We were thawing out some barrels that were frozen inside a warehouse with a generator and kerosene heaters. The doors were closed and the EE's were going in and out of the warehouse only to check barrels. The IE was going in and out approx. 5 hours. He told his supervisor and was sent home. He went to the doctor @8:00p.m. and was released from hospital @ 9:30 a.m.