Wet-Bulb Globe Temperature and Human Heat Limits

What WBGT actually is

The Wet-Bulb Globe Temperature is a single number that combines four aspects of a hot environment: air temperature, humidity, air movement, and radiant heat (from the sun or hot surfaces). It uses three thermometers: a shaded dry-bulb thermometer for air temperature, a naturally ventilated wet-bulb thermometer wrapped in a wet wick for humidity and wind, and a black-globe thermometer for radiant heat. For outdoor use, the formula is WBGT = 0.7 × T_nwb + 0.2 × T_g + 0.1 × T_db. Indoors or without solar load the dry-bulb term is dropped and the wet-bulb weight rises to 0.7 with a globe weight of 0.3.

WBGT was invented in the 1950s by Constantin Yaglou and Commander David Minard as part of a successful campaign to reduce heat casualties at US Marine Corps recruit training centers. The Marine Corps program showed that limiting drill and physical conditioning above certain WBGT values sharply reduced heat-related illness while preserving training. WBGT is now used by militaries, occupational safety agencies (NIOSH, OSHA, ISO 7243), and athletic bodies (NCAA, high-school athletic associations) to guide activity in the heat.

The US National Institute for Occupational Safety and Health (NIOSH) publishes recommended WBGT limits by workload and acclimatization status. Recommended Exposure Limits (RELs) apply to workers acclimatized to heat over days to weeks; Recommended Alert Limits (RALs) apply to unacclimatized workers and are lower by about 2-3°C.

The 35°C wet-bulb limit and what really happens

In a 2010 paper in PNAS, Sherwood and Huber argued that heat imposes a thermodynamic upper bound on human survival that is independent of clothing, hydration, or acclimatization. Because skin temperature must stay near 35°C for the body to shed metabolic heat by sweating, a wet-bulb temperature (T_w) above 35°C would eliminate the required temperature gradient and force core temperature to rise. In today's climates, they noted, peak T_w observed anywhere in the world essentially never exceeds 31°C; the 35°C ceiling is not currently reached but would begin to be reached in some regions with about 7°C of global warming, and would spread with 11-12°C.

That theoretical limit was later put to the test. In the Penn State HEAT Project (Vecellio 2022), 24 young, healthy adults aged 18 to 34 performed minimal activities of daily living in an environmental chamber while researchers slowly raised temperature or humidity. Uncompensable heat stress (core temperature rising continuously) began well below 35°C. Averaged across humid conditions the empirical critical wet-bulb temperature was about 30.55°C. In hot-dry conditions the picture was different: dry heat gained from the environment outpaced evaporative cooling, and critical wet-bulb values were even lower (roughly 25-28°C).

In short, the theoretical 35°C is a physics-only ceiling; the practical limit for real human physiology is meaningfully lower, and depends strongly on humidity and workload. The Penn State authors note that older adults, people on certain medications, and those with chronic conditions likely have limits below the young-adult figure. As of the study's publication, wet-bulb temperatures approaching or briefly exceeding the empirical limit had been recorded in South Asia and the Persian Gulf.

The measured, recommended, and theoretical numbers are not the same thing. The chart shows the empirical critical wet-bulb in humid conditions, the NIOSH light-work WBGT limit, and the Sherwood-Huber theoretical wet-bulb ceiling.

WBGT and wet-bulb temperature are not the same

This is one of the most common points of confusion. The Wet-Bulb Globe Temperature is a composite occupational heat index that combines wet-bulb, black-globe, and dry-bulb readings; typical outdoor WBGT values in summer are in the mid-20s to mid-30s in °C. The pure wet-bulb temperature (T_w) is a physical quantity representing the lowest temperature achievable by evaporating water in the air at that humidity; global peak T_w historically maxed out near 31°C. A NIOSH WBGT limit of 30°C is not the same as the 30°C wet-bulb limit in the Penn State study, though the numbers happen to be close.

For everyday interpretation: WBGT tells you when a work or sport environment is unsafe for continued activity. The wet-bulb ceiling tells you when the outdoor environment itself, at rest, has become incompatible with sustained human physiology. Both matter, and both depend on how humid the air is, how well the body can sweat, and how much heat the person is producing internally.

The published limits are population values. Several factors shift an individual's tolerable heat load in either direction.

How to read these numbers

Three cautions keep the figures honest. First, all limits are population values with individual variation; the young-adult empirical wet-bulb limit does not apply directly to older adults or people with chronic conditions. Second, WBGT limits assume continuous work; work-rest cycles allow higher exposures. Third, the theoretical 35°C wet-bulb ceiling is a physics-based extreme, not an everyday operational limit; the practical limit is lower and depends on activity and humidity.

The dependable takeaways are: heat stress is well described by combining temperature, humidity, wind, and radiant load (which is what WBGT does); acclimatized workers can tolerate meaningfully higher WBGT than unacclimatized ones; and current empirical evidence puts the wet-bulb tolerance limit for young, healthy adults performing minimal activity at roughly 30-31°C in humid conditions, below the theoretical 35°C. This page is educational; workplaces and athletic programs should follow the guidance of qualified safety and medical professionals.

