Lumen Sites for mine & construction sites

WBGT heat stress calculator

Estimate the wet bulb globe temperature (WBGT) from the air temperature and humidity, and see how it compares with your site’s heat stress levels.

29.5° WBGT

Caution: at or above your caution level of 28°

Vapour pressure 19.0 hPa

What WBGT measures

Wet bulb globe temperature is the heat stress index behind the international standard ISO 7243 and many Australian workplace heat stress procedures, especially in mining. It combines the four things that decide how easily the body can shed heat: air temperature, humidity, radiant heat from the sun and hot surfaces, and air movement.

A WBGT meter measures it directly with three sensors: a natural wet bulb, cooled by evaporation so it responds to humidity and wind; a black globe, which soaks up radiant heat; and an ordinary air thermometer. Outdoors in the sun they are combined as 0.7 × wet bulb + 0.2 × globe + 0.1 × air temperature.

How this calculator estimates it

Without a meter you usually have only the weather: temperature and humidity. The Bureau of Meteorology’s simplified formula turns those two readings into an estimate:

WBGT ≈ 0.567 × T + 0.393 × e + 3.94

T is the air temperature in °C and e is the water vapour pressure in hectopascals, worked out from the temperature and relative humidity. The formula assumes moderate sunshine and a light wind, so it can read low in full sun with no breeze, and high in shade or a stiff wind. It is the same estimate Lumen Sites uses for its hourly heat stress forecasts.

WBGT chart

The estimate for common conditions. Amber cells reach 28°, the caution level Lumen Sites starts with; red cells reach 32°, its stop level.

Estimated WBGT (°C) by air temperature and relative humidity. A dash marks combinations that almost never happen (a dew point above 28°). Values above 35 show as 35+: well past any stop level, where the simple formula is least reliable.
Air temp.	10% RH	20% RH	30% RH	40% RH	50% RH	60% RH	70% RH	80% RH
24°	18.7	19.9	21.1	22.2	23.4	24.6	25.7	26.9
26°	20.0	21.3	22.6	23.9	25.3	26.6	27.9	29.2
28°	21.3	22.8	24.3	25.7	27.2	28.7	30.2	31.7
30°	22.6	24.3	25.9	27.6	29.3	30.9	32.6	34.2
32°	23.9	25.8	27.7	29.5	31.4	33.3	35+	–
34°	25.3	27.4	29.5	31.5	33.6	35+	35+	–
36°	26.7	29.0	31.3	33.7	35+	35+	–	–
38°	28.1	30.7	33.3	35+	35+	–	–	–
40°	29.5	32.4	35+	35+	35+	–	–	–
42°	31.0	34.2	35+	35+	–	–	–	–
44°	32.4	35+	35+	35+	–	–	–	–
46°	34.0	35+	35+	–	–	–	–	–

Hot, dry days still reach the stop level: 40° at 20% humidity estimates 32.4. And humidity matters more the hotter it gets: at 32°, going from 30% to 60% humidity lifts the estimate from 27.7 to 33.3.

What the levels mean

There is no single WBGT at which all work must stop. A safe level depends on how hard people are working, what they are wearing, whether they are used to the heat and how long they go between breaks, which is why the numbers belong in your site’s heat stress procedure. Australian work health and safety laws don’t set a maximum working temperature: employers have to manage the risk of heat illness, and WBGT levels are a common way to do it.

Lumen Sites starts every site at a caution level of 28° and a stop level of 32°. Change the two levels above to match your own procedure and the calculator uses them.

Signs of heat illness

Heat exhaustion: heavy sweating, paleness, dizziness, headache, nausea, weakness or muscle cramps. Move the person somewhere cool, give them water and keep watching them.
Heat stroke: confusion, slurred speech, collapse or seizures, with a high body temperature. It is a medical emergency: call triple zero (000) and start cooling the person straight away.

Plan the day, not just the moment

A calculator tells you where things stand now. On a remote site the more useful question is which hours will cross your levels tomorrow, so heavy work can start early and breaks can be planned around the hottest part of the day. Lumen Sites forecasts estimated WBGT for every hour of the next 48 hours at each site, against that site’s own levels.

To see how often it gets that hot, every Australian town page on Lumen shows the average number of heat-stress days in each month, for example Port Hedland, Newman and Kalgoorlie.

Limits of an estimate

This is an estimate from two weather readings, not a measurement. It cannot see radiant heat from plant, rock faces or enclosed spaces, and it knows nothing about workload, clothing or acclimatisation. Measure WBGT on site when it matters, and follow your heat stress procedure.