The Definition of Wet Bulb Globe Temperature (WBGT)

Definition of Wet Bulb Globe Temperature (WBGT)

Wet bulb globe temperature (WBGT) is a measure of heat stress that accounts for the four environmental factors that affect how hot a person actually gets: air temperature, humidity, radiant heat from the sun or hot surfaces, and air movement. It is the index most occupational heat stress guidelines use to decide how hard and how long people can safely work in hot conditions.

WBGT was developed for the U.S. military in the 1950s after heat casualties during recruit training, and it is now used by ACGIH, NIOSH, the military, sports bodies and the international standard ISO 7243. It differs from the heat index used in everyday weather reports, which only combines air temperature and humidity and is calculated for shade. Because WBGT includes direct sun and wind, it gives a more complete picture for outdoor work.

How WBGT Is Calculated

A WBGT meter combines three readings:

• Natural wet bulb temperature (Tnwb): a thermometer covered in a wet wick and exposed to natural air movement. It reflects humidity and the cooling effect of evaporation.
• Globe temperature (Tg): a thermometer inside a black globe. It reflects radiant heat from the sun or hot equipment.
• Dry bulb temperature (Tdb): ordinary air temperature.

Outdoors in sunlight, WBGT = 0.7 Tnwb + 0.2 Tg + 0.1 Tdb. Indoors or without direct sun, WBGT = 0.7 Tnwb + 0.3 Tg. The heavy weighting on the wet bulb reading shows how much humidity matters: when sweat cannot evaporate, the body loses its main way of shedding heat.

Using WBGT to Set Work Limits

A WBGT reading on its own does not say whether work is safe. Guidelines adjust it for three factors:

• Workload: heavy work produces far more internal heat than light work, so limits are lower.
• Acclimatization: workers who have built up heat tolerance over one to two weeks can work at higher WBGT values than new or returning workers.
• Clothing: coveralls, chemical protective suits and other heavy or vapor-barrier clothing trap heat. ACGIH publishes clothing adjustment factors that are added to the measured WBGT.

The ACGIH threshold limit values and NIOSH recommended exposure limits use these inputs to set work and rest cycles, such as 45 minutes of work and 15 minutes of rest each hour at a given WBGT and workload. Military and athletic programs use flag systems that change activity limits as WBGT rises.

Regulations

Federal OSHA has no final heat standard and enforces heat hazards under the General Duty Clause. OSHA published a proposed heat injury and illness prevention rule in 2024 that used heat index triggers and allowed WBGT as an alternative. Several states, including California, Washington, Oregon, Colorado and Maryland, have their own heat rules, and the trigger measure differs by state.

Example of WBGT

A refinery planned a turnaround in August. On the first morning, the safety team measured a WBGT of 29°C (84°F) in the sun near the units, while the heat index on the weather app looked moderate. Workers in fire-resistant coveralls doing heavy work were already near the ACGIH action limit after the clothing adjustment.

The site set work and rest cycles tied to hourly WBGT readings, moved the heaviest tasks to early morning, added shaded cooling stations with water and electrolyte drinks, and put new arrivals on a gradual acclimatization schedule. Supervisors were trained to spot early signs of heat stress and to stop work when a worker showed symptoms.

Frequently Asked Questions: Wet Bulb Globe Temperature (WBGT)

Is WBGT the same as the heat index?

No. The heat index combines air temperature and humidity and assumes shade. WBGT also includes radiant heat and wind, so in strong sun it can show a higher risk than the heat index suggests.

Do I need a special meter to measure WBGT?

For accurate readings, yes. WBGT meters measure all three temperatures at the work location. Some weather services and apps provide WBGT forecasts, which are useful for planning but do not replace measurements at hot spots such as near furnaces or on roofs.

What WBGT is too hot to work?

There is no single number. The safe limit depends on workload, acclimatization and clothing. A WBGT that is acceptable for light work by acclimatized workers may call for long rest breaks or stopping heavy work by new workers in protective suits.

How often should WBGT be measured?

On hot days, many programs measure at least hourly at each work area, and more often when conditions change quickly, such as when clouds clear or work moves near a heat source.