Wet bulb globe temperature (WBGT) is a heat stress index that combines air temperature, humidity, wind speed and radiant heat, such as sunlight, into a single number. It is the measure most widely recommended by occupational health organizations for evaluating heat stress at work and setting work and rest schedules in hot environments.
Heat illness can progress from heat rash and cramps to heat exhaustion and life-threatening heat stroke. Because WBGT reflects more of the conditions that affect the body's ability to cool itself than air temperature alone, it gives employers a better basis for deciding when to add breaks, water, shade and other protections.
What Is Wet Bulb Globe Temperature?
WBGT is a composite temperature reading. A CDC report describing occupational heat limits notes that WBGT incorporates four environmental factors: air temperature, relative humidity, wind speed and radiation. The National Weather Service describes WBGT as a measure of heat stress in direct sunlight and notes that military agencies, OSHA and many nations use it as a guide to managing workload in direct sunlight.
WBGT is calculated from three measurements:
- Natural wet bulb temperature (Tnwb): measured with a thermometer covered by a wet wick that is exposed to natural air movement. It reflects the cooling effect of evaporation and is strongly influenced by humidity and wind.
- Globe temperature (Tg): measured inside a black globe. It reflects radiant heat from the sun or from hot surfaces such as furnaces and ovens.
- Dry bulb temperature (Tdb): ordinary air temperature measured with a shielded thermometer.
WBGT Formulas
Outdoors with a solar load, WBGT is calculated as:
WBGT = 0.7 Tnwb + 0.2 Tg + 0.1 Tdb
Indoors, or outdoors without a solar load, the dry bulb term is dropped:
WBGT = 0.7 Tnwb + 0.3 Tg
The heavy weight on the natural wet bulb reading shows how important humidity and evaporative cooling are to heat stress. A humid, still day can produce a higher WBGT than a hotter but drier, breezy day.
WBGT vs. Heat Index
The heat index, which is commonly reported in weather forecasts, combines air temperature and relative humidity and is calculated for shady conditions. WBGT adds wind and radiant heat, so it better reflects conditions for workers in direct sun or near hot equipment. The National Weather Service notes this difference: heat index considers only temperature and humidity for shady areas, while WBGT accounts for sun angle, cloud cover and wind.
Heat index is still useful because it is widely available. A CDC review of outdoor heat-related illnesses from 2011 to 2016 found that WBGT-based occupational exposure limits were exceeded in all 14 fatalities studied, while the heat index was below 91°F in 12 of the 25 cases reviewed. The authors suggested that a heat index of 85°F could serve as a screening threshold when WBGT is not available.
How WBGT Is Used to Set Heat Limits
WBGT alone does not tell you whether work is safe. Heat stress depends on both environmental heat and the metabolic heat produced by the work itself, as well as clothing and the worker's acclimatization. Occupational limits therefore combine WBGT with workload.
- NIOSH: NIOSH's 2016 criteria document on occupational exposure to heat and hot environments sets a Recommended Alert Limit (RAL) for unacclimatized workers and a Recommended Exposure Limit (REL) for acclimatized workers. Both are expressed as WBGT values that decrease as workload increases.
- ACGIH: the American Conference of Governmental Industrial Hygienists publishes a Threshold Limit Value (TLV) for acclimatized workers and an Action Limit for unacclimatized workers, also based on WBGT and work intensity. The CDC report notes that the NIOSH limits are consistent with those of ACGIH.
Both approaches also call for adjustments when workers wear clothing that limits heat loss, such as coveralls or chemical protective suits, and lower limits for workers with personal risk factors.
WBGT and OSHA
No Specific Federal Heat Standard Yet
Federal OSHA does not currently have a specific standard for heat. OSHA enforces heat hazards under the General Duty Clause, Section 5(a)(1) of the OSH Act, which requires employers to keep the workplace free from recognized hazards that are causing or likely to cause death or serious physical harm.
Proposed Federal Heat Rule
On August 30, 2024, OSHA published a proposed rule, Heat Injury and Illness Prevention in Outdoor and Indoor Work Settings. The proposal would use an initial heat trigger of a heat index of 80°F and a high heat trigger of 90°F, with equivalent WBGT values allowed as an alternative. OSHA held an informal public hearing that concluded in July 2025, and the post-hearing comment period closed on October 30, 2025. As of late 2026, the rule had not been finalized. Check OSHA's heat rulemaking page for the latest status.
