OSHA's heat illness prevention framework references both heat index and wet bulb globe temperature (WBGT), but applies them differently depending on work setting, exposure type, and the regulatory document in question. Understanding which metric carries compliance weight, and when, is not an academic distinction. It determines whether your facility's heat response plan is calibrated to what enforcement and injury prevention guidance actually demands.
This article covers all three metrics, how they differ mechanically, when they produce materially different readings, and what OSHA's current guidance and 2024 proposed rulemaking say about each. Also check out our free heat index calculator for specific information on your environment.
Key Takeaways
- Heat index uses only air temperature and relative humidity (RH) as inputs, and assumes shaded conditions, light wind, and a resting adult, conditions that do not reflect most industrial or physical labor environments.
- Wet bulb globe temperature incorporates four variables: air temperature, humidity, solar radiation via a black globe thermometer, and wind speed, weighted so that humidity carries 70% of the composite value.
- Wet bulb temperature and wet bulb globe temperature are not the same measurement. Conflating them is a measurement error with real safety consequences.
- OSHA's proposed Heat Injury and Illness Prevention rule, published in August 2024, uses heat index as its primary trigger metric at 80°F and 90°F thresholds, while National Institute for Occupational Safety and Health (NIOSH) heat stress guidelines recommend WBGT-based exposure limits for physical labor under radiant conditions.
- In indoor facilities with high radiant heat from furnaces, ovens, or industrial equipment, heat index will systematically underestimate worker heat stress because it ignores radiant load entirely.
- Relative humidity is the dominant variable in both metrics. In enclosed facilities, the humidity level the facility maintains directly affects where both readings land.
What Each Metric Actually Measures
All three terms appear in heat safety guidance, sometimes within the same document. They are not interchangeable, and treating them as rough synonyms creates measurement error that can leave workers exposed above actual safety thresholds.
Heat Index: Inputs, Assumptions, and Limitations
Heat index is a NOAA-derived apparent temperature that combines air temperature and relative humidity into a single value representing how hot conditions feel to a human body. The National Weather Service heat index calculation was developed under a specific set of assumptions: a shaded environment, light wind around 5.8 mph, and a resting or lightly active adult.
Those assumptions matter because they are the source of heat index's core limitation in occupational contexts. The moment a worker is performing physical labor, standing in direct sun, or working near hot equipment, those baseline conditions no longer hold. Heat index will underestimate actual heat stress in proportion to how far real conditions deviate from those assumptions.
Wet Bulb Temperature vs. Wet Bulb Globe Temperature: Not the Same Metric
Psychrometric wet bulb temperature is the temperature a thermometer reads when its sensing element is covered by a wet wick and exposed to airflow. It measures evaporative cooling potential only. It does not account for radiant heat, direct solar load, or wind speed as a comfort variable.
Wet bulb temperature is used in heating, ventilation, and air conditioning (HVAC) engineering and dew point calculations. It is not a standalone heat stress metric for occupational outdoor or industrial settings. Wet bulb globe temperature (WBGT) is a four-variable composite that includes radiant heat and is a fundamentally different measurement. Using the two terms interchangeably in a safety plan is an error.
WBGT: Formula, Components, and What the Weightings Mean
WBGT was developed in the 1950s by the U.S. military to reduce heat casualties during training. The formula is: WBGT = 0.7 × natural wet bulb temperature + 0.2 × black globe temperature + 0.1 × dry bulb temperature.
The 70% weighting on natural wet bulb temperature reflects a physiological fact: sweat evaporation is the body's primary heat dissipation mechanism, so atmospheric moisture has the greatest influence on physiological heat strain. The globe thermometer's 20% weight captures radiant heat from the sun or from hot surfaces. Dry bulb temperature, the standard air temperature reading, carries only 10% of the composite value.
When Heat Index and WBGT Diverge, and Why It Matters
Readers researching these metrics already know the two numbers can differ. What they need is a clear account of when the divergence is large enough to affect safety decisions, and what mechanism drives it in each scenario.
Outdoor Sun vs. Shade: Where Heat Index Underestimates Most
Solar radiation is the primary driver of divergence between heat index and WBGT in outdoor settings. Under direct sun, a black globe thermometer can read 10 to 20°F above ambient air temperature. That radiant load flows directly into the WBGT calculation at 20% weight, pushing WBGT materially above what heat index shows.
