A Northern California school district is under fire after it disclosed Monday that nearly 200 drinking fountains and water faucets across multiple schools had elevated levels of lead that tested above the district's standards.

The Oakland Unified School District, California's 11th-largest public school district, said in a letter to families that it had tested over 1,000 faucets and fountains at 40 TK-12 schools to assess lead levels in the water. While nearly 83% of the faucets and fountains tested were under the school district's limit of 5 parts per billion, or ppb, about 17% were above the limit.

The district's standard of 5 ppb is more strict compared to the state and federal guidelines of 15 ppb, which the U.S. Environmental Protection Agency set as the action level for lead in drinking water.

Out of the 1,083 faucets and fountains tested, the district identified 116 that tested above 5 ppb but remained below 15 ppb. And another 70 fixtures surpassed the state and federal limit.

Fixtures that show elevated levels were taken out of service for repairs and will be retested until lead concentrations are at or below 5 ppb, according to the district. A total of 61 fixtures have been fixed so far and are currently waiting to be retested, the district said.

The district has advised families to make sure children carry reusable water bottles to access filtered water stations on campuses.

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Families, staff outraged over lack of communication

Despite public records showing that schools were tested for lead levels between late March and June, families were only notified of the results on Monday. The district also acknowledged that some schools had learned of the water quality testing over the past week and said more will be notified this week.

"Despite our attention to detail and care for our sites, we did not communicate effectively to the members of each school community as the testing launched, as we received the results, and as fixes were being implemented," the school district said in a letter to families.

"We are putting systems in place to ensure a lack of effective communication does not occur again, and that school communities receive quick notice when this kind of testing is taking place on their campuses," the district added.

Monday's announcement has raised concerns among parents and teachers who say they were left in the dark about the lead concentration testing. Staff at Frink United Academy of Language, one of the schools that tested above the district's standards, called for transparency and noted that students have been exposed to lead since April, according to KTVU and KGO-TV.

"We are demanding safe drinking water for our students immediately," Frick Academy Counselor Catherine Cotter said at last week's school board meeting, KTVU and KGO-TV reported. "Transparency about what's happening and why we were not told this until the other day. Since April, the kids have been there all summer drinking water with lead in it."

Stuard Loebl, a 6th-grade teacher at Frink Academy, told CBS News that he's outraged and heartbroken by the results. "So students have been drinking lead water this whole time; obviously before the report was released but since as well, which is infuriating," Loebl said.

Health effects of lead exposure

Lead exposure can cause serious health effects for children and is particularly harmful for young children, according to the World Health Organization (WHO). The Centers for Disease Control and Prevention (CDC) warns that even low levels of lead in blood can cause negative health effects for children.

"Exposure to lead can affect multiple body systems," the WHO said. "Young children are particularly vulnerable to lead poisoning because they absorb 4–5 times as much ingested lead as adults from a given source."

High levels of exposure can lead to brain and nervous system damage; slowed growth and development; learning and behavior issues; and hearing and speech problems, according to the CDC. These effects can lead to lower IQ, decreased ability to pay attention, and underperformance in school, the CDC added.

"The neurological and behavioral effects of lead are believed to be irreversible," the WHO said.

Lower levels of exposure are often difficult to detect as there are no obvious symptoms, according to the CDC and WHO. Public health agencies advise parents to talk to their healthcare provider about potential blood lead testing.