Methodology and limitations

This page draws on: the original Marine Corps report and 1957 JAMA article by Yaglou and Minard establishing WBGT; a peer-reviewed history of WBGT and its limitations (Budd 2008); the NIOSH Criteria for a Recommended Standard: Occupational Exposure to Heat and Hot Environments (DHHS/NIOSH Publication No. 2016-106, February 2016) for RELs and RALs; the OSHA Heat Hazard Recognition guidance for a simplified table adapted from NIOSH; the Sherwood and Huber (2010) PNAS paper for the theoretical 35°C wet-bulb ceiling; and the Vecellio et al. (2022) J Appl Physiol paper for the empirical critical wet-bulb temperature in young healthy adults. ISO 7243 is cited for international standardization of WBGT.

Limitations: WBGT is a general environmental index and does not fully capture individual factors such as fitness, hydration, clothing, and health. Empirical wet-bulb tolerance data come from a small (n=24) young, healthy cohort under specific conditions; older adults, medicated individuals, and those with chronic disease likely have lower tolerance. Sherwood and Huber's 35°C limit is a physics-only ceiling and should not be treated as an operational threshold. This page is general educational information, not medical or occupational-safety advice.

Frequently asked questions

What is the Wet-Bulb Globe Temperature (WBGT)?

WBGT is a composite heat-stress index used by workplace and athletic safety agencies. Outdoors, it is calculated as 0.7 × natural wet-bulb + 0.2 × black-globe + 0.1 × dry-bulb temperature, combining humidity, radiant heat, and air temperature. It was developed in the 1950s at Marine Corps Recruit Depot Parris Island to prevent recruit heat casualties.

How is WBGT different from wet-bulb temperature?

WBGT is a weighted average of three thermometer readings and represents heat stress at a work site. Wet-bulb temperature (T_w) is a physical quantity: the lowest temperature achievable by evaporating water in ambient air. WBGT and T_w are numerically different; a NIOSH WBGT limit of 30°C is not the same as a 30°C wet-bulb limit.

What are the NIOSH heat-exposure limits?

For acclimatized workers, NIOSH recommends WBGT ceilings of 30°C for light work, 28°C for moderate, 26°C for heavy, and 25°C for very heavy work. Limits for unacclimatized workers are about 2-3°C lower. These assume continuous work; work-rest cycles allow higher WBGT.

Is the human wet-bulb limit really 35°C?

That is a theoretical ceiling. Sherwood and Huber (2010) argued that a wet-bulb temperature above 35°C makes it thermodynamically impossible to shed metabolic heat. In laboratory experiments with young, healthy adults performing minimal activity, however, uncompensable heat stress began at about 30.55°C wet-bulb in humid conditions and even lower in hot-dry conditions.

How close is the world to the wet-bulb limit today?

Global peak wet-bulb temperatures historically maxed near 31°C. Wet-bulb values briefly approaching or exceeding the empirical young-adult limit of about 31°C have been recorded in parts of South Asia and the Persian Gulf. The theoretical 35°C ceiling has not been reached, but models project it would begin to be crossed in some regions with several degrees more global warming.

What raises or lowers an individual's tolerance?

Acclimatization, fitness, and hydration raise tolerance; higher workload, heavier clothing, older age, chronic cardiovascular, respiratory or kidney disease, and certain medications (anticholinergics, some antidepressants, diuretics) lower it. Published limits are population values, not individual guarantees. This page is educational, not medical or occupational-safety advice.

Sources

Primary peer-reviewed studies and official sources first, then reviews and institutional framing (secondary).

1. Yaglou CP, Minard D. Control of heat casualties at military training centers. AMA Arch Ind Health. 1957;16(4):302-316. Original WBGT description and Marine Corps Recruit Depot Parris Island program. PubMed
2. Minard D. Prevention of Heat Casualties. JAMA. 1957;165(14):1813-1818. Companion Marine Corps report defining WBGT. DTIC report
3. Budd GM. Wet-bulb globe temperature (WBGT): its history and its limitations. J Sci Med Sport. 2008;11(1):20-32. History and interpretation of WBGT. PubMed
4. National Institute for Occupational Safety and Health (NIOSH). Criteria for a Recommended Standard: Occupational Exposure to Heat and Hot Environments. DHHS (NIOSH) Publication No. 2016-106, February 2016. WBGT-based RELs (acclimatized) and RALs (unacclimatized). NIOSH
5. Sherwood SC, Huber M. An adaptability limit to climate change due to heat stress. Proc Natl Acad Sci USA. 2010;107(21):9552-9555. Proposes 35°C wet-bulb as theoretical human ceiling. PNAS
6. Vecellio DJ, Wolf ST, Cottle RM, Kenney WL. Evaluating the 35°C wet-bulb temperature adaptability threshold for young, healthy subjects (PSU HEAT Project). J Appl Physiol. 2022;132(2):340-345. Empirical critical wet-bulb of 30.55°C in humid environments; lower in hot-dry. PubMed

How to cite this page

Related reading on this site

• Climate change and heat-related mortality
• Heat-related illness in the US: key statistics
• Workplace heat and productivity: the evidence
• Heat acclimatization and sweat adaptation
• Maximum human sweat rate
• Dehydration during exercise: the evidence