Should you use WBGT or the heat index for workplace heat stress?

Short answer: WBGT — wet bulb globe temperature — is the occupational instrument: it reads all four environmental drivers of heat stress (air temperature, humidity, radiant heat, and air movement), it is what NIOSH, ACGIH, and the U.S. military base their exposure limits on, and OSHA calls on-site WBGT measurement the accurate approach, with heat index "a less desirable substitute." The heat index — temperature plus humidity, computed for shade at rest — is a legitimate screening tool for many outdoor settings without added radiant sources. The two are different quantities on different scales: there is no conversion between them, and a phone weather reading is neither one.

WBGT vs heat index: which should you use for workplace heat stress? (2026)

Two numbers dominate workplace heat conversations, and they are routinely confused: the heat index that weather apps report, and the WBGT that occupational limits are actually written against. Confusing them is not cosmetic — a crew "under the advisory threshold" by heat index can be far over the NIOSH limit by WBGT when sun, a furnace, heavy work, or coveralls enter the picture, because the heat index cannot see any of those. This guide is for the safety manager or supervisor deciding what to measure and what to do with the number: what each metric actually contains, why radiant heat and workload move the danger without moving a thermometer, the clothing adjustments that turn measured WBGT into effective WBGT, and where each tool is — and is not — defensible.

Why this matters.
OSHA's heat hazard recognition guidance is direct: workplace environmental heat should be measured on-site with WBGT meters, heat index is "a less desirable substitute," and heat-related illnesses occur at heat index values below the public heat-advisory thresholds. The NIOSH/OSHA Heat App itself is labeled a screening tool that "does not replace a more accurate WBGT-based hazard assessment which is the core tool used by occupational health professionals." Crews have been hurt inside the gap between those two numbers.

What the heat index measures

The heat index is a National Weather Service construct: it combines air temperature and relative humidity into a "feels like" value representing conditions in the shade, at rest. High humidity suppresses evaporative cooling — sweat that cannot evaporate cools nothing — so the index climbs steeply as moisture rises. Those are its entire contents. By design it does not account for:

• Sunlight or any radiant source — full sun alone can add roughly 15°F to how conditions load the body;
• Wind — which in most situations helps workers shed heat;
• Workload — the metabolic heat of the task, which is the dominant driver in most occupational cases;
• Clothing — coveralls and vapor barriers that block the evaporation the index assumes.

Used honestly — as a screening number for outdoor work without significant added radiant load — it earns its place: it is free, instant, and the basis of the NIOSH/OSHA Heat Safety Tool app and our own heat stress calculator, which maps temperature and humidity to the OSHA-NIOSH risk bands with the actions each band expects.

What WBGT measures

Wet bulb globe temperature is an instrument reading, not a formula on two weather numbers. A WBGT meter carries three thermometers and blends them into one value:

Because the instrument reads the actual environment, placement is part of the method: OSHA's guidance says to put the WBGT meter close to the work — in the sun if the work is in the sun — and to follow the manufacturer's instructions for setup, calibration, and settling time. NIOSH, ACGIH, the U.S. military, and major athletic organizations all specify WBGT for heat-stress assessment, and the NIOSH Recommended Exposure Limits are written in WBGT degrees, not heat index.

Why radiant heat matters, and why a phone reading is not a measurement

Radiant heat loads the body directly without warming the air in between: sun, furnaces, ovens, molten material, engines, asphalt, and sun-baked metal all pour energy onto workers that a dry thermometer never registers. That is exactly the term the black globe exists to capture — and exactly what the heat index formula assumes away by defining itself in the shade. Indoors, radiant equipment is often the hazard: a bakery, foundry bay, or warehouse under a hot metal roof can be dangerous at an air temperature that sounds tame.

The related trap is outsourcing the number to a weather app. OSHA's guidance is specific about why worksite conditions diverge from the weather station generating that reading: the station may be miles away — error grows with distance — and site features create local heat the station cannot see: still air in trenches and bowl-shaped structures, reflective water and metal bouncing sun onto crews, dark surfaces re-radiating all afternoon. A phone forecast is a planning input for tomorrow; the measurement OSHA describes is an on-site WBGT reading where the crew is actually standing today.

Metabolic workload and the NIOSH limits

Most heat illness strikes people doing strenuous work — the body's own metabolic heat stacks on top of the environment. That is why the occupational limits are written as a matrix of WBGT against workload, not a single cutoff. OSHA's guidance publishes a simplified screen and the workload table adapted from NIOSH's 2016 criteria — the unacclimatized column is NIOSH's action-limit territory (its Recommended Alert Limit), the acclimatized column its Recommended Exposure Limit, which ACGIH's TLV mirrors:

Read the direction of the table: heavy work drops the allowable WBGT into the 70s°F, and a brand-new worker digging in 75°F WBGT conditions is already past the alert line — numbers a heat-index screen would wave through. OSHA's simplified screen says the same thing in one row: above 77°F effective WBGT, strenuous work is high-risk for unacclimatized workers and "possibly unsafe" even for acclimatized ones. The acclimatization ramps that separate those two columns are laid out in how to work safely in extreme heat.

Clothing adjustment factors: measured WBGT vs effective WBGT

PPE that blocks evaporation makes the environment effectively hotter than any instrument reads. The occupational method handles this with clothing adjustment factors added to the measured WBGT, producing the effective WBGT the limit tables are entered with — values OSHA adapts from NIOSH guidelines:

The last row is the one that rewrites job plans: a vapor-barrier suit turns a mild 72°F-WBGT day into an over-the-limit environment for almost any sustained work. Which suit fabric is which is decoded in disposable coverall types explained, the physiological-burden side (including respirators) in respirator medical evaluation requirements, and offsetting gear — phase-change vests that cool inside sealed suits where evaporative gear cannot — in the cooling gear collection.

