Objectives: The objectives of this study were to assess the physiological and perceptual responses of healthcare workers to a chemical, biological, or radiological (CBR) training exercise in tropical conditions, and to compare the effective ness of four cooling methods on core body temperature. IPRED 2010 http://pdm.medicine.wisc.edu s41 Methods: Sixty volunteer participants, all of whom were healthcare workers (predominantly doctors and nurses) were randomized into four cohorts: (1) Shade; (2) Ice Vest; (3) Water Immersion; and (4) Crushed Ice Ingestion).The participants were matched for body mass index (BMI).The exercise consisted of triaging, resuscitating, transporting and decontaminating real weight manikins while dressed in level-3 personal protective equipment (PPE) in field conditions (mean outdoor wet-bulb globe temperature of 31.4°C) for a total of three hours, alternating work/rest phases every 30 minutes. Cohort 2 wore ice vests under their PPE during the active phase and then rested in the shade. During the cooling phase, Cohort 1 rested quietly in the shade, Cohort 3 were immersed in large tubs of water at 25°C and Cohort 4 ingested T.SmL.kg"1 body mass of crushed ice. All four groups had ad libitum access to fluids. An ingestible telemetry pill permitted measurement of core temperature throughout the exercise, while tympanic temperature, heart rate, blood pressure, subjective thermal sensation, and thermal discomfort ratings were recorded periodically throughout the cooling phase. Results: The peak core body temperature of 40.8°C was observed, with three participants recording temperatures above 40.4°C during the study. Overall, the participants that wore the ice vests during the active phase and those that undertook water immersion during the rest phase stored less heat during their active phases. Participants in both of these groups performed tasks more effectively and had less muscle soreness the following day. Conclusions: This first-of-its-kind study was performed in tropical conditions in the field, with healthcare workers wearing impermeable PPE during a realistic CBR incident scenario. The study confirmed that CBR responders are at risk of hyperthermia in these conditions. The authors were able to extrapolate work/rest ratios in hot and humid conditions that may be used safely in healthcare workers with a wide range of BMIs and background fitness. Keywords: Australia; chemical, biological, and radiological; cooling methods, core body temperature; hyperthermia