A public health expert said the city of Baltimore should commit to additional Legionella testing after documents obtained by The Baltimore Sun showed that Legionella retest results from several city buildings, including City Hall, may be less reliable than previously thought.

Legionella is a naturally occurring bacteria found in water that causes Legionnaires’ disease, a severe form of pneumonia that can be fatal.

The city closed the buildings that it found or suspected had Legionella contamination for treatment, but quickly reopened them during a burst of testing late last year.

After treating the pipes with chlorine, the city retested several buildings, which showed little to no bacteria. But the retest samples were late to the lab, raising questions about whether the results were accurate.

“The levels they found initially were so high they should be re-testing (even after negative results) on a quarterly basis to know that the treatment was effective in the long term,” Natalie Exum, an assistant professor of public health at Johns Hopkins University, told The Sun in an email.

Exum believed the initial results were high enough to warrant regular testing “regardless of whether or not … they violated the laboratory protocol.”

Symptoms for Legionnaires’ disease include coughing, fever, headaches and muscle aches, according to the Centers for Disease Control. Symptoms can develop between two and 14 days after exposure. Older people, the immunocompromised and current or former smokers at greater risk, the CDC says.

While the first round of tests showed bacteria levels up to hundreds of colony-forming units per milliliter — an amount a public health expert called unsafe — the retest results from the city buildings that came under scrutiny in December showed at most 0.5 colony-forming units per milliliter, the smallest amount detectable by the lab.

However, those retest results carry a note from the lab saying they were received by analysts “past holding time,” the amount of time that passes between the collection of a sample to when it is tested.

The city collected and shipped the samples during the week of the Christmas holiday, which it offered as an explanation for the sample’s tardiness.

A 2020 study published in the scientific journal Microorganisms found “a statistically significant reduction in Legionella counts after 48 hours,” though the study did note that the reduction in bacterial load was “substantially less than the variability of sampling locations in a building.”

It is unknown how long the cultures from the Baltimore buildings were held past holding time, though the test results said samples should be analyzed within two days.

Experts contacted for this story said they could not speculate on what impact the wait time had on test results without knowing the lab’s protocols.

George Young, a certified Legionella water management plan consultant who works at Environmental Safety Technologies, a Legionella testing firm in Kentucky, told The Sun that samples received after 48 hours are unreliable and can give false negatives or false positives depending on the conditions inside the water.

Typically, Young said he recommends additional testing for clients whose samples arrived late to the lab, to ascertain the reliability of the results.

It is not clear if the buildings still have the bacteria present in their water supplies.

Though potentially disquieting, public health expert Exum said she believes knowing about contamination means people and municipalities can protect themselves.

“Even if there’s intermittent testing … I think it’s unreasonable to expect that large building water systems are going to always be free of … pathogens,” Exum said.

Though there is no federally mandated level of Legionella concentration that requires remediation, private nonprofit research and policy group founded by Abraham Lincoln, the National Academies of Science, Engineering and Medicine, believe a level of 50 colony-forming units per milliliter is “high enough to warrant serious concern and trigger remediation in a building water system.”

Nine samples across four buildings in Baltimore registered a level higher than 50 colony-forming units per milliliter.

“Water quality in city buildings is guided by state and federal regulations. The city will continue to comply with those regulations fully, as well as go beyond what is required, to ensure the health and safety of city residents and employees,” John Riggin, a spokesman for the city’s Department of General Services said.

The samples were not too old for testing, Riggin said.

“The Department of General Services has no reason to question the validity of the results,” he said. “Such a delay will not cause a negative result or disappearance of the bacteria.”

Highly chlorinated water can kill off Legionella in samples that are too late, Young said, potentially causing a false negative.

In a December email to The Sun, Baltimore Department of General Services said new water samples were taken from the two city circuit courthouses and People’s district courthouse “upon completion” of their chlorine treatments.

It was unclear when samples were taken from the other buildings.

The testing lab director at the time, Janet Stout, is “a leading authority on Legionella,” Riggin told The Sun. She co-authored the study on hold times that appeared in the journal Microorganisms.

Stout retired at the end of 2024, according to Special Pathogens Lab. She was not immediately reachable for comment. The lab declined to comment on the results from Baltimore City.

Special Pathogens Lab and its parent company did not respond to a request to speak with its current lab director Friday.

The city did not provide a water management plan when asked by The Sun; Riggin said the city plans to incorporate periodic testing as part of a water management plan, which will be implemented “in the coming weeks.”

On Dec. 30, the mayor’s office promised the public “the city will continue to conduct precautionary testing in city-owned facilities across Baltimore.”

Baltimore Mayor Brandon Scott’s office, as well as multiple state delegates and senators representing Baltimore City, did not respond to requests for comment.

Five buildings were re-tested. They detected no Legionella growth in City Hall, the Abel Wolman Building and the E. Fayette Street district court. Even though the samples were received past holding time, the other buildings showed low levels of bacteria.

The state began proactively testing for the bacteria as part of a new water quality testing regimen last fall, after which the city later followed suit.

The aging infrastructure found across the city has been a point of concern at the state level, as “many state-owned buildings are old and facing deteriorating maintenance issues,” the state said in a press release.

Legionella is not specifically regulated in Maryland statutes, though it is listed as a reportable disease in the Code of Maryland Regulations. Building owners must inform tenants of outbreaks being investigated.

“We need to require vigilance of our facilities managers to … have healthy building water practices,” Exum said.

It is easier and less expensive for building managers to test for the conditions that can promote Legionella growth rather than testing for the bacteria, Exum said.

The city’s legal department said on Jan. 17 that the department was “working on a schedule for future water quality testing in buildings” and securing a “long-term contract” with the testing and remediation company.

The legal department said the general services department voluntarily notified tenants of the results even though it was not required by law.

Have a news tip? Contact Racquel Bazos at rbazos@baltsun.com, 443-813-0770 or on X as @rzbworks.