Heat Stress: a burning issue

Heat stress describes the total burden that hot environments place on the human body, combining air temperature, humidity, sun/thermal radiation and wind into a single physiological challenge. When heat stress is high, the body struggles to shed metabolic heat, core temperature can rise, and the risk of heat exhaustion or heat stroke increases.

What is Wet Bulb Globe Temperature (WBGT)?

Wet Bulb Globe Temperature (WBGT) is one of the most widely used indices for assessing environmental heat stress in occupational, military and sports settings. It was developed in the 1950s by the U.S. military to address training casualties and later adopted in international standards.

For outdoor conditions with solar radiation, WBGT combines three temperatures:

• Dry‑bulb temperature (Ta) — shaded air temperature.
• Natural wet‑bulb temperature (Tnwb) — a wetted thermometer exposed to air flow and radiation, reflecting cooling by evaporation.
• Black‑globe temperature (Tg) — a thermometer inside a 15 cm black sphere, representing radiant heat from sun and surroundings.

The heavy 0.7 weighting on wet‑bulb temperature emphasises the importance of humidity and evaporative cooling, while globe temperature (0.2) captures radiant heat and wind effects, and dry‑bulb (0.1) completes the thermal picture.

Black‑globe temperature and radiant heat

The black‑globe thermometer (Tg) is a hollow, matte‑black sphere with a temperature sensor at its centre, typically 150 mm in diameter. Because it absorbs radiation from all directions, Tg rises well above air temperature in direct sun or near hot surfaces such as artificial turf, concrete, or industrial equipment.

Tg responds to three main factors:

• Solar radiation — direct sun and reflected light increase Tg relative to air temperature.
• Thermal radiation — hot surroundings (walls, ground, equipment) raise Tg even in shade.
• Wind speed — stronger wind cools the globe more effectively, reducing Tg for a given radiation load.

How dry‑bulb temperature, humidity and wind shape heat stress

Dry‑bulb temperature (Ta)

Dry‑bulb temperature is the regular shaded air temperature measured in a ventilated screen. Higher Ta increases both Tnwb and Tg, and directly raises WBGT through the 0.1 weighting. However, WBGT treats Ta as the least influential of its three components; what matters more is how that heat interacts with humidity and radiation.

Relative humidity and wet‑bulb temperature (Tnwb)

Natural wet‑bulb temperature is measured by wrapping a thermometer bulb in a wetted wick, exposing it to air and radiation, and allowing evaporation to cool it. The cooling depends strongly on:

• Relative humidity — at high humidity, evaporation is inefficient, so Tnwb is close to Ta; at low humidity, evaporation is strong, so Tnwb can be much cooler than Ta.
• Wind speed / air movement — more airflow over the wet wick enhances evaporation, lowering Tnwb.

Because WBGT weights Tnwb at 0.7, humidity and airflow dominate the index. Environments with the same WBGT can pose different physiological stress depending on how easily sweat can evaporate, highlighting that WBGT is a useful guide but not a perfect predictor of strain.

Wind speed

Wind influences heat stress in two ways:

• It enhances convective cooling, helping remove heat from skin and from the black globe, and lowering Tg.
• It boosts evaporation, lowering Tnwb and improving the body's ability to lose heat via sweat.

At the same temperature and humidity, a breezy environment will typically have lower Tnwb and Tg, and thus lower WBGT, than a still one.

Putting it together: why WBGT (and its limits) matter

WBGT condenses air temperature, humidity, radiation and wind into a single number that is relatively easy to measure and interpret, which is why it underpins many heat‑safety guidelines. However, its limitations are important:

• It can under‑ or over‑represent humidity and air movement effects in some environments.
• Devices that omit true globe temperature, or that rely on non‑standard wet‑bulb setups, can produce misleading WBGT values.
• Different surfaces and microclimates can have very different WBGT even when air temperature is the same, so local measurement is crucial.

In practice, understanding how dry‑bulb temperature, relative humidity, wind speed and radiation interact through Tnwb and Tg helps explain why two days with the same thermometer reading can feel completely different — and why robust, well‑measured WBGT remains a valuable, if imperfect, tool for managing heat stress.