On a 95°F day at 60% RH with full solar exposure and light wind, WBGT may cross NIOSH action thresholds for moderate or heavy work while heat index registers in a lower risk category. A heat response plan calibrated to heat index alone may not trigger rest breaks or workload reductions at the point when WBGT indicates it is physiologically necessary.
In full shade with low wind, the two metrics converge most closely. That is the only outdoor condition where heat index is a reasonable safety proxy, and even then its limitations during physical exertion persist.
Indoor Radiant Heat Environments: Why Heat Index Fails in Facilities
Indoor environments eliminate solar radiation but introduce a different radiant source: hot equipment surfaces. Foundries, commercial kitchens, steel mills, and warehouses with furnace equipment can generate intense radiant heat loads with no direct sunlight involved.
Heat index ignores all radiant heat, regardless of source. A facility relying solely on heat index in these environments will systematically underestimate heat stress. The appropriate metric is WBGT calculated with indoor weighting, which removes the solar component. This indoor WBGT approach is specified in ISO 7243:2017. Relative humidity remains the single largest variable in both indoor and outdoor WBGT, meaning the humidity level your facility maintains is a direct input to where the metric lands.
What OSHA Actually References, and What the Proposed Rule Says
This is the question driving most of the searches that land on this topic. The answer is more specific than most guidance documents make clear, and the distinction between OSHA's enforcement posture and NIOSH's occupational health recommendations has real consequences for how you structure a heat illness prevention plan.
OSHA's Heat Index Action Levels Under Current Guidance
OSHA's current Heat Illness Prevention campaign references four National Weather Service heat index risk categories and ties recommended employer actions to each. The categories are: Caution (below 91°F), Extreme Caution (91 to 103°F), Danger (103 to 124°F), and Extreme Danger (above 125°F). At higher categories, OSHA's guidance calls for increased monitoring, rest breaks, shade access, and emergency protocols.
This guidance is not a mandatory standard under a specific Code of Federal Regulations (CFR) section. Enforcement occurs under the General Duty Clause of the Occupational Safety and Health Act, which requires employers to protect workers from recognized hazards. Heat index is the metric OSHA's existing campaign uses for hazard recognition, which is why it appears in most employer-facing safety communications.
NIOSH WBGT Thresholds: The More Stringent Occupational Standard
The National Institute for Occupational Safety and Health (NIOSH) recommends WBGT-based exposure limits in its Criteria for a Recommended Standard: Occupational Exposure to Heat and Hot Environments, published in 2016. NIOSH recommends WBGT rather than heat index specifically because heat index does not account for radiant load or wind speed, both of which significantly affect body core temperature and physiological heat strain during physical work.
NIOSH WBGT thresholds by work intensity are:
- Light work: WBGT limit of 86°F (30°C) for unacclimatized workers
- Moderate work: WBGT limit of 82°F (28°C) for unacclimatized workers
- Heavy work: WBGT limit of 79°F (26°C) for unacclimatized workers
- Very heavy work: WBGT limit of 77°F (25°C) for unacclimatized workers
Acclimatized worker limits are higher. These NIOSH heat stress guidelines represent the more rigorous occupational standard. For industrial facilities with physical labor and radiant heat sources, aligning to NIOSH thresholds may be the more defensible compliance posture, even when OSHA's enforcement framework is built around heat index.
The 2024 OSHA Proposed Rule: Heat Index as the Regulatory Trigger
OSHA published a proposed Heat Injury and Illness Prevention in Outdoor and Indoor Work Settings rule in the Federal Register in August 2024. This is a proposed rule, not a final standard. Its requirements may change before any final version is issued, and safety managers should monitor OSHA's rulemaking updates directly.
As proposed, the rule establishes an initial heat trigger at a heat index of 80°F and a high heat trigger at 90°F. At the initial trigger, employers would be required to provide water, rest, and shade or a cool rest area. At the high heat trigger, additional requirements apply, including observation for signs of heat exhaustion and heat stroke, buddy systems, and emergency response procedures. The rule uses heat index, not WBGT, as its primary threshold metric.
OSHA's stated rationale is that heat index is measurable without specialized equipment, making it more enforceable across the full range of employer types and facility sizes.
Which Metric to Use and When: A Practical Framework for Facility Managers
The practical question is which metric your facility should use for monitoring, documentation, and response planning. The answer depends on your work setting, the nature of physical activity, and whether radiant heat sources are present.
