Sept. 14 (HealthDay News) -- If your immune system is weakened, you may want to rethink that daily shower.

New research suggests that ordinary showerheads are awash in germs, particularly a type that can cause lung disease in people whose immunity to illness is compromised.

The germs could be "blasted out of the showerhead and inhaled by the person showering," said study co-author Leah M. Feazel, a researcher at the University of Colorado's department of molecular, cellular and developmental biology.

But Feazel said showerheads shouldn't pose a threat to most people. And while the new findings do raise questions, it's not clear if showerheads are any more germ-friendly than other places around the house, such as faucets, counters and toilets, she said.

Feazel and her colleagues decided to look at showerheads because they seem like an ideal place for germs to grow.

The inside of a showerhead provides ideal conditions for microbial growth, Feazel said. "It is moist, warm, protected from disturbance, and frequently fed with nutrient resources in the tap water. Also, most people have noticed discoloration on their showerheads. This 'soap-scum' is actually microbial growth."

The researchers analyzed germs found in the film formed in 45 showerheads from nine U.S. cities. They found a variety of bacteria in showerheads, most of which don't cause illness in people. But they also found germs called mycobacteria, which are common and can cause lung disease in people with compromised immune systems, Feazel said.

The levels of certain germs that could spell trouble were 100 times above what they were in water before it made its way to the showerhead, the researchers said.

The unique thing about showerheads is that the germs could be inhaled. People are unlikely to inhale other kinds of household germs that fit into the category known as biofilms, with the exception of those produced by humidifiers, according to the study.

The findings were published in this week's online issue of the Proceedings of the National Academy of Sciences.

Feazel stressed that most people shouldn't be concerned about showerheads.

"If a person is worried about the risk of lung infection from showering, they have several options," she said. "Bathing, rather than showering, is probably best for those who are at risk. The size of the water droplets produced in bathing is too large to go deep into the lungs, whereas showering creates tiny particles that can go very deep and cause disease."

An all-metal showerhead -- not a plastic one with a metal coating -- is another alternative, as is replacing a showerhead several times a year, Feazel said.

"Cleaning the inside of a showerhead is very difficult and may be only partially effective," she explained.

George A. O'Toole, an associate professor in the department of microbiology and immunology at Dartmouth Medical School, noted that germs lurk everywhere.

"I imagine that if you looked at the kitchen sink, faucet and drain, the insinkerator, your dishwasher, the toilet, your washing machine and the hose in the yard, you might find similar pathogens," he said.

In the case of showerheads, he said, "people with good immune systems really don't need to worry about this. People with bad immune systems probably do, but they also need to worry about every encounter with microbes."

People with weakened immune systems include those infected with HIV, cancer patients undergoing chemotherapy, and recent transplant recipients.

SOURCES: Leah M. Feazel, researcher, department of molecular, cellular and developmental biology, University of Colorado, Boulder; George A. O'Toole, Ph.D., associate professor, department of microbiology and immunology, Dartmouth Medical School, Hanover, N.H.; Sept. 14-18, 2009, Proceedings of the National Academy of Sciences, online