The bacteria police are at it again, having already discovered that handbags, money, and recyclable shopping bags teem with bacteria. Now they have a new and surprising item to add to the gross-out list. Ironically, it’s an item you most likely think of as antibacterial. After unscrewing 50 showerheads across the country and examining the contents, researchers from the University of Colorado discovered that a full third of them harbored dangerous bacteria that could cause lung infections and breathing problems. It turns out that showerheads are a perfect environment for the formation and growth of biofilm — i.e, warm, moist, and protected.

So much for getting disinfected in the tub. The researchers found 15 different types of bacteria hiding in the fixtures, including a particularly high concentration of Mycobacterium avium, a relative of the tuberculosis microbe. This species of bacteria thrives in city water supplies that chlorinate, because it’s resistant to chlorine and so can proliferate wildly when competitive bacteria get wiped out. It causes a nasty, antibiotic-resistant infection that can linger for years, with symptoms ranging from persistent cough to fatigue and breathessness. Doctors have been seeing steadily increasing numbers of people with infections caused by Microbacterium avium, particularly women who have compromised immune systems. They postulate that the spike in these lung infections has everything to do with showers — with the fact that showers have become more popular than baths, and also because of the chlorinated water issue in municipal systems. In fact, four of the shower heads tested came from rural homes supplied by private wells, and no Microbacterium avium were found in those.

Just how bad is the problem? To give some perspective, put this in your shower-cap: when in New York, the researchers tested the air in city subways and found that breathing subway air is healthier than breathing shower vapors, at least from a microbial point of view. The showerheads the team tested had 100 times more Mycobacterium avium than tap water from the faucet. The dark, moist environment provided by the showerhead creates a perfect breeding place for bacteria, and so they thrive there. This means that when you first turn on the shower, beware. That first blast most likely contains the highest concentrations of bacteria, fresh from the nozzle.

“If you are getting a face full of water when you first turn your shower on, that means you are probably getting a particularly high load of Mycobacterium avium, which may not be too healthy,” one of the researchers, Norman Pace, said.

Apparently, the bacteria suspend themselves in droplets of water and when you inhale, you take those bacteria deep into your lungs, where they can wreak havoc if you’re immune system isn’t buff. “The water droplets formed when a shower is turned on are very, very small and can go deep into your lungs and carry bacteria deep into your lungs, which is how you get disease,” said study director Leah Feazel. “A bath, on the other hand, doesn’t usually have the aerator on it to create these tiny particles and is therefore safer.”

Does this mean that you should either walk around grimy rather than chance a shower or draw a bath if you want to get clean?

Not necessarily. First, the scientists say that metal showerheads are far less bacteria-friendly than plastic ones, so you can switch to metal to minimize problems. In fact, you should replace showerheads a few times a year, they say. Cleaning out the old fixture probably won’t do it; the researchers found that bleach did not kill the microbes (remember, avium is resistant to cholorine). “We tested bleaching shower heads and we actually found that we had more of the mycobacteria after bleaching than we did before,” said Ms. Feazel. She suggests letting the water run for 30 seconds or so before immersing yourself — although this, admittedly, isn’t an environmentally friendly solution. You do have the option of using a whole house filtration system. And there are even some dedicated shower filters that incorporate silver to actively kill bacteria.

Then again, the researchers claim that if you have a strong immune system, you can probably ignore all of the above. “This really shouldn’t concern average, healthy people. The main concern is for people who are immune-compromised,” the researchers say, athough they admit that even healthy people can succumb. On the other hand, the more you force your immune system to fight the effects of your morning shower, the less immune function is available to fight everything else you face, from swine flu to cancer.

In any event, the shower-head is just one of many dangers lurking in the bathtub. I recently wrote about PVC in vinyl shower curtains, which can cause serious damage to the liver as well as to the nervous, reproductive, and respiratory systems. Then, if your water is chlorinated, there are the carcinogenic properties of chlorine to fret about.

It’s enough to make you want to return to the middle ages when people bathed once a year and carried flowers to the altar to cover the smell.

:hc