A 12-year-old boy died in a storage room and his mother was rescued from the same storage room, where many apples were being stored. The atmosphere in the room was strictly controlled and the concentration of oxygen (O2) was recorded. The cause of death was suffocation. According to the records, the concentration of O2 was about 2.7% when the boy died. On the other hand, it was about 3.4% when his mother entered the room and she survived, although she remained unconscious for about four hours. These findings show that the absolute concentration of O2 causing acute death due to O2 deficiency is approximately 2.5-3.0%.