Naval ratings serving in a warship in the tropics answered a questionnaire concerning their sensations of warmth at meal times on the mess-decks during two investigations carried out at an interval of 6 months when the ship was proceeding between tropical and more northerly waters. Temperature measurements were recorded at the same times as the questionnaires were completed by the subjects.

It is proposed that the upper level of warmth to be accepted in the design of ventilation systems for accommodation spaces should be those conditions when either more than 80% of persons feel reasonably comfortable or less than 20% observe the presence of unevaporated sweat on their skin, which corresponds to effective temperatures of 79 and 78° F. respectively, according to the majority opinion in these two investigations. The latter figure of 78° F. is suggested as the upper level to be accepted for men living under these conditions.

The upper level of the optimum or ideal zone for comfort, when equal proportions of persons reported that they were either cool or warm, was observed to be at 74° F. effective temperature.

The proportions of men who were uncomfortably warm at different levels of warmth were related to the proportions who reported that their skin was wet with unevaporated sweat. Very few observed that they were sweating, whatever the air velocity or their clothing, when the dry-bulb temperature was below 83° F., and the wet-bulb temperature below 75° F.

Brisk circulation of the air on the mess-decks made an increasingly large contribution to the comfort of these very lightly clad men as the air temperature rose from 85 to 89° F., but as the temperature fell below 83° F. variations in air Thermal comfort in the tropics 431 movement had little effect on the proportions of persons who were reasonably comfortable under these warm conditions.

Thermal discomfort was reported by an increasing number of men within a temperature zone where others have shown by experiment that there will occur a statistically significant decrease in the skilled performance of men who are acclimatized to working at high temperatures.

This paper is published by the permission of the Admiralty and the Medical Research Council's Royal Naval Personnel Research Committee. The investigations described were made possible by the active interest of Captain H. S. Hopkins, C.B.E., Royal Navy, and Commander W. Elliot, D.S.C., Royal Navy, the excellent facilities which they placed at the disposal of the observers, and the wholehearted co-operation of the naval ratings who acted as subjects. I am indebted to Dr T Bedford for advice and criticism and to Mr Cheng Swee Guan and Sick Berth Attendant W. F. Montague, Royal Navy, for care in tabulating the data, preparing the illustrations and technical assistance.