A jury in Washington State decided on Monday that Monsanto should pay $857 million to former students and parent volunteers who said they had been exposed at a school to dangerous chemicals made by the company and later became sick, according to court records.

The Superior Court jury in Seattle said Monsanto must pay $73 million in compensatory damages and $784 million in punitive damages to five students who had attended Sky Valley Education Center in Monroe, Wash., northeast of Seattle, and two parents who had volunteered there.

The former students and parents said that they had become sick from chemicals known as polychlorinated biphenyls, or PCBs, that had leaked from light fixtures at the school, according to Henry Jones, a lawyer for the plaintiffs. The chemicals in the fixtures were made by Monsanto, which Bayer bought in 2018.

The verdict, which will be reviewed by a judge, would add to billions of dollars in similar amounts awarded by juries that have troubled Bayer in the years since its Monsanto acquisition.

Mr. Jones said in an email on Monday after the verdict, “No one who heard this evidence would ever change places with any of these people in exchange for all the money the jury awarded.”

Monsanto said in a statement on Monday that it planned to appeal to overturn the verdict and challenge the “constitutionally excessive damages awarded.”

“The objective evidence in this case, including blood, air and other tests, demonstrates that plaintiffs were not exposed to unsafe levels of P.C.B.s, and P.C.B.s could not have caused their alleged injuries,” the company said.

The plaintiffs include former students and parent volunteers who were at the Sky Valley Education Center starting in 2005, according to court documents. They claim they suffered neurological, neurophysiological, endocrine and autoimmune issues after being exposed to chemicals at the school, according to court documents.

The Monroe School District, which includes the Sky Valley Education Center, in Washington State did not immediately respond to requests for comment on Monday afternoon.

PCBs were once regularly found in commercial products and industrial equipment, such as lighting, until they were banned in the United States in 1979 amid concerns that they harmed people and the environment, according to the Environmental Protection Agency.

Products that include PCBs are no longer commercially produced in the United States, but the chemicals may still be present in products made before they were banned, according to the E.P.A.

Conclusive evidence has found that PCBs can cause cancer in animals, as well as harm their immune, reproductive, nervous and endocrine systems, according to the E.P.A. The chemicals have been classified as “probably carcinogenic” to humans, according to the agency.

Since Bayer purchased Monsanto, the company has been troubled with costly legal battles over concerns about chemicals produced by Monsanto, such as Roundup, the weed killer.

Bayer agreed to pay $10 billion in 2020 to settle claims that Roundup caused cancer, one of the largest of such settlements. The company has said that it has set aside an additional $6 billion for ongoing lawsuits and others that could be filed later.