Polychlorinated biphenyls, or PCBs, are industrial chemicals so toxic that Congress banned them 40 years ago. Research has shown that they can cause a range of health concerns, including cancer and neurological problems such as decreased IQ. And yet, because they were commonly used in building materials for decades, they continue to contaminate classrooms in between 13,000 and 26,000 schools nationwide, according to Harvard researchers.

No one knows exactly how many schools are affected — nor how many children are being exposed to these toxic chemicals — because many schools don’t test for PCBs. Under federal law, they don’t have to.

Now activists are mounting a campaign to change that, lobbying Congress to close what they say is a dangerous loophole that could be harming millions of children. The effort comes in the aftermath of the Flint water crisis, amid new scrutiny of schoolchildren’s exposure to another toxic substance for which schools are not required to test: lead in drinking water fountains.

“Parents have the right to know what their children are being exposed to in school,” said Jennifer deNicola, a parent in Malibu, Calif., who helped lead a years-long effort to rid that community’s schools of PCBs in window caulk and other materials, eventually filing a lawsuit against the school district. People can be exposed to the chemicals when they touch contaminated substances, eat contaminated food or breathe air contaminated with PCB-laden dust. The caulk in Malibu schools had concentrations of PCBs that in some instances were thousands of times higher than the federal limit.

The legal battle in that tony seaside community — which ended last month, when a federal judge ordered the school district to remove PCBs from its schools by Dec. 31, 2019 — drew national attention to the issue, not least because supermodel Cindy Crawford pulled her children out of the school system and became a spokeswoman for the cause.

“This isn’t my normal day job, but it just didn’t make sense to me, and it didn’t seem fair,” Crawford told reporters Wednesday morning. “My children are being homeschooled, but that is not an option for most people.”

Schools also have grappled with the problem elsewhere, including in New York, Massachusetts and Washington. A school in Hartford, Conn., was closed indefinitely for cleanup last year after it was found to have airborne PCB levels nearly 2,000 times greater than the federal limit, according to the Hartford Courant.

Now deNicola and Crawford are turning their attention to undetected PCBs in schools nationwide. “We need to make this a political issue bigger than us,” said deNicola, who started the nonprofit America Unites for Kids to address the problem.

They have joined forces with the Environmental Working Group, a research and advocacy organization that specializes in public health. They are lobbying Congress to make clear that the Environmental Protection Agency must require schools built between 1950 and 1979 — when PCBs were commonly used not only in window caulk, but also in school fluorescent lighting fixtures, paint and floor finishes — to test for contamination.

They have an ally in Sen. Ed Markey (D-Mass.), who on Wednesday released a report on the extent of PCB contamination in the nation’s schools and called on Congress to provide the money that schools need to test for, and respond to, the problem. Markey also called for mandatory PCB testing in schools, for parents to be notified when PCBs are found in their children’s schools, and for the Environmental Protection Agency to begin urging inspections of schools built or renovated during the PCB era.

According to Markey’s report, EPA has received 286 reports of PCB contamination in 22 states during the past decade, affecting thousands of schools. But that likely represents just a tiny fraction of the problem because testing is random and piecemeal, according to the report.

“Absent a requirement to test or inspect schools for PCB contamination, the discovery of PCB hazards in schools occurs by chance,” the report says.

“Right now at the rate of current enforcement and inspection activities by states and the EPA, it would take at least 32 years to inspect schools that may have PCB-containing caulk,” Markey said Wednesday, and even longer to inspect all potentially contaminated materials.

EPA officials said they do not currently recommend inspections at all such schools because that blanket approach would not be appropriate or effective at every school. Instead, they recommend reducing chances of contamination by removing PCB-based fluorescent light fixtures, by removing caulk and other materials during planned renovations, and by keeping PCB-laden dust to a minimum by mopping and using wet rags.

That approach is the most effective way for schools to use limited resources to deal with potential PCB contamination, said Jeff Morris of EPA’s Office of Pollution Prevention and Toxics, who also emphasized that current laws and regulations do not give EPA the authority to require testing. “We’re not saying that this is a trivial thing at all,” he said. “It is in a lot of buildings, it’s true.”

EPA is working to create a new rule that would require schools to get rid of all fluorescent lights with PCB-based ballast. And — highlighting how much remains unknown about the chemicals despite their long-standing reputation as dangerous — government scientists are working to better understand what should be considered acceptable levels of PCB air contamination.

Schools must stay below thresholds of between 100 and 600 nanograms per cubic meter, depending on the age of the students, according to the EPA. But some experts — including one that the Malibu parents hired to fight the school district there — argue that research has shown evidence that PCBs can be harmful to children at far lower levels.

Morris, the EPA official, said the levels the agency has set are based on the best available PCB science. “But the big caveat is that PCB science needs to be improved,” he said.

EPA guidance is critical because schools turn to the agency for help in understanding and responding to environmental health risks while complying with the law. The Malibu school district was found to have violated the Toxic Substances Control Act because of the illegally high levels of PCBs in window caulk. But district officials said that they had consistently complied with EPA guidance and requirements, and the judge in the case agreed.

“We’re education experts,” said Gail Pinsker, spokeswoman for the Santa Monica-Malibu Unified School District, who said the district is confident that its classrooms are, and always have been, safe. “The district is a public agency that is following the law and following the EPA guidelines that have been sent to us.”