You may not be alone in the shower.

A new study finds some showerheads have a dirty side and can deliver potentially dangerous bacteria along with the hot water meant to scrub you clean.

The surprising new study from the University of Colorado at Boulder finds that showerheads, particularly plastic ones, provide a dark, wet, and warm environment for bacteria to not just live but to grow.

Those potentially harmful microbes get sprayed out of the showerhead as an aerosol that people can inhale into their lungs, where it can lead to lung disease in some people.

The research team led by molecular biologist Prof. Norman Pace made their findings after analyzing more than four dozen showerheads in nine cities across the U.S.

They found a slimy "biofilm" inside many of the showerheads that harboured the DNA of a bacterium called Mycobacterium avium, or MBA for short.

"We found about 30 per cent of the showerheads had significant loads of MBA cells imbedded in film inside the showerhead, in the interior surfaces. So when you turn on the shower, some of them wash out," Pace explained to CTV News.

M. avium is part of a family of bacteria called nontuberculous mycobacteria (NTM). They live everywhere in the environment, particularly in wet soil and marshland and in the rivers that feed the drinking water supplies of many cities.

The researchers used PCR analysis to compare the amounts of M. avium in municipal water before it entered the showerhead and then again after it flowed out.

They found the water from the showerheads contained levels of pathogens, including non-tuberculous mycobacteria, at levels 100-fold higher that levels in pre-shower water.

The study's results appear in the online edition of the Proceedings of the National Academy of Sciences journal.

While NTM are generally harmless to most people, they can cause lung infections in those with damaged immune systems and in the elderly.

When NTM-filled water droplets suspend themselves in the air, they can easily be inhaled into the deepest parts of the lungs, potentially leading to infection that causes a dry cough, shortness of breath and weakness.

A few years ago, the U.S. Centers for Disease Control reported of a case of an entire family becoming ill with NTM-related lung infections that was traced back to a rarely-cleaned family hot tub. All the family members recovered but only after months of antibiotic treatment.

Infections disease expert Dr. Andy Simor says there have been the occasional report of people becoming ill from NTM infections traced back to dirty showers, but he says he's not concerned a daily shower poses a huge public health risk.

"Most people who are in good health are exposed to these bacteria all the time and it doesn't cause problems," Simor says.

"It really only causes problems in people with severely damaged immune systems: people who have HIV/AIDS, people who are on high-dose steroids because they've got an organ transplant, or who are on chemotherapy. That's the population at risk."

He adds: "I don't think these findings should change anything the way we do anything at all. People should continue to have showers and baths the way we normally do."

Still, if you have health issues or are immune compromised and you want to be safe, you should consider taking baths instead since they don't create the aerosol that showers do.

Shower-lovers should also make it a habit to run warm water through their showers for a few seconds before stepping in. Even better, changing a showerhead regularly should reduce the risk.

"Look at your showerhead and if it has a lot of crusty stuff depositing, you should throw it away and get a new one. I recommend all-metal," Pace says.