Outdoor direct sun with physical exertion (construction, agriculture, utility work)
- Recommended metric: WBGT measured with a full instrument set (natural wet bulb, black globe thermometer, and dry bulb thermometer)
- Why heat index is insufficient: Solar radiation is not captured in heat index; divergence from WBGT is greatest under direct sun and exertion
- Applicable standard: NIOSH 2016 Criteria Document; American College of Sports Medicine position stands on exertional heat illness
Outdoor shade or overcast with moderate exertion
- Recommended metric: Heat index is an acceptable proxy when WBGT instruments are unavailable
- Limitation to document: Heat index still does not account for exertion-driven increases in physiological heat strain; note this limitation explicitly in your heat plan
- Applicable standard: OSHA Heat Illness Prevention campaign guidance
Indoor facilities with physical labor and radiant heat (foundries, kitchens, industrial plants)
- Recommended metric: Indoor WBGT per ISO 7243:2017, which removes the solar component from the calculation
- Why heat index is insufficient: Heat index ignores all radiant heat sources; it will underestimate stress from furnaces, ovens, and hot equipment surfaces
- Applicable standard: ISO 7243:2017; NIOSH 2016 Criteria Document
Indoor office, data center, or light-activity facilities
- Recommended metric: Heat index is generally adequate; radiant load and exertion are low
- Why WBGT is unnecessary: Without significant radiant heat sources or physical exertion, the additional variables WBGT captures add limited value
- Applicable standard: OSHA Heat Illness Prevention campaign guidance
Measuring WBGT Without a Black Globe Thermometer
A full WBGT instrument set costs several hundred dollars and is not universally available. Validated mathematical models can estimate WBGT from standard meteorological inputs including air temperature, relative humidity, solar radiation, and wind speed. Sports medicine researchers, including in ACSM position stands on exertional heat illness, have relied on this kind of estimate when a full instrument set isn't available in the field.
Estimated WBGT is a reasonable starting point for planning and monitoring. Measured WBGT from a calibrated instrument is preferable for formal compliance documentation and incident defense. If your facility must demonstrate due diligence in a post-incident review, measured data is more defensible than an estimated figure derived from a weather station feed.
How Humidity Control Affects Heat Stress Metrics in Industrial Facilities
For facility managers, the connection between humidity and heat stress metrics is not intuitive until you look at the underlying formulas. Once you do, the relationship is direct and significant.
Relative Humidity as the Dominant Variable in Both Metrics
WBGT weights natural wet bulb temperature at 70%, and that term is driven almost entirely by how much moisture is in the air. Heat index, the metric OSHA's proposed rule uses as its regulatory trigger, takes only two inputs: air temperature and relative humidity. In both cases, RH is the variable with the greatest influence on the measured result.
For more on the mechanics of how moisture in the air behaves, see our guides on relative humidity and how humidity is measured. Understanding dew point vs humidity also clarifies why RH fluctuations at a fixed temperature shift both metrics.
A facility running at 75°F and 65% RH will produce materially higher heat index and WBGT readings than the same facility at 75°F and 45% RH. In an enclosed industrial space, the humidity level the facility maintains is therefore not just a process variable: it is a direct input to occupational safety and health calculations.
Precision Humidity Control as an Environmental Management Tool
Precision humidity control systems that maintain RH within a known, stable range make humidity a controlled input rather than a fluctuating condition your safety plan cannot account for. When a facility's industrial humidification systems hold RH within plus or minus 1 to 2% continuously, the humidity component of both heat index and WBGT is predictable at any given time.
Smart Fog's precision humidification systems are designed to maintain this level of stability across enclosed industrial environments. For facilities documenting heat stress monitoring for OSHA compliance or NIOSH threshold alignment, knowing the RH with precision supports more accurate metric calculations and more defensible safety documentation. This is not a substitute for a heat illness prevention plan. It means that one of the two inputs in heat index, and the dominant variable in WBGT, is a known quantity rather than an estimate.
Facilities with specialized occupational safety and health requirements, including those operating under humidity control in defense and military facilities protocols or healthcare facility humidification standards, often require both precision humidity control and documented environmental monitoring. Stable RH supports both.
Final Thoughts
The core answer to the compliance question is this: OSHA's enforcement and proposed rulemaking use heat index because it requires no specialized equipment. NIOSH recommends WBGT because it is the more accurate metric for physical labor under real industrial conditions. Both frameworks are active simultaneously, and neither fully replaces the other.
