Parents in Newark, New Jersey learned this week that students at half of the city's public schools may have been exposed to elevated levels of lead in their drinking water.

The water at half of the district's buildings was shut off over safety concerns. However, parents remain unsure about just how long their kids were exposed to unsafe water. As it turns out, no one really does.

On Monday, March 7, Newark Public Schools (NPS) officials notified the state's Department of Environmental Protection that annual test results, taken in December, showed lead levels in 30 district buildings ranging from undetected, to higher than the accepted level of 15 parts per billion, the New York Times reported. The water was shut off two days later.

Department officials said that no building turned up more than four samples with lead above the so-called action level. But school officials say they are unsure if previous samples were adequately examined, and have asked environmental protection officials to conduct an analysis of previous reports.

Lead exposure, especially in young children, can have irreversible effects on brain development.  

One official told WNYC that they were unsure if the sample results from each of the district's 65 schools had actually been examined. A NPS spokesperson did not respond to ATTN:'s request for comment. 

Others within the schools have questioned the effectiveness of the testing and analysis system set up to prevent shut offs over unsafe water. A 2014 memo released by the Newark Teacher's Union highlighted precautionary measures to block against lead contamination — but it also underscored the system's apparent failings.

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"Lead levels are tested regularly and results shared with administrators who are trained to read and respond to the results. What happened here?" NTU president John Abeigon told NJ Spotlight. "Who received the results over the last several years and what did they do with them?"

Contamination in the Newark schools was likely traced to aging infrastructure, WNYC reported. Students are currently drinking bottled water.

The Newark water supply, environmental protection officials said, was not contaminated.