Enlarge / Part of the Brigham and Women's Hospital in Boston on Dec. 16, 2021. (credit: Getty | John Tlumacki )

Water purification systems installed in two ice machines in a Boston hospital were supposed to make the water taste and smell better for patients on a surgery floor—but it ended up killing three of them, an investigation found.

The purification systems inadvertently stripped chlorine from the municipal tap water, allowing bacteria normally found at low levels to flourish and form biofilms inside the machines. This led to infections in four vulnerable cardiac-surgery patients who had prolonged stays on the hospital floor. Three of them died of their infections.

Researchers detailed the case cluster and ensuing investigation in a study published Monday in the Annals of Internal Medicine .