Abstract: On July 12, 2011, four employees of US Foodservice Incorporated were working in the cold storage area of a grocery warehouse. The employees reported flu like symptoms to the facility's shift supervisor. The fire department was called, and they determined the employees were overexposed to carbon monoxide (CO). CO levels up to 600 ppm in the cold storage area and 100 ppm in other areas of the warehouse were detected. The source of the CO could not be identified. The warehouse was evacuated and ventilated. The four employees were treated at Creighton Medical Center for carbon monoxide poisoning.