A US jury on Monday ordered chemical company Monsanto to pay $857 million to seven people at a school in the western state of Washington who said they were sickened by chemicals leaking from light fittings.

The ruling is the latest legal setback for Monsanto, which is already grappling with hefty legal bills after losing court cases over its glyphosate-based weedkiller Roundup.

The jury said the company, owned by German pharmaceutical giant Bayer, had sold the polychlorinated biphenyls (PCBs) found in the fittings that the group of students and parent volunteers at Sky Valley Education Center in the town of Monroe in Washington state claimed had made them ill.

It ordered the firm to pay a total of $73 million in compensation and $784 million in punitive damages to the plaintiffs in the case.

Lawyer Felix Luna, representing the plaintiffs, told the jury Monsanto had engaged in years of subterfuge to cover up what they knew about the harmful effects of PCBs.

"Monsanto… never warned anyone that (PCBs) would outlast whatever they're put in," Luna said in his closing argument, according to a transcript of the case.

"They never warned anyone that when it gets in the body that they're metabolites for life, that they're neurotoxic… a hazard, or (that) PCBs could lead to systemic poisoning."

The jury found the company negligent and ordered varying amounts of compensation to the seven plaintiffs, with each also being awarded $112 million in punitive damages.

A spokesperson for Monsanto said the firm would appeal the ruling.

"We disagree with the verdict and will pursue posttrial motions and appeals to get this verdict overturned and to reduce the constitutionally excessive damages awarded," a statement said.

"The objective evidence in this case, including blood, air, and other tests, demonstrates that plaintiffs were not exposed to unsafe levels of PCBs, and PCBs could not have caused their alleged injuries."