For outdoor direct sun and indoor radiant heat environments, WBGT is the more rigorous and more appropriate metric. For facilities that cannot instrument WBGT, understanding the conditions under which heat index underestimates stress is essential for building a defensible heat plan. Relative humidity is the variable with the greatest influence on both metrics, which means the humidity level your facility maintains has a direct bearing on where both readings land.
If your facility manages heat stress compliance in an enclosed industrial environment and you need humidity to be a controlled, documented variable, contact Smart Fog engineers to discuss precision humidity control for your specific facility type and heat monitoring requirements.
FAQ
What is the difference between wet bulb globe temperature and heat index?
Heat index is a two-variable apparent temperature based on air temperature and relative humidity only. Wet bulb globe temperature (WBGT) is a four-variable composite that adds solar radiation via a black globe thermometer and wind speed, with humidity carrying 70% of the calculated value. WBGT is the more complete measure of heat stress on a physically working body, particularly under direct sun or near radiant heat sources that heat index cannot capture.
Does OSHA require employers to use WBGT or heat index for heat illness prevention?
OSHA's existing Heat Illness Prevention campaign guidance uses heat index, with action levels tied to National Weather Service categories. OSHA's proposed Heat Injury and Illness Prevention rule, published in August 2024, also uses heat index as its primary trigger metric at 80°F and 90°F thresholds. NIOSH, a separate federal agency, recommends WBGT-based exposure limits for occupational heat stress, particularly for physical labor and radiant heat environments. Neither WBGT nor heat index is currently required under a specific mandatory CFR standard.
Can heat index be used instead of WBGT for OSHA compliance?
Heat index aligns with OSHA's current enforcement posture and the framework of the 2024 proposed rule, so using it is consistent with OSHA's guidance. However, heat index underestimates heat stress in settings with direct solar exposure, high radiant heat from equipment, or heavy physical exertion. For those environments, NIOSH recommends WBGT because it accounts for the variables that drive physiological heat strain in real work conditions. Using heat index alone in high-radiant or physically demanding settings may meet OSHA's stated guidance while falling short of NIOSH's more rigorous occupational standard.
What WBGT level is considered dangerous for outdoor workers?
According to NIOSH's 2016 Criteria for a Recommended Standard, WBGT exposure limits for unacclimatized workers range from 77°F (25°C) for very heavy work to 86°F (30°C) for light work. Above these thresholds, heat exhaustion and heat stroke risk increases significantly. Acclimatized workers have higher limits. These thresholds vary by work intensity, so your heat plan should specify which category applies to each task or worker group.
Why does WBGT include solar radiation but heat index does not?
Heat index was developed by NOAA as a public weather communication tool for general outdoor conditions, not for occupational safety during physical labor. Solar radiation significantly increases the heat load on a working body by adding radiant energy on top of ambient temperature and humidity. WBGT was developed specifically for occupational and athletic contexts where workers are exposed to direct sun or hot surfaces. Including solar radiation via the black globe thermometer makes WBGT the more physiologically accurate measure for those settings.
How do you measure WBGT without a black globe thermometer?
Validated mathematical models can estimate WBGT from standard meteorological data including air temperature, relative humidity, solar radiation, and wind speed. The American College of Sports Medicine has referenced these estimation approaches in position stands on exertional heat illness. Estimated WBGT is a reasonable planning tool but is less defensible than measured WBGT from a calibrated instrument set for formal compliance documentation or post-incident review.
What are the NIOSH recommended WBGT exposure limits by work intensity?
NIOSH's 2016 Criteria Document recommends the following WBGT limits for unacclimatized workers: light work at 86°F (30°C), moderate work at 82°F (28°C), heavy work at 79°F (26°C), and very heavy work at 77°F (25°C). Work intensity is defined by metabolic rate. Tasks involving sustained lifting, carrying, or vigorous movement fall in the moderate to heavy categories for most industrial workers.
How does indoor humidity affect a facility's heat index and WBGT readings?
Relative humidity is the dominant input in both metrics. Heat index uses only air temperature and RH as its two inputs. Natural wet bulb temperature, which carries 70% of the WBGT calculation, is primarily driven by atmospheric moisture. In an enclosed facility, the RH level you maintain directly determines where both metrics land. A facility at 75°F and 65% RH will produce materially higher heat index and WBGT readings than the same facility at 75°F and 45% RH, which has direct implications for whether OSHA action levels or NIOSH thresholds are triggered.
