes tested. A meta-analysis of 13 studies published in 2016 in the Journal of Applied Microbiology also documented shoe contamination in healthcare facilities: after hospital rounds, 65% of doctors' shoes tested positive for MRSA or Enterococcus faecalis. Another study found at least one pathogenic bacterium on 48% of pairs of shoes used in operating rooms.
Floor coverings matter
In a 2023 study on the Northeastern University campus in New York, scientists compared water samples from street puddles, shoe soles and floors inside a building. Sidewalk puddles contained an average of 30,000 fecal bacteria per 100 milliliters of water. All examined soles had varying levels of such bacteria, while carpeted areas proved especially contaminated.
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The study's author, Alessandra Leri, noted that carpets can serve as a reservoir for bacteria, while shoe soles are a significant route for transferring fecal bacteria from the outdoor environment indoors. Infants and young children, who crawl and play on surfaces, have the greatest contact with the floor.
Not only microorganisms
Shoes can also bring PFAS, known as “forever chemicals,” lead dust and pesticides into the home. The Indiana Department of Health launched the Shoes Free Protects You and Me campaign, focusing in particular on the risk of bringing lead into homes.
According to a 2023 CBS News/YouGov poll, 63% of Americans regularly remove their shoes after returning home, while 24% ask guests to do so. Among respondents aged 18 to 29, 44% had introduced this rule for guests, compared with 7% among people older than 65.
