Walking through the front door with shoes on may bring far more than dirt onto the floor. Research on shoe contamination shows that soles routinely carry fecal bacteria, antibiotic-resistant organisms and environmental toxins picked up from sidewalks, public restrooms, hospitals and rural paths, turning the entryway of a home into the most contaminated zone in the house.
One of the most frequently cited studies came from Charles Gerba, a professor of virology at the University of Arizona, who examined 26 pairs of shoes worn outside for three months. The shoes harbored between 3,600 and 8 million colony-forming units of bacteria, both inside and out. E. coli, or fecal bacteria, was the most common organism found. Gerba told TIME that roughly 80 percent of shoe bottoms carried fecal bacteria, largely picked up from public restroom floors, though dogs, birds and outdoor surfaces also contribute. By his count, shoes are considerably germier than public toilet seats, which average about 3,200 colony-forming bacterial units.
E. coli is not the only threat. Researchers at the University of Houston found that 40 percent of shoes tested carried C. difficile, an intestinal bacterium that causes diarrhea. A 2016 meta-analysis in the Journal of Applied Microbiology reviewed 13 studies tracing where shoes travel and what they collect. One small study in that group found that after physicians made hospital rounds, 65 percent of their shoes tested positive for either antibiotic-resistant Staphylococcus aureus, known as MRSA, or Enterococcus faecalis, another gastrointestinal bacterium. Another study found that 48 percent of shoes worn in operating rooms carried at least one pathogenic bacterium, most commonly Staphylococcus.
Rural environments bring their own risks. A 2009 study in rural Alaska had participants put on sterilized boots, walk along a randomly selected outdoor path, then step onto sterilized linoleum and remove the boots. After that single walk, 70 percent of the boots tested positive for multiple gut bacteria. When people then wore the contaminated boots across a sterile floor, bacteria transferred to the flooring in half of the walks.
Cities are not exempt. Dog walking on sidewalks and in parks leaves traces of fecal bacteria and pools of urine along heavily traveled routes, even where owners pick up after their pets. A 2023 study published in Design, Architecture, and Ergonomics by researchers at Northeastern University in New York City sampled sidewalk rain puddles, shoe bottoms and flooring inside a campus building. Puddles averaged 30,000 gut bacteria per 100 milliliters of water. Every shoe sole sampled from student and teacher volunteers carried varying levels of the same bacteria, and the building interior was contaminated as well, especially in carpeted areas.
«The surface of your flooring makes an enormous difference,» said Alessandra Leri, professor of chemistry and biology at Northeastern and the study's lead author. «If you have carpets, they function as a reservoir of bacteria.» Regardless of the floor surface, she said, «shoe soles are a serious vector of transmission of fecal bacteria from the outdoor environment to the indoor environment.»
Shoes also ferry chemical contaminants. PFAS, the so-called forever chemicals that persist in the environment and the body, are regularly picked up by shoes and are used in manufacturing soles. Lead dust, often stirred up during remodeling of older buildings, can cling to shoes, as can pesticides used on farms and in home gardens.
The risk is concentrated near the door. Gerba said the first four steps inside a home are the germiest floor area. His tracer studies, which applied dye and tracer viruses to shoe bottoms, showed how shoes leave a trail across the floor. Contaminants then pose the greatest danger to household members who have the most contact with the floor, typically babies and small children who crawl and play on it.
Public health messaging has begun to reflect the findings. This year, the Indiana Department of Health launched a «Shoes Free Protects You and Me» advertising campaign promoting the benefits of a no-shoes policy at home, with a particular focus on tracked-in lead. A 2023 CBS News/YouGov poll found that 63 percent of Americans regularly remove their shoes when they get home, while 24 percent ask guests to do the same.
Leri follows a no-shoes policy at home and leaves her shoes by the door. She does not ask guests to remove theirs, especially at parties. «If you are having a cocktail party, you are sensitive to guests feeling like shoes are a part of their ensemble,» she said. Instead, she rolls up her rugs before company arrives, making the floors easier to clean afterward.