The full-face respirator is frequently employed by workers. Research results indicated the respirators are available and in use in only 20-30% of the work phases evaluated as required [1,5]. Failure to use a respirator may be caused by thermo stress [2,3]. This study aimed to evaluate the physiological workloads and subjective discomforts for the full-face respirators under thermo and humid work conditions. Sixteen physically fit male university students (mean age 22.0±3.0)participated in the study, which was approved by the Institutional Review Board for study ethical review. A full-face respirator (MSA Advantage 3000) with an organic canister and a low respiratory resistance quarter-face mask as the control condition were employed. The evaluated independent variables included respirator type, workload (WL, 45W and 85W), environmental temperature (ET, 25 and 35 centigrade) and relative humidity (RH, 50% and 70%). The main dependent variables were summarized in table 1. All subjects carried out the experiments at five different days with at least one day off. On the first day the maximal oxygen consumptions were measured using a revised protocol [2]. The total 8 experiment conditions were completely randomized. The physiological variables were analyzed by repeated measure ANOVAs. The subjective RPE was analyzed using Wilcoxon test. Results indicated that working in higher ET imposed significant physiological and psychological strain shown by significantly increased heart rate, working pulse, breathing frequency, peak inspiratory and expiratory pressure, temperature in mask measured, and RPE and decreased expiratory time. Compared with work at 25 centigrade, work at 35 centigrade and equipped with the full-face respirator, the heart rate significantly increased 9 to 10 beats per minute for light work load and 12 to 15 beats per minute for moderate workload. Increased relative humidity was reported with significantly increased oxygen consumption, work pulse, breathing frequency, and RPE. Similar results were also reported by Chongvisal [1]. It is suggested that the work-rest ratio should be decreased when wearing respirator under thermo and high relative humidity work conditions.