If you think you’re just getting clean by taking a long, hot shower – think again. A study says if you have an immune system that’s been compromised by chronic illness or by medication, germs lurking in your shower head could be making you sick.

In a study published in 2009 in The Proceedings of the National Academy of Sciences, researchers at the University of Colorado, in Boulder, report that shower heads are a dark, wet and warm breeding ground where infectious microbes can grow.

The devices then spray potentially harmful pathogens down as aerosols on people who inhale them.

“I think it is surprising,” says Leah Marie Feazel, the first author of the study and a professional research assistant. “I really hope this raises awareness for those who are immunocompromised, that their household may be harboring potential disease organisms. And I hope it raises awareness in the medical community about this risk as well.”

Some kinds of arthritis medications, including corticosteroids like prednisone, disease-modifying anti-rheumatic drugs like methotrexate and biologics like infliximab (Remicade) and etanercept (Enbrel), can weaken the body’s natural defenses, leaving people who take them more vulnerable to infections.

Researchers analyzed shower heads at 45 sites in nine U. S. cities, with major sampling campaigns focused on the New York and Denver metropolitan areas. Through DNA sequencing, they discovered about 30 percent of the shower heads they tested had Mycobacterium avium, a relative of the organism that causes tuberculosis.

The symptoms of a Mycobacterium avium complex, or MAC, infection are fever, fatigue, loss of appetite, weight loss and possibly diarrhea and abdominal pain.

They then used a more sensitive testing method that only looked for that organism. Doing that, they discovered 80 percent of shower heads had Mycobacterium avium, and those fixtures contained more than 100 times the number of disease-causing germs as found in pre-shower water.

Scientists only found the organisms in shower heads from municipal sources, not private wells.

They think that’s because this family of bacteria survives the chlorination process that kills most other germs – and then has a wide-open space to grow.  

Researchers say it is not dangerous for healthy individuals to take showers, but it could be a cause for concern among people with weakened immune responses because shower aerosol particles could carry bacteria into airways.

“People on immunosuppressants should be very cautious. You should talk to your doctor about possibly switching to bathing rather than showering. Showering can create tiny particles that you are breathing into your lungs and you could develop a lung infection. Bathing doesn’t create these cells because the water’s all coming out in one stream, rather than being forced through tiny holes that creates micro particles,” Feazel says.

Researchers tried cleaning the shower heads with bleach, and they say that didn’t work at all. In fact, it only enriched the bacteria, creating more of it. So scientists say if you have a compromised immune system and want to shower, their best recommendation is to change your shower head frequently  - between two and four times a year.  

They say metal shower heads are less prone to bacteria growth than plastic ones. But beware – many shower heads that look metallic are actually plastic coated with a shiny material, so read the packaging carefully.

Researchers also say it’s not worth trying to test your own shower head because it’s expensive and virtually impossible for someone without access to scientific tools.