Heat National Emphasis Program
OSHA first launched its National Emphasis Program (NEP) on outdoor and indoor heat-related hazards on April 8, 2022. On April 10, 2026, OSHA issued a revised heat NEP (CPL 03-00-024) that remains in effect for five years. Under the NEP, OSHA conducts proactive heat inspections in targeted high-risk industries, and on heat priority days, when the heat index is 80°F or higher, inspectors are directed to ask whether employers have a heat-related hazard prevention program.
State Requirements
Several states with their own OSHA-approved plans, such as California, Washington, Oregon, Minnesota and Maryland, have their own heat-related rules, and some use WBGT or heat index thresholds. Employers should check the rules in each state where they operate.
How to Measure and Use WBGT Step by Step
- Get the right instrument. Use a WBGT meter that measures natural wet bulb, globe and dry bulb temperatures, or use a reliable WBGT forecast for outdoor work as a planning tool.
- Measure where people work. Take readings at the work location and at about the height of the worker's torso, not in an air-conditioned office or a shaded parking lot.
- Allow time to stabilize. Follow the manufacturer's instructions so the globe and wet bulb sensors reach equilibrium before recording.
- Estimate workload. Classify tasks as light, moderate, heavy or very heavy work, because limits depend on metabolic heat.
- Compare to limits. Use NIOSH or ACGIH tables, adjusted for clothing and acclimatization, to decide whether added controls are needed.
- Apply controls. Increase rest breaks, provide shade and cool water, schedule heavy work for cooler hours, rotate workers and use cooling equipment as needed.
- Record and repeat. Log readings and actions taken, and re-measure as conditions change during the day.
Protecting Workers in the Heat
- Acclimatize new and returning workers. OSHA notes that nearly half of heat-related worker deaths happen on the first day on the job and over 70 percent during the first week. OSHA recommends that new workers work no more than 20 percent of the normal duration on the first day and increase by 20 percent each day.
- Provide water, rest and shade. Make cool drinking water easy to reach and encourage regular breaks in shaded or cool areas.
- Train workers and supervisors. Teach the signs of heat illness and what to do, including calling for emergency help for suspected heat stroke.
- Use a buddy system. Have workers watch each other for symptoms, especially in high heat.
- Plan for emergencies. Include heat illness response in your emergency action plan and make sure workers know how to get first aid and medical care.
Examples
- A roofing contractor checks a WBGT forecast each morning and moves heavy tear-off work to early hours when afternoon WBGT is expected to exceed its threshold.
- A foundry measures WBGT near furnaces and finds that radiant heat drives readings far higher than air temperature suggests, so it adds reflective shielding and more frequent rest breaks.
- A landscaping company uses a modified schedule for new hires during their first week to allow acclimatization.
How SMS360 Helps
The SMS360 Risk Assessment module helps teams assess heat stress for specific tasks and locations, document controls such as work and rest schedules, acclimatization plans and cooling measures, and assign follow-up actions. Linking heat assessments to industrial hygiene data and incident records helps safety teams see where heat risks are highest and whether controls are working.
Frequently Asked Questions
What is a dangerous WBGT level?
There is no single dangerous WBGT value for all work. Safe limits depend on workload, clothing and whether workers are acclimatized. NIOSH and ACGIH publish WBGT-based limits that fall as work becomes more strenuous, with lower limits for unacclimatized workers and for workers wearing heavy or impermeable clothing.
Is WBGT better than heat index?
For workers in direct sun or near radiant heat sources, WBGT is generally a better measure because it includes wind and radiant heat, while heat index considers only temperature and humidity in the shade. Heat index can still be useful as a screening tool when WBGT is not available.
Does OSHA have a WBGT limit?
Federal OSHA does not currently have a heat standard with a WBGT limit. OSHA enforces heat hazards under the General Duty Clause and through its heat National Emphasis Program. A proposed federal heat rule published in August 2024 had not been finalized as of late 2026.
How is WBGT measured?
WBGT is measured with an instrument that records natural wet bulb temperature, black globe temperature and dry bulb air temperature. The readings are combined using a weighted formula: outdoors with sun, 0.7 times the natural wet bulb plus 0.2 times the globe plus 0.1 times the dry bulb temperature.
