The Exel warehouse used to process returns from Walmart was evacuated for a toxic contamination in late August.Hundreds of workers at an Indianapolis Walmart returns processing center may have been contaminated with a toxic substance last month. While the center was evacuated and employees are now undergoing medical tests, one employee has filed a lawsuit against the mega-retailer. The center, where logistics company Exel processes merchandise returned from Walmart stores, has been empty since August 20 when the building was evacuated after it was confirmed that the toxic substance PCB, or polychlorinated biphenyl, was present, WTHR-TV reports. PCB is a synthetic organic chemical compound that is highly toxic and classified as a “probable human carcinogen.” Upon clearing the center, officials with Exel told employees they would receive their full pay and benefits, but would not return to the center until they were told they could come back. The 600 full-time employees and contract workers were notified five days after the initial evacuation that Walmart had discovered the highly toxic substance in their center. PCBs, banned in the United States for decades, were once commonly used as coolants and stabilizers in products such as fluorescent light ballasts, transformers, paints, cements, electrical components, pesticides, lubricating oils and sealants A spokesperson for Exel says the contamination was found by accident, when equipment was being moved inside the center.