What happens when you arrive at someone's home and the host asks you to remove your shoes? Right after I check whether there's a hole in my sock I wonder, “Should we worry about carrying bacteria on our shoes into our homes?” Does this request relegate bacteria to the outside? Turns out the answer is no. The number of bacterial cells already inside the body is estimated at 10 times the number of human cells, and the surface of our skin is home to Staphylococci, Streprococci, and Corynebacterium. Scientists describe us as “more bacteria than human."

True, the average shoe harbors hundreds of thousands of bacteria per square inch. According to Jonathan Sexton, an environmental microbiologist at the University of Arizona:

“The soles of our shoes are a meet and greet for these microbes, and with each step we take, we pick up new attendants. But most of us don't spend a lot of time on the ground, where shoe bacteria dwell.”

While some might launch into a house-cleaning frenzy at the thought of a shoe entering their homes, the reality is that the load of bacteria shoes bring indoors won’t make us sick. Kevin Garey, professor at the University of Houston College of Pharmacy agrees:

"A healthy person needs to contact thousands of microbes from one harmful bacterial strain to become infected.”

The one exception:

"I might be concerned about a child crawling on the floor.”

This brings to mind the many times I’ve dropped food on the floor, picked it up and put it in my mouth. Am I the only one who learned the five-second rule – a kooky hygiene concept that allows a 5-second window of time to pick up food (and sometimes cutlery) after it has been dropped? Feeling sheepish, I researched the validity of the five-second rule and confirmed why my children and I are alive and thriving.

It seems 70% of women and 56% of men use the five-second rule. It’s so common that during a six-week internship in the Food Science and Nutrition Department at the University of Illinois, Jillian Clarke did this supervised study:

"Jillian swabbed the University floors in the lab, hall, dormitory, bathrooms, and cafeteria to see how many organisms she could isolate. She examined the swabs, and there were few microorganisms. That surprised her supervisor who told her to do it again. The results were the same. They concluded that the floors were clean from a microbiological point of view, because floors are dry, and most pathogens (like salmonella, listeria, or E. coli) can't survive without moisture. All bets are off when it comes to damp floors or wet food.”

For her work, Clarke was awarded an Ig Nobel prize at Harvard University – a special prize given for research that first makes you laugh, then makes you think.

I’m intrigued by how many bacteria live in, on, and around us. We know floors are dirty, but it’s reassuring to know the important role our immune systems play in maintaining good health. Still, in spite of the good news in Clarke’s study, I’m abandoning the 5-minute rule!

How about you?

Shoes on or off when we enter your abode?