Researchers studying the microbiology of water systems at 45 sites in seven different states across the U.S. found unusually high levels of bacteria in showerheads—almost 100 times more than any other place in the home. Although bacteria levels were high in all showerheads, some sample sites in the New York City and Denver areas had large amounts of Mycobacterium avium, a relative of the bacteria that causes tuberculosis and leprosy.

“If you’re 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,” says study author Norman Pace, professor of biology at the University of Colorado.

Mycobacteria are found in soil, water, and decaying matter. Certain Mycobacteria, such as Mycobactrium avium, can cause digestive, lymphatic, and pulmonary infections. In the shower, these bacteria are delivered as aerosols. Particles are small enough to get into the lungs, and when inhaled or swallowed, can lead to a host of pulmonary problems.

The study, part of a bigger study that began as a class project, used DNA barcode testing to extract rRNA (bacterial DNA) from different parts of the shower and showerhead. Researchers sampled water flowing from the showerhead and obtained swab samples from the inside of the showerhead. Finally, samples were collected from water flowing out of the pipe after the showerhead was removed.

Homes with well water systems did not have Mycobacteria in their showerheads, a factor authors believe may be due to Mycobacteria’s chlorine resistance. Although they suggest that bathing may be safer, researchers assure that it’s not dangerous to shower.

“We did see some organisms that might be of concern if you have immune issues (such as) cystic fibrosis, AIDS, recent organ transplant, substance abuse, or are pregnant or otherwise immune compromised,” the authors wrote.

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