Are you bathing in bacteria?

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Can taking a shower be hazardous to your health? Perhaps so, according to a study published online today in the Proceedings of the National Academy of Sciences.

We’re not talking about anything as mundane as slipping and falling in a wet, soapy shower stall. Nope, this is much creepier. We’re talking about invisible microbes that live inside your shower head and bombard you in aerosolized form as you bathe.

(Cue the ominous music …)

“Shower aerosol particles can be sufficiently small to carry bacteria deep into the airways,” according to a group of researchers from the University of Colorado at Boulder. Unsuspecting shower-takers could be placing themselves at risk for contracting asthma, bronchitis and pulmonary diseases. Some scientists have even speculated that a recent rise in certain kinds of bacterial infections can be traced to the increasing popularity of showers instead of baths.

So the Colorado researchers took samples of biofilms that formed inside 45 shower heads from nine cities, including New York and Denver. (No California cities were included in the analysis.) Generally speaking, the microbes they found resembled the populations known to exist in each locale’s water supply, but many of those bugs were more than 100 times as plentiful in the shower heads, according to the study.

The researchers were surprised to find an abundance of bacteria belonging to the Mycobacterium genus. One such species, M. avium, is a noted “opportunistic pathogen” that appears to thrive inside shower heads. Small amounts of various other bacterial species known to cause respiratory diseases were also identified in the study.

On the plus side, the researchers only found minuscule amounts of Legionella pneumophila, the organism that causes Legionnaire’s disease.

To put things into perspective, the researchers noted that there are 10,000,000 bacteria in a typical liter of tap water and another 1,000,000 bacteria in a cubic meter of indoor air. “In our daily lives, we humans move through a sea of microbial life that is seldom perceived,” they assured readers.

Still, they wrote, their census lends support to other reports linking M. avium infections to shower heads. Perhaps some people with compromised immune systems or weak lung function should forgo showers and take baths instead, they advised.

-- Karen Kaplan