Heat Index
Quick answer
The heat index combines air temperature and humidity into a single 'feels-like' number. It is a much better guide to heat risk than temperature alone, and it is the number most workplace heat guidance uses to trigger breaks, hydration and monitoring.
What the heat index is
The heat index — sometimes called the 'apparent temperature' or 'feels-like temperature' — combines the air temperature and the relative humidity to describe how the heat actually feels on the human body. On a humid day, sweat evaporates less easily, so the body cools less effectively and the effective heat load is higher than the thermometer suggests.
Why employers should use it, not the thermostat
A workplace with an office temperature of 30°C but 70% humidity produces a heat index above 40°C. Workers experience the higher number, not the lower one. Using heat index as the trigger for water, rest and shade protocols aligns your response with what your team's bodies are actually experiencing.
Practical trigger points
OSHA and many national safety bodies use rising heat-index bands to trigger increasingly protective controls: general awareness at moderate risk, mandatory hydration and rest at high risk, and stopping non-essential outdoor work at very-high or extreme risk. The specific thresholds vary by country and role, but the principle is the same: as heat index rises, your controls should escalate automatically — not on personal discretion.
Common mistakes
Reading only the temperature
A 32°C day at 20% humidity is very different from a 32°C day at 80% humidity. Only the heat index captures that.
Frequently asked questions
- Is the heat index the same as WBGT?
- No. WBGT (Wet Bulb Globe Temperature) is a more precise specialist measurement used by regulators for outdoor and industrial settings. Heat index is a widely understood proxy that is easier to use day-to-day.