The teachers, who worked at the Sky Valley Education Centre in Monroe, Washington, said they suffered brain damage from exposure to polychlorinated biphenyls, or PCBs, in the fluorescent lighting at the school.

The law firm that represented the teachers, Friedman Rubin, said a jury returned the verdict Tuesday in King County Superior Court.

"This is a big step in holding Monsanto accountable," the teachers' attorney, Rick Friedman, said in a statement.

Bayer, which bought Monsanto in 2018, said the company disagreed with the verdict and may appeal.

"The undisputed evidence in this case does not support the conclusions that plaintiffs were exposed to unsafe levels of PCBs at the Sky Valley Education Centre (SVEC) or that any exposure could have possibly caused their claimed injuries," Bayer spokesperson Susan Skiles Lukes said in a statement.

"These are historic claims that relate to products Monsanto has not produced in more than 40 years," Lukes said.

This was the first of 22 trials involving teachers, parents and students who spent time at the Sky Valley Education Centre.

PCBs were banned four decades ago over concerns the chemicals could cause cancer and other illnesses.

Many older buildings still have caulk, ceiling tiles, floor adhesives and paint made with PCBs, which sometimes have been found at levels far higher than allowed by law.

PCBs are mixtures of compounds manufactured by Monsanto Co. and widely used as coolants and lubricants in electrical equipment until they were banned in 1979.

Monsanto has also faced a deluge of litigation over its weed killer Roundup. Last year, Bayer said it would pay more than $10 billion to settle thousands of lawsuits claiming Roundup causes cancer. The company said the settlement involves about 125,000 filed and unfiled claims.

Australian Associated Press