Six Factors, Not One

The body maintains a core temperature near 37 °C, losing heat mainly by evaporating sweat. Heat stress occurs when heat gained exceeds heat lost, and six factors determine that balance.

Environmental factors

• Air temperature — the obvious one, and the least useful on its own.
• Humidity — the critical factor. Sweat that does not evaporate provides no cooling, so 35 °C at 80% humidity is far more dangerous than 40 °C in dry air.
• Radiant heat — direct sun, furnaces, hot surfaces, engines. Radiant load can exceed air temperature effects entirely.
• Air movement — accelerates evaporation, unless the air is hotter than skin, in which case a fan adds heat.

Personal factors

• Work rate — metabolic heat from the muscles is often the largest single heat source.
• Clothing — impermeable coveralls, chemical suits and fire-retardant layers block evaporation and can make a moderate environment dangerous.

The most dangerous heat jobs are often indoors: heavy work in a chemical suit inside a plant, not light work in the sun.