US researchers analyzed 50 shower heads from nine different cities and found 30 percent harbored significant levels of a pathogen linked to lung disease called mycobacterium avium.
While the pathogen is common in municipal water systems, the levels found clinging to shower heads in slimy "biofilms" were more than 100 times higher than the "background" levels in the water.
"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," said lead author Norman Pace, a microbiologist at the University of Colorado at Boulder.
Pace's team began studying shower heads after research at the National Jewish Hospital in Denver found that recent increases in pulmonary infections from so-called "non-tuberculosis" mycobacteria species like M. avium may be linked to people taking more showers and fewer baths.
That's because water spurting from shower heads can distribute pathogen-filled droplets that are easily inhaled into the deepest parts of the lungs.
"There have been some precedents for concern regarding pathogens and shower heads," said Pace. "But until this study we did not know just how much concern."
Immune-compromised people like pregnant women, the elderly and those fighting off other diseases are most at risk of developing pulmonary disease caused by M. avium.
The symptoms include tiredness, a persistent, dry cough, shortness of breath, weakness and "generally feeling bad," Pace said.
The researchers sampled shower heads in public facilities, houses and apartment buildings in New York, Illinois, Colorado, Tennessee and North Dakota.
They found lower levels of pathogens in smaller towns and cities which used well water rather than municipal water.
They also found that metal shower heads harbored far fewer pathogens than plastic shower heads.
The results do not indicate that people should switch from showers to baths, said co-author Laura Baumgartner, also of the University of Colorado.
"Is it dangerous? Getting out of bed is dangerous," she said in a telephone interview.
"Everywhere you go there are microbes."
Switching to a metal shower head, especially one with a filter that can be changed regularly, can help reduce the buildup of pathogens.
Stepping outside the room for a minute after turning the shower on can also reduce the likelihood of inhaling pathogens that get pushed out of the shower head with the first burst of water, she said.
"For the average person, you shouldn't not be worried at all."
The study was published in the Proceedings of the National Academy of Sciences.