About 13 employees at a warehouse on Obici Industrial Boulevard were being treated Thursday at Sentara Obici Hospital after a carbon monoxide incident earlier in the day.

Suffolk Fire and Rescue responded to the scene at 2:09 p.m. after being advised of an unconscious patient at the Ark Group warehouse, according to a press release from city spokeswoman Diana Klink.

After emergency workers assessed the patient and observed symptoms, they performed an air quality test of the warehouse and found levels of carbon monoxide that became higher the farther they went inside, Klink stated.

Employees had been operating forklifts with the doors closed due to the cold temperatures, she wrote, and the fumes built up during the course of the afternoon.

The building was cleared of all employees, and they began complaining of headaches and dizziness, Klink said. Eight patients were taken to Sentara Obici Hospital by ambulance, and five went there in their own vehicles. All had injuries not considered life-threatening, Klink said.

Crews used fans on engines to help ventilate the building, and Norfolk Fire and Rescue brought in additional fans, Klink said. A medic unit from Chesapeake Fire and Rescue assisted in covering the northern part of Suffolk due to the number of units that responded to the incident, Klink said.