Respiratory protective devices (face masks) are required to protect to wearer from particles and vapors in the ambient air when other means of air purification are unavailable. But many workers who ought to wear these devices fail to put them on, or else take them off when they should be wearing them. Using an artificial air supply to the mask, set at different combinations of temperature and humidity, we recently defined the thermal comfort zone, i.e. the temperature conditions of the mask acceptable to the wearer. In preparation for the present application, we have examined porous masks, and masks with filters and valves, to find out how they interfere with heat loss from the face. Factors that appear to affect the thermal acceptability of such masks are: 1) Radiant heat loss from the face to the inner surface of the mask, 2) heat transfer through the wall of the mask, 3) heat loss from the outer surface of the mask to the environment, 4) heat transfer between the respiratory tract, and inspired air and 5) heat transfer between the mask air, mask wall, the expired air and environment. The equipment used to measure heat transfer and temperature gradients of face masks consists of thermocouples, thermistors, thermal radiometers, humidity sensors, heat transfer discs, and air flow meters. These are used to establish quantitative thermal profiles and heat transfer rates inside and outside the masks. A constant temperature room and cycle ergometer are used to set the conditions and work load for the subjects. Psychophsyical rating scales are used for assessment of thermal comfort of the subjects. The masks are modified to change the patterns of airflow and heat transfer, with reassessment of comfort and acceptability, and objective measurements of heat transfer rates and thermal gradients inside and outside the mask. The hypothesis to be tested is that thermal discomfort of the face is a major factor in workers not wearing respiratory protective devices.