The germs that shoes pick up

While published research on shoe contamination is limited, some studies have found shoes can track in a surprising amount of germs.

In one study, published in 2008, Charles Gerba, a professor of virology at the University of Arizona, looked at 26 pairs of shoes that had been worn for three months outside and found the shoes carried between 3,600 and 8 million colony-forming units of bacteria on both the inside and outside of the shoes. The most common bacteria found was E. coli, or fecal bacteria.

"[About] 80% of the bottoms of the shoes had fecal bacteria on them, largely picking them up from the floors of public restrooms, which always have fecal bacteria on them," Gerba said. "They may also pick them up outside, in places that dogs and birds are."

Based on Gerba's numbers, shoes contain significantly more germs than public toilet seats, which have been found to have an average of 3,200 colony-forming bacterial units.

And it's not just E. coli that's being tracked around by shoes. Research from the University of Houston found that 40% of shoes tested carried C. difficile, an intestinal bacterium that causes diarrhea.

Healthcare settings pose an even greater risk of picking up germs. One meta-analysis from 2016 published in the Journal of Applied Microbiology looked at 13 studies, tracing where shoes traveled and what they picked up.

Of those studies, one small study found that after physicians did rounds in hospitals, 65% of their shoes tested positive for either antibiotic-resistant Staphylococcus aureus or Enterococcus faecalis, another gastrointestinal bacterium. Another study found that 48% of shoes worn in operating rooms tested positive for at least one pathogenic bacterium, mainly Staphylococcus.

Where removing your shoes matters the most

Some types of flooring can pick up more germs than others. In one 2023 study, published in Design, Architecture, and Ergonomics, researchers at Northeastern University sampled sidewalk rain puddles, shoe bottoms, and flooring inside one of the school's buildings on campus. On average, they found 30,000 gut bacteria per 100 milliliters of water in puddles on the neighborhood sidewalks.

Every shoe sole that was sampled from both students and teacher volunteers carried various levels of the same bacteria, and the interior of the building was also contaminated, especially in carpeted areas.

"The surface of your flooring makes an enormous difference," said Alessandra Leri, a professor of chemistry and biology at Northeastern and the study's lead author. "If you have carpets, they function as a reservoir of bacteria."

However, no matter the interior floor surface, "shoe soles are a serious vector of transmission of fecal bacteria from the outdoor environment to the indoor environment," Leri added.

In the majority of homes where people don't remove their shoes upon entering, the area closest to the front door is usually the most contaminated. "We found that the first four steps at the entrance to the household are the germiest floor area in the home," Gerba said. "We also did tracer studies putting dye and tracer viruses on the bottom of shoes and could see how they left a trail on the floor."

Whenever any contaminants spread throughout the house, they pose the most danger to household members with most contact with the floor, especially babies and small children who crawl and play on surfaces.

More and more people, especially younger people, are enforcing shoe bans in their home, according to the CBS News/YouGov poll. Although only 7% of people over the age of 65 and 13% of people ages 45 to 64 required guests to remove their shoes before entering, 44% of people ages 18 to 29 did the same.

"I find that many people have already adopted that custom," said Leri. "It is a more hygienic way of living."