If you enjoy soaking in a whirlpool bathtub as part of your organic lifestyle, you may be exposed to some of the filthiest water in the world, according to Dr. Rita B. Moyes, a microbiologist and immunologist at Texas A&M University.

Her recent study reveals that whirlpools are often a breeding ground for dozens of different bacteria that can cause a variety of infectious diseases.

Dr. Moyes tested 43 water samples from whirlpool bathtubs—both private and public—and found that all had bacterial growth ranging from mild to dangerous. Frighteningly, 95% showed the presence of fecal-derived bacteria, while 81% had fungi and 34% contained staphylococcus, which can cause deadly staph infections.

“Whirlpool baths are almost always a prime area for potentially harmful microbes,” Dr. Moyes explains. “The main reason is the lining of the pipes. They are full of inaccessible air, and water in these pipes tends to get trapped, often for long periods of time. When the jets are then switched on, this water with harmful bacteria gets blown into the tub where a person is soaking, and then trouble can start.”

To put the issue in context, a normal teaspoon of tap water contains an average of about 138 bacteria, with many samples showing no bacteria at all, Dr. Moyes says. By contrast, the same teaspoon of whirlpool tub water contains an average of more than 2.17 million bacteria.

“The stagnant water in a whirlpool bathtub pipe is a great place for bacteria to grow and grow,” she says, making users prime candidates for urinary tract infections, septicemia (blood poisoning), pneumonia and skin infections. A whirlpool’s aerosol mist can also force microbes into the lungs or open cuts.

Early studies never showed bacterial levels to pose problems for whirlpool users, but “that’s probably because a hot tub or whirlpool as a source of infection can’t be clearly distinguished from other sources,” Dr. Moyes says. “An example might be when you develop a respiratory infection. The doctor can tell you that you do have a respiratory infection, but he or she can’t tell you how you got it.

“The best way to prevent such bacteria from forming is to clean out the pipes,” she adds. “The pipes in a whirlpool bathtub need to be scraped and cleaned just like you need to brush your teeth with toothpaste.”