The average vacuum cleaner's suction and rotating-beater brush don't usually reach the bottom of the carpet, and that area is a haven for bacteria—about 200,000 per square inch, according to research conducted by Philip Tierno, Jr., PhD
, a microbiologist and immunologist at New York University Langone Medical Center. In his book, The Secret Life of Germs
, Tierno explains that hundreds of thousands of different types of species feast on human skin cells that fall onto rugs, plus tiny pieces of food, pollen and pet dander. Most carpet manufacturers say the best way to deep clean is to use steam, aka hot-water extraction, every 18 months.