Atmospheric conditions—primarily temperature, humidity, and air flow—constitute an important problem in industrial psychology and human engineering, since they have considerable influence on productivity, accident rate, and morale. They also affect our comfort at home and in the open air.In general, workers are most comfortable, have the fewest accidents, and are most productive when the temperature is around 70°. However, the optimum range is largely determined by air movement, type of work, and humidity. High humidity is extremely uncomfortable because it prevents the body from using its major mechanism for maintaining heat equilibrium, that is, evaporation of perspiration. Studies show that a temperature of 90° with a humidity of 10 per cent is as comfortable as 80° with 60 per cent humidity or 75° with 100 per cent humidity. We also feel more comfortable at high temperatures if the air is flowing than if it is stationary.Heavy work can be carried on at lower temperatures than light work because our metabolic rate is greater and more body heat is produced. The hand skin temperature is the most critical factor; it should be no lower than 55° to 60°. The optimum general temperature for light sedentary work in winter is 68° to 73° and in summer 75° to 80°; for moderate hard work it is 65° and for strenuous physical work, it is 60° throughout the year (Baetger, 1944). It is an interesting fact that individual differences are quite small. Nine out of ten workers in different parts of the country and during different seasons of the year have reported that they feel comfortable within a 66° to 77° range when engaged in office and light physical work (Ryan, 1947). This makes the job of the ventilation engineer easier.Some investigators have found that high temperature has less effect on mental than on physical activity. In an experiment conducted during World War II, Mackworth (1946) found that the number of errors made by telegraph operators was about the same at 19°, 85°, and 88°, but that the error rate rose slightly at 92° and precipitously at 97°. Another investigator, Davis (1957), has shown that optimum ventilation does not always solve the problem of comfort. Workers in an aircraft plant in Texas complained bitterly of stuffiness, humidity, and heat even though the air-conditioning equipment was working and the atmospheric conditions were perfect. The reason for the complaint was traced to the fact that the plant had no windows and the air vents were high above the floor; as a result the workers, who came from rural areas, felt “cooped up” because they were used to being in the open air where they felt a breeze. The managers of the plant conceived the idea of tying streamers to the vents so they could see that the air was moving —and the complaints quickly died down.