Experts are advising people to break the habit of rinsing their mouths with water from old showerheads, warning of possible exposure to nontuberculous mycobacteria (NTM) that build up inside the fixtures. Most healthy people experience no symptoms, however, and the risk is concentrated among patients with chronic lung disease.
Lim Jae-joon, a professor of pulmonology at Seoul National University Hospital, explained that bacteria attach to showerheads and can spread during showering, according to recent accounts from the medical community. He recommended that people stop rinsing their mouths with water from long-used showerheads and replace the heads at least once every six months.
Unlike the tuberculosis bacterium, NTM lives widely in the environment, including in water and soil. Lung disease accounts for more than 90% of NTM infections. In South Korea, the Mycobacterium avium complex and the Mycobacterium abscessus complex are the most common causative bacteria.
Patient numbers are rising. An analysis of Health Insurance Review and Assessment Service data, cited in a review published in volume 67, issue 1 of the Journal of the Korean Medical Association in 2024, found that the crude prevalence of NTM rose fivefold, from 11 cases per 100,000 people in 2010 to 57 in 2021. Over the same period, tuberculosis fell from 131 to 52 per 100,000, meaning NTM surpassed tuberculosis for the first time in 2021. NTM lung disease is not a reportable condition, however, so no official national statistics exist. Researchers say advances in testing technology and greater awareness among doctors may also have contributed to the increase.
Overseas research supports the link to showerheads. A study published by researchers at the University of Colorado in the international journal mBio in October 2018 analyzed 606 samples from inside showerheads in the United States and 50 in Europe. Regions of the United States with high rates of NTM lung infections overlapped with areas where pathogenic mycobacteria were frequently detected. The correlation was measured at the regional level, however, and does not establish individual transmission routes.
No study has been identified that directly tests the risk of rinsing the mouth. Inhaling aerosols generated during a shower is most often cited as a transmission route, and reflux of swallowed water from the stomach into the airway has also been suggested. A study in South Korea found a higher incidence of NTM lung disease among patients with gastroesophageal reflux disease.
Doctors say it is rare for NTM disease to develop from showering or tap water use alone. Patients with chronic obstructive pulmonary disease or bronchiectasis, people whose lungs were damaged by past tuberculosis and those receiving immunosuppressive treatment face relatively higher risk. Experts advise this group to clean the inside of showerheads frequently, replace them regularly and avoid extending shower times beyond what is necessary.
