Summary A number of construction trades are exposed to contaminants such as asbestos, crystalline silica and moulds. In these working environments, when control at source and administrative and engineering measures prove inadequate, the use of a respiratory protective device (RPD) is essential to reduce the risks of exposure to airborne contaminants. In some work situations with high ambient temperatures, however, wearing an RPD could make tasks harder to perform, causing workers to want to take them off, even when they are still exposed to contaminants. The goal of this study was to quantify the physiological, physical and perceptual-motor constraints associated with wearing a P100 half-facepiece RPD, depending on the temperature and relative humidity, when physical exertion is required. There were three parts to the study. In the first two parts, progressive effort tests (30% to 80% of relative cardiac cost) were run with and without P100 half-facepiece RPDs on eight male subjects on a treadmill in a controlled environment chamber. In part 1, the participants did the tests in three ambient temperature conditions (23°C, 29°C and 35°C) at a constant relative humidity of 50%. In part 2, the participants did the tests under three ambient relative humidity conditions (30%, 50% and 80%) at a constant temperature of 29°C. In part 3, four different models of P100 filter half-facepiece RPDs, among the most commonly used in the Quebec construction industry, were assessed under controlled ambient conditions (temperature of 29°C and relative humidity of 50%). During the assessments, perceptual-motor tasks were performed before, during and after the treadmill tests. A number of physiological variables related to RPD use were measured: heart and respiratory rates, oxygen and carbon dioxide concentrations inside the RPD, brain oxygen saturation, temperature inside the RPD and internal body temperature. Psychophysical perception scales were used to assess perceived exertion during treadmill tests. In part 3, the discomfort due to extended wearing of the four models of RPD was also assessed over a period of eight hours, by means of visual analogue scales. The results showed that, aside from the temperature of the air inhaled inside the RPD, an increase in ambient temperature, at constant relative humidity, does not seem to cause any effect of interaction with the wearing of the RPD. The use of an RPD does cause an increase in the psychophysical perception of effort, however, and this increase is comparable for the three temperature levels examined. Under high relative humidity conditions, a significant increase in heart rate and psychophysical perception of effort was observed for an effort of the same intensity while wearing an RPD. An increase in relative humidity seems to have a greater effect on physiological indices than does ambient temperature when an RPD is being worn. The results of the first two parts show that oxygen and carbon dioxide concentrations inside the RPD are, respectively, lower than and greater than the concentrations found in ambient air. The increase in respiratory rate seems to enable an increase in pulmonary ventilation to renew the air inside the RPD. These respiratory changes probably help maintain the concentrations of these gases inside the RPD despite the increase in physical exertion. The results of the third part of the study suggest that the filter types used can have a minor effect on certain physiological variables. When the pancake and rigid shell models are compared, the results reveal significant, though small, differences with respect to psychophysical perception of effort, heart rate, brain oximetry, internal body temperature and end-expiratory CO2 pressure. Wearing a respirator for an entire day caused discomfort around the nose for all four models of RPD, although it was slightly less for the silicone models. Generally speaking, however, the silicone RPDs were no more comfortable to wear than the other models when it came to other parts of the face, according to the participants. The results of this study, conducted in a controlled situation, show that certain environmental conditions can affect physiological parameters and cause discomfort when wearing an RPD. An increase in effort did not amplify the effects measured when wearing an RPD. Conversely, relative humidity seems to be the only factor that amplifies the exertion perceived by the participants and their heart rate when wearing an RPD. Other studies will be necessary to gain a better understanding of the effect of this variable under different conditions of RPD use similar to those found in the workplace.