Few if any buildings are immune to the challenges of reopening after a months-long shutdown caused by COVID-19. Even the U.S. Centers for Disease Control and Prevention faced a hurdle when property managers found Legionella bacteria in the water systems in some of the buildings it leases in the Atlanta area. The bacteria can cause Legionnaires’ disease, defined by Mayo Clinic as a severe form of pneumonia usually triggered by inhaling the bacteria from water or soil. Although no CDC employees were sick, the buildings had to be temporarily closed in August after reopening earlier, reported CNN Health.

Different types of commercial structures—including large office buildings, hotels, and some factories—can be vulnerable after a protracted closure, said Chris Edens, an epidemiologist on the CDC’s Legionella team quoted in the article.

The bacteria can develop in stagnant water that hasn’t been properly disinfected. When that happens and building occupants turn on faucets or flush toilets, the bacteria can travel through the air and be inhaled, according to a New York Times article on the Atlanta closure. Chlorination and other disinfecting procedures can kill the bacteria, but the bacteria can also create sludge inside pipes that’s hard to remove, Edens said.

Although turning on taps and sending fresh water through the building, called flushing, are known to help, the length of the recent lockdown may call for more, said Andrew Whelton, an associate professor of civil, environmental, and ecological engineering at Purdue University quoted in the article. He explained that specific actions may be needed in addition to CDC general guidance. For example, some buildings may require more chlorine than usual, more flushing, or flushing throughout the entire building.