The following investigation was made, at the request of the Mine Rescue Research Committee of the Safety in Mines Research Board, as to the upper limit of temperature at which air can be breathed without distress under various conditions. During the use of self-contained breathing apparatus in rescue work, the air circulating in the apparatus becomes heated and saturated with moisture, and the question arises of the limiting temperature of inspired air that a man can tolerate while resting and while doing work, and which is therefore permissible for an apparatus.