Although many observations have been made on the upper limits of heat tolerated by working men (1 to 6), little work has been published on the specific effects of clothing on man's performance in these limiting hot environments. Gagge, Winslow, and Herrington's calorimetric studies (7) on the effect of clothing were done on resting men in environments with operative temperatures below 1010 F. However, Robinson, Turrell, and Gerking (8) have studied this problemby comparing the most severe environments in which both nude and clothed men could maintain thermal equilibrium after the second hour of a six hour work period. They showed experimentally that men wearing a single layer of lightclothing (windbreak poplin) could maintain thermal equilibrium only when the environments were less severe than those tolerated by nude men. The clothing had the same effect as lowering the limiting wet bulb temperature of the environment 20 to 60F., de-pending on the work rate and environment. A previous report from this laboratory (6) described the upper limits of heat that could be tolerated by highly acclimatized nude men working for four hours at approximately 250Cal. per hour. The present investigation was undertaken to ex-tend these data and to study the role of clothing by determining certain of the most severe environments in which highly acclimatized clothed men could work at this rate.

PROCEDURE All experiments were conducted during January, Feb-ruary and March 1945 in a laboratory hot room (35'X 22'x 14'). A Carrier 15-T6 air processing unit permitted control to within+ or-10 F. of the desired dry or wet bulb air temperature. For eachset of conditions the air temperature was maintained for four to five days before any tests were conducted so that walls and air were