Mayor Bill de Blasio's administration failed to conduct lead tests in thousands of apartments where children had been exposed to the toxic substance, due to a bureaucratic breakdown between two city agencies, according to a new report.

An investigation by Comptroller Scott Stringer, released on Thursday, found that the city Health Department was aware of thousands of private apartments in which 11,972 children had been exposed to levels of lead deemed dangerous under federal standards. But that information was never shared with the Housing and Preservation Department (HPD), which is responsible for inspecting the buildings and ensuring compliance from landlords.

As a result of what the report calls "zero communication" between the two agencies, nearly 10,000 apartments that should have been tested—including residences where multiple children had been exposed to high levels of lead—were never inspected.

"At its core, the investigation exposes a clear failure by the City to leverage its own data related to lead exposure and utilize that data to precisely and methodically inspect buildings and areas most likely to pose a threat to children," the report states.

In 2012, the Center for Disease Control toughened their standard for dangerous lead exposure to just 5 micrograms per deciliter. But the city's threshold remained at 15 micrograms until last year. Asked about the report on Friday morning, Mayor de Blasio cited the discrepancy for the failure to inspect apartments that warranted it.

"It's very complex," he told WNYC's Brian Lehrer. "I think the reality is there were different standards being held...they were not aligned, and trying to figure out what the right thing to do took some time."

The report also found that the housing department has significantly neglected its responsibility to ensure landlords are proactively addressing lead paint threats. The investigation revealed that the agency did not issue a single violation between January 2013 and September 2018 to landlords who did not make the required annual notifications and inspections.

A spokesperson for HPD did not immediately respond to Gothamist's inquiries about the enforcement lapse.

The report comes as Mayor Bill de Blasio has touted a "Vision Zero"-style approach to eradicating lead paint hazards. According to the city, the number of young children exposed to lead has decreased by nearly 90 percent since 2005.

But the administration has also struggled to meet its own deadlines to address the issue. Earlier this week, the New York City Housing Authority acknowledged that a plan to test 135,000 apartments for lead paint would not be completed by the end of the month, as initially promised.

A recent investigation by WNYC also revealed high levels of lead paint in numerous public elementary schools, with some samples showing lead levels more than 100 times the current safety standard. The Department of Education subsequently released data showing lead paint hazards at hundreds of schools; concern among parents about a lack of testing has remained high as children returned to school this month.

Asked whether there was a backlog of private apartments that have still not received proper lead inspections, Mayor de Blasio said he could not say for sure, but would provide that information later today.

A spokesperson for the Mayor's Office could not immediately provide that data.