Core body temperature (t b ) of market-size male broilers (46 ± 3 d; 2.8 ± 0.1 kg; Ross × Ross breed) was continuously measured by telemetry during acute, 90- to 240-min exposures to 18 thermally challenging conditions. The thermal conditions consisted of 18 factorial combinations of three dry-bulb air temperatures (t db ; 35°C, 38°C, and 41°C), two dewpoint temperatures (t dp ; 19.4°C and 26.1°C), and three air velocities (V; 0.2, 0.7, and 1.2 m s -1 ). Based on t b rise after 90-min exposures to the thermal conditions, a temperature-humidity-velocity index (THVI) was developed to delineate the synergistic effects of the thermal components on the birds, having the form of THVI = (0.85t db + 0.15t wb ) × V -0.058 , where t wb = wet-bulb temperature. The homeostasis state of the bird was classified as normal, alert, danger, or emergency, which correspond to a t b rise threshold of 1.0°C, 2.5°C, 4.0°C, or > 4.0°C, respectively. These different homeostasis states were functionally and graphically expressed in terms of THVI and exposure time. For example, if the broilers were acutely exposed to a thermal condition for 90 min, then the THVI threshold for the normal, alert, danger, and emergency state would be about 35°C, 38°C, 40°C, and >40°C, respectively. If the exposure duration was increased to 120 min, the THVI threshold would drop to 34°C, 37°C, 38°C, and >38°C, respectively. The results of this study serve as a scientific basis for making management decisions and risk assessment associated with market-size broiler production and handling under thermally challenging conditions.