Dangerous heat stress begins at a wet-bulb temperature of 26.7°C (80°F), with absolute human survivability limits reached at 35°C (95°F).
Occupational health and athletic organizations use the Wet Bulb Globe Temperature (WBGT)—which also accounts for humidity, wind, and solar radiation—to determine safe activity limits.
| WBGT Range | Danger Level | Activity Guidelines |
|---|---|---|
| < 26.7°C (< 80°F) | Safe | No restrictions on outdoor work or exercise, though unconditioned individuals should be monitored. |
| 26.7 – 29.4°C (80 – 85°F) | Caution | Strenuous work causes stress after 45 minutes. Take a 15-minute mandatory break every hour. |
| 29.4 – 31.1°C (85 – 88°F) | High Risk | Stress occurs after 30 minutes. Take a 30-minute break every hour. |
| 31.1 – 32.2°C (88 – 90°F) | Severe | Stress occurs after 15–20 minutes. Take 40 to 45-minute breaks each hour. |
| > 32.2°C (> 90°F) | Extreme / Cancel | Extreme exertion becomes dangerous rapidly. All outdoor activities should be suspended or heavily modified. |
Note on Humidity: In 100% humidity, the dry air temperature equals the wet-bulb temperature. In drier air, the ambient temperature can be significantly higher before it reaches a dangerous wet-bulb threshold. For example, a wet-bulb temperature of 35°C equals roughly 46°C (115°F) in 50% humidity.