Concerned parents planned to protest Tuesday Malibu High School’s reopening after summer break because elevated levels of carcinogens that can cause cancer and other negative healths effects were discovered last year.

The campus faced controversy when three teachers were diagnosed with thyroid cancer as of November 2013 and about 20 others expressed health concerns.

Potentially harmful levels of polychlorinated biphenyls; which can cause immune system suppression, liver damage, endocrine disruption, and damage to the reproductive and nervous systems; were found at the campus, Environmental Protection Agency Senior Policy Analyst Hugh Kaufman told KTLA in November.

PBC made its way into many schools built in the 1950s to the 1970s through caulk, a flexible material used to seal gaps in many part of windows and doors, according to the EPA website.

Malibu High School’s classrooms have been cleaned and EPA guidelines for testing have been met, according to Superintendent Sandra Lyon.

But the school should not be inhabited until the source of the elevated levels of PBC is found and removed, Kaufman said.

“They have to remediate those building materials so that the rooms can be safe according to EPA’s regulations,” he said.

The school planned to resume classes Tuesday, Aug. 18.

School district officials could not be reached for comment.

Parents planned to protest on Tuesday at 6 p.m. outside the Malibu Library.