A hot day is not the same as a dangerous day. Two sites can both read 38 degrees on the thermometer, and one can be much harder on the body than the other, because of humidity, sun and the heat given off by machines. WBGT is the number safety people use to capture all of that in one figure.
Enter your readings below, and the calculator will give you the WBGT. If you add your own limit, it will tell you if you are above it.
What is WBGT?
WBGT stands for Wet Bulb Globe Temperature. It is a heat stress index. Unlike a normal air temperature reading, it combines the things that decide how hot the body feels at work: the humidity, the heat from the sun or hot surfaces, and the air temperature.
That matters because the body cools itself by sweating. When the air is humid, sweat does not dry well, and the body overheats faster. WBGT takes that into account. A dry 40 degrees and a humid 35 degrees can be very different in terms of risk, and the WBGT shows the difference.
The three readings you need
- Natural wet bulb temperature (NWB): shows the effect of humidity and air movement. It uses a wet wick on the thermometer bulb, left in the open air.
- Globe temperature (G): shows the heat that comes from radiation, such as sunlight, furnaces and hot walls. It is read inside a black globe.
- Dry bulb temperature (DB): the normal air temperature. You need it only for outdoor work in the sun.
Take the readings with a proper WBGT meter, at the place where people work, and at about the height of the body. Do not use the weather forecast for this.
The WBGT formulas
Outdoors with sun: WBGT = 0.7 x NWB + 0.2 x Globe + 0.1 x Dry bulb
Indoors or no sun: WBGT = 0.7 x NWB + 0.3 x Globe
The natural wet bulb carries the most weight, 70 percent, because humidity is such a big part of heat stress.
Worked examples
Outdoors. A road crew works in the open on a summer afternoon. The natural wet bulb is 27 degrees, the globe is 45 degrees and the dry bulb is 36 degrees.
WBGT = 0.7 x 27 + 0.2 x 45 + 0.1 x 36 = 18.9 + 9 + 3.6 = 31.5 C
Indoors. A boiler room has a natural wet bulb of 28 degrees and a globe of 36 degrees.
WBGT = 0.7 x 28 + 0.3 x 36 = 19.6 + 10.8 = 30.4 C
Both are high. The next step is to compare them with a safe limit for the type of work that people are doing.
What is a safe WBGT?
There is no single safe number. The limit depends on how hard the work is, the clothing people wear, and whether workers are used to the heat. A person walking slowly can tolerate a higher WBGT than someone carrying loads uphill. Workers who are new to the heat, or who have been away from it for some days, can tolerate much less.
Use the limit from the heat stress standard your company or client follows, such as ACGIH or NIOSH. You can type it into the calculator as your reference limit. If the result is above it, apply controls straight away.
What to do when the WBGT is high
- Change the schedule: do the hardest work early in the morning or late in the day
- Add rest breaks: short and often is better than one long break, and the rest area should be cool and shaded
- Provide water: put it close to the work, and remind people to drink before they feel thirsty
- Give shade: shelters, umbrellas, reflective screens and fans all help
- Reduce the workload: use tools, rotate people and share heavy tasks
- Let people get used to it: new workers should build up their hours over about a week or two
- Check clothing: heavy PPE and coveralls trap heat and make the real stress higher than the WBGT shows
Know the warning signs
Everyone on a hot site should know what heat illness looks like.
- Heat exhaustion: heavy sweating, headache, dizziness, nausea, cramps and tiredness. Move the person to a cool place, give water and let them rest.
- Heat stroke: confusion, slurred speech, hot skin, fits or collapse. This is an emergency. Call for medical help at once and start cooling the person while you wait.
Never leave a person with these signs to “walk it off”.
Step by step
- Decide if the work is outdoors in the sun or indoors.
- Take the natural wet bulb and globe readings, and the dry bulb outdoors.
- Enter them in the calculator.
- Compare the WBGT with the limit for the type of work.
- Apply controls if you are close to or above the limit.
- Check again during the day, because heat changes hour by hour.
Common mistakes
- Using the weather forecast temperature instead of a WBGT reading
- Using an ordinary wet bulb reading instead of the natural wet bulb
- Using the outdoor formula for indoor work, or the other way round
- Measuring in the shade when workers are in the sun
- Forgetting that heavy clothing makes people hotter than the number suggests
- Checking once in the morning and never again
Linking it to your wider plan
Heat should be part of your normal risk assessment. Score it with the risk matrix calculator. If your workplace is also loud, check the noise exposure calculator. Poor airflow adds to the heat in closed rooms, and the air changes per hour calculator shows how well a room is being ventilated.
Frequently asked questions
What does WBGT mean?
It means Wet Bulb Globe Temperature. It is a heat stress index that combines humidity, radiant heat and air temperature.
Is WBGT the same as air temperature?
No. Air temperature only shows how hot the air is. WBGT also shows the effect of humidity and radiant heat.
Which formula should I use?
Use the outdoor formula for work in direct sun. Use the indoor formula for work indoors or outdoors without a solar load.
Do I need a special meter?
Yes. A WBGT meter measures the natural wet bulb and globe temperatures. A normal thermometer cannot do this.
What WBGT is too high to work?
It depends on the work rate, clothing and how used to the heat the workers are. Follow the limit from your heat stress standard.
Learn this properly
Heat stress and working in hot conditions are covered in NEBOSH, IOSH and OSHA courses. ISH Institutes teaches these in Peshawar. Chat with us on WhatsApp and we will help you pick the right course.