The number of children potentially exposed to lead paint in New York City's public housing buildings is three times greater than previously thought, the federal monitor tasked with overseeing NYCHA revealed on Thursday.

Six thousand additional apartments with children under the age of six are believed to contain dangerous lead paint — on top of the 3,000 units previously identified, according to Bart Schwartz, the federal monitor.

The discovery was made public in a short update posted to Schwartz's website on Thursday. He said the housing authority reported the additional apartments earlier this week, after portable X-ray inspections turned up "a significantly larger number of apartments where children under six reside who might be exposed to lead risks."

Lead is a neurotoxin that can cause permanent brain damage to children if imbibed or inhaled, typically in the form of dust from lead paint or paint chips. Research shows that even relatively low levels of lead exposure can cause loss of IQ, hyperactivity and other behavioral problems. The Centers for Disease Control has determined that no level of lead exposure is safe.

Asked about the findings on Thursday, Mayor Bill de Blasio said he was hearing the news for the first time. "I'm concerned," he said. "We're seeing a steady decrease but if there's anything where we've got to double back and go farther then we will."

The mayor has faced repeated accusations of downplaying the extent of the lead crisis in public housing. Under his tenure, NYCHA was caught lying to the federal government about conducting inspections — a cover-up that continued even after the mayor was made aware of the problem.

The federal monitor was appointed to oversee reforms of the agency in January of last year, as a result of the lead paint scandal, as well as the rampant heat outages, broken elevators, rats, leaks and other issues long-plaguing the housing authority. That agreement stipulated that the city spend $2.2 billion on public housing over the next decade. A month later, de Blasio vowed to inspect roughly 135,000 apartments for lead by September of this year — a far higher total than the authority previously said was necessary.

It's unclear whether the city has completed all of those inspections. A spokesperson for the agency did not respond to Gothamist's inquiries.

More than 1,100 children tested positive for elevated lead levels between 2012 to June 30, 2018. City officials point out that childhood lead levels have declined in recent years, and that kids living in public housing typically have lower rates of lead poisoning than those in private housing.

But some NYCHA tenants have long suspected that the true number of children exposed to lead is much higher than the city was admitting.

In his statement, the monitor said the agency will now begin the work of remediating those apartments for lead.

"With this new information, the importance of quickly and thoroughly implementing all necessary actions must be emphasized," he said. "I am committed to seeing these lead safety steps be taken as soon as possible. Protecting these children is a central part of my mission as Monitor."