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MONDAY, Sept. 14, 2009 (Health.com) — If the shower scene in Alfred Hitchcock’s Psycho scared you, here’s another reason to scream: A new study says that potentially disease-causing germs can get trapped in showerheads and grow into biofilm, or coats of slime that deliver a bacteria blast along with your hot water.

Although the classic horror film gave legions of moviegoers a fear of showering, the new study shouldn’t do the same, experts say. The bacteria probably don’t pose a threat to most people, although they could be problematic for those with weakened immune systems.

Showerheads are dark, wet, and warm—the ideal environment for bacteria that cause lung diseases to thrive. As we turn on the faucet to get clean, the showerhead may spray our bodies and the air around us with such opportunistic bugs as Mycobacterium avium and other germs known as non-tuberculosis mycobacteria, according to the new research in the Proceedings of the National Academy of Sciences. In fact, levels of these bacteria are more than 100-fold higher than levels found in the pre-shower water, according to the researchers, who analyzed the germs in the biofilm of 45 showerheads from nine U.S cities, including New York and Denver.

“If you are immune compromised or are susceptible to pulmonary infections, take a bath instead of a shower,” says lead researcher Leah M. Feazel, of the University of Colorado, in Boulder. “If you are healthy and your immune system is functioning properly, you should not worry about the germs in your showerhead.”

Mycobacterium avium, found in 20% of study samples, can cause lung infections in both healthy people and those with weakened immune systems, particularly smokers, alcoholics, people with chronic lung disease, and others with conditions that make it difficult to fight off infection. Symptoms include fatigue, a chronic dry cough, and shortness of breath.

The good news is that L. pneumophila, the water-loving germ that causes Legionnaires’ disease, was rare in the study. Only 3 out of 6,000 genetic sequences tested were L. pneumophila. Legionnaires’ disease is a severe type of pneumonia, and outbreaks have been linked to L. pneumophila–contaminated water in large central-air-conditioning systems, whirlpool spas, and other sources of water droplets.

“[The study] is nothing to freak out about because most germs don’t hurt you,” says Philip M. Tierno Jr., PhD, the director of clinical microbiology and immunology at New York University Langone Medical Center. People come into contact with 60,000 types or groups of bacteria on a regular basis, says Tierno, who is also a clinical professor of microbiology and pathology at the New York University School of Medicine in New York City. “Only one or two percent are pathogenic,” he explains.

That said, cleanliness is next to godliness. “The new study emphasizes the need for us to periodically get rid of biofilm on our showerheads,” he says. Change the showerhead once a year or more frequently, like they do in hospitals, to prevent mineral deposits and biofilm, he suggests.

Metal showerheads appear to be less likely than plastic showerheads to grow biofilm, according to the researchers. Tierno recommends taking a steel brush and good cleaning solution to wash out metal showerheads.