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Areas of the United States where certain bacteria are most prevalent in showerheads are the same areas where bacterial lung infections called nontuberculous mycobacterial (NTM) lung infections are most common, according to a study from the University of Colorado, Boulder.

Most bacteria that thrive in showerheads and household water systems are harmless, but the mycobacterium that causes NTM lung infections is a serious threat to public health. For this study, the researchers examined showerheads in households throughout the U.S. and Europe. They found that bacteria vary depending on geographic area and water source and chemistry, and showerheads in homes with chlorine-treated water have an abundance of certain mycobacteria.

According to the American Lung Association, nontuberculous mycobacteria are naturally found in water and soil. Most people do not become ill from exposure, but for some — including older adults — inhaling the bacteria can cause serious, chronic lung infections that require long-term antibiotic treatment.