If those residences are home to small children, pregnant women or anyone with a compromised immune system, the water should be tested, said George Hawkins, general manager of D.C. Water.

The CDC concluded that homeowners who had pipes only partially replaced may have made the problem worse. The center also confirmed that children living in the District were exposed to an increased risk of lead poisoning from 2000 to 2006 as an inadvertent result of efforts to disinfect the water supply that caused lead pipes to corrode and leach into the water that flowed through them.

The findings are a sharp reversal by the federal health agency, which initially said it had found no evidence of measurable or significant harm to public health. A congressional inquiry concluded in May that the CDC knowingly used false data in making a “scientifically indefensible” claim that the water was safe to drink.

The report marks the first time the CDC has publicly acknowledged that there was measurable health risk from the city’s lead crisis and that the primary remedy appears to have been flawed.

The CDC said it “found that children living in housing where a lead service line was partially replaced after 2003 were more likely to have [elevated blood lead levels] than children living in housing without a lead service line.” It went on to say that “partial lead service line replacement was not effective in decreasing risk for [higher blood lead levels],” with the risk similar to that for people who never had their lead lines replaced.

“This is the CDC telling us something we knew and acted upon,” said D.C. Council member Jim Graham (D-Ward 1), who has been critical of partial pipe replacements. “Not only did we know this, but we stopped it.”

The new CDC report reopens an issue that many residents thought was resolved when the city spent $93 million to replace thousands of service lines. From 2004 to 2008, the District replaced water lines serving 17,600 homes, Hawkins said. Homeowners were responsible for the portion of the pipes on their property. In 14,800 of those homes, owners chose not to make any additional repairs.

“Partial lead service line replacements don’t always work and in fact can cause sometimes more harm than good,” Hawkins said. “We thought it was a good idea until the data started showed that it wasn’t.”

There is no blood lead level that is considered safe for children. The new CDC report found that children in homes where lead pipes had been partially replaced were three times as likely to have elevated lead levels than those whose homes never had lead pipes.

It is the process of unearthing and cutting the lead service line that is thought to cause the increased risk for lead exposure in homes that have undergone partial replacements. The risk decreases over time after the repair is done, Hawkins said. He added that D.C. Water conducts regular testing of households that have had partial lead service line replacements; none of the tests have raised concerns, he said.

“We encourage the notion of testing your water and seeing what’s there,” Hawkins said. “If it’s only adults in the house, it’s probably still a judgment call, but less health-imperative than if there’s small children and pregnant women.”

The federal government banned the use of lead pipes almost 25 years ago, and the District embarked on an ambitious plan to replace lead service lines and to encourage homeowners to eradicate lead plumbing from their homes.

Water suppliers may have unwittingly exacerbated the problem in 2000, when the Washington Aqueduct began to use the chemical chloramine, rather than clorine, to purify the water supply. Although its use complied with federal requirements to reduce carcinogenic byproducts, many experts think it corroded pipes and caused lead to leach into the water.

The new CDC report found that elevated lead levels in children peaked in 2003, a year when chloramine was the only disinfectant used.

That year, residents were warned of potentially dangerous lead levels in their tap water. In 2004, it was reported that most D.C. homes with lead pipes had lead levels above a threshold set by the Environmental Protection Agency. But the worries of many homeowners were assuaged later that year when the CDC reported that no children it had tested suffered from lead poisoning.

CDC Director Thomas R. Frieden acknowledged in June that those findings were wrong and that “children living in homes serviced by lead water pipes were twice as likely” to have elevated lead levels.

The utility continues to replace lead service lines if they are connected to a water main that is being replaced or if a customer is replacing the private portion of the line.

After a lead service line is replaced, D.C. Water monitors the lead levels in the household’s water for at least 51/2 months, Hawkins said. The utility also provides filters while lead levels remain elevated.

“This report to us confirms and supports the steps we’ve been taking, and it helps the scholarship, but we’re sort of already there in what we’re doing in the system,” Hawkins said. “We’re hitting this like a full-court press and are doing everything we think is warranted.”

Hawkins said he was concerned about one class of households not included in the study: those that had their entire lead service lines replaced but also have interior plumbing made of galvanized steel. Research has suggested that lead from service lines can become embedded in galvanized plumbing and continue to leach into the water after the service line is replaced.

Frank Borris, 45, was one of thousands of D.C. residents living in an area where the city elected to do a partial pipe replacement in 2004. He, like many of his Shepherd Park neighbors, decided to pay out of pocket for a full replacement because of concerns for the health of his family.

With two neighbors, he hired a contractor to replace the pipes leading into their houses at roughly $1,000 per house, he said, “and that was cheap, really cheap.”

“I had concerns about their health and also mine and my wife’s,” Borris said. “A partial replacement is only a partial solution. I really didn’t see the value in doing a partial replacement at that time.”

halseya@washpost.com debonism@washpost.com

Staff writers Carol D. Leonnig, Nikita Stewart and Nathan Rott and staff researcher Madonna A. Lebling contributed to this report.