Use local weather guidance
Heat index is a planning indicator. Real risk also depends on sun exposure, wind, exertion, hydration, age, health conditions and local public-health alerts.
Example
At 90 F and 60% relative humidity, the apparent temperature can be much higher than the air temperature because sweat evaporates less efficiently.
Common mistakes
Do not use heat index as the only safety measure. Direct sun, protective clothing, exercise and poor hydration can make conditions riskier.
Limitations
This is not medical advice. Follow local weather warnings and health guidance for heat exposure, especially for vulnerable people or strenuous work.
Last reviewed: 2026-05-17