When the heat index is reasonable, and when WBGT is the answer

• Heat index is a reasonable screen when: the work is outdoors without significant added radiant sources beyond the sun you can adjust for, workload is light to moderate, clothing is ordinary, and you treat it as a trigger for precautions rather than a clearance. NIOSH frames its app exactly this way — useful for many outdoor environments, while recommending WBGT-based limits where available.
• WBGT is the method when: work is indoors near radiant equipment, crews are in coveralls or vapor barriers, workload is heavy, the site has trenches, reflective surfaces, or dead air, or you are running a program against the NIOSH/ACGIH limits — which only accept WBGT as input.
• Never bridge them arithmetically. They are different quantities: one is a shade-at-rest comfort construct, the other a four-factor instrument reading. No defensible formula converts a heat index into a WBGT, and this site's heat stress calculator deliberately stays on the heat-index side and says so.

Worked example: choosing the method for a warehouse and a paving crew

Two jobs, same 88°F August afternoon, opposite answers:

1. Define each exposure. Job A: order pickers in a non-air-conditioned warehouse under a metal roof, moderate workload, ordinary clothing. Job B: a paving crew over fresh asphalt in full sun, heavy workload, high-vis over cotton.
2. Screen with the heat index — once. Temperature and humidity put the day at a heat index near 93°F: the moderate band on our heat stress calculator. That legitimately starts the water-and-rest cadence for both crews — and it is the last thing the heat index can contribute, because neither job matches its shade-at-rest assumptions.
3. Measure Job A where the pickers work. A WBGT meter at head height in the hot aisle — not at the shipping office — captures the roof's radiant load and the dead air between racks. Reading: 79°F WBGT. Against the table, moderate work is over the unacclimatized limit and brushing the acclimatized one: rotate aisles, add airflow, and hold the new hire to the ramp schedule.
4. Measure Job B in the sun over the mat. The globe thermometer drinks in what the crew feels: 87°F WBGT over the asphalt. Heavy work is far past both columns — this is aggressive work-rest territory with soaked evaporative vests like the Ergodyne Chill-Its 6665 evaporative cooling vest, shaded recovery, and the heaviest raking shifted to morning.
5. Apply clothing factors where they bite. If Job A had been a tank-cleaning task in vapor-barrier coveralls, the +19.8°F adjustment would put even a 70°F-WBGT reading over every limit in the table — the day pivots to a phase-change vest under the suit, like the Ergodyne Chill-Its 6260 phase change cooling vest, and short timed entries.
6. Log the readings and re-measure as conditions move. Sun angle, humidity, and workload all drift through a shift; the number that governs is the current one, taken where the work is.

Frequently asked questions

What is the difference between WBGT and heat index?

The heat index combines air temperature and relative humidity into a "feels like" value defined for shade at rest — it sees two of the four heat factors. WBGT is an instrument reading from three sensors that captures all four: temperature, humidity, radiant heat, and air movement. Occupational exposure limits from NIOSH and ACGIH are written in WBGT; the heat index is a screening tool.

Can you convert heat index to WBGT?

No. They are different physical quantities on different scales, and no formula defensibly converts one into the other — a heat index of 90°F tells you nothing certain about WBGT because radiant load and wind are missing from it entirely. If a decision requires a WBGT value, the answer is a WBGT measurement at the work location.

Is a WBGT of 90 the same as a heat index of 90?

Not remotely — a 90°F WBGT is a vastly more dangerous environment than a 90°F heat index. The NIOSH-based limits top out below 90°F WBGT even for light work by acclimatized workers, while a 90°F heat index sits in the moderate screening band. Treating the scales as interchangeable is the classic way to underestimate a hot job.

Can I use my phone's weather app to judge worksite heat?

Only as a planning screen. The reading comes from a weather station that may be miles away — error grows with distance — measured in shade over grass, not over your asphalt, inside your trench, or beside your oven. OSHA's guidance calls for on-site measurement, and lists exactly the site features (still air, reflective surfaces, local heat sources) that make remote readings wrong where it matters most.

Do I need an actual WBGT meter?

For a program judged against the NIOSH or ACGIH limits, or any regular work indoors near radiant sources, in heavy PPE, or at heavy workloads — yes: the limits only accept WBGT, and only an instrument produces one. Follow the manufacturer's setup, calibration, and settling instructions, and place it where the work happens, in the sun if the work is in the sun. For light outdoor work in ordinary clothing, a heat-index screen with conservative triggers is a defensible starting layer.

What is effective WBGT?

The measured WBGT plus a clothing adjustment factor — the number you actually enter the limit tables with. Ordinary work clothing and cloth coveralls add nothing; SMS coveralls add about 0.9°F; polyolefin about 1.8°F; double-layer clothing about 5.4°F; and limited-use vapor-barrier coveralls add roughly 19.8°F, which is why sealed-suit work overwhelms mild days.

What does the heat index miss indoors?

Almost everything that makes indoor heat dangerous: radiant load from ovens, furnaces, machinery, and hot roofs, and the dead air that blocks evaporative cooling. Indoor heat assessment is WBGT territory — and it is inside the scope of OSHA's heat enforcement, as the indoor half of the National Emphasis Program described in OSHA workplace heat requirements.

How much does full sun change the picture?

Full sun can add roughly 15°F to how conditions load the body compared with the shade value the heat index reports — which is why screening tools tell you to add for sun, and why the WBGT method solves the problem physically instead: put the globe in the sun the workers are standing in and measure it.

What is the NIOSH REL and RAL for heat?

NIOSH's Recommended Exposure Limit (REL) is the effective-WBGT ceiling for acclimatized workers at a given workload, and its Recommended Alert Limit (RAL) is the lower ceiling for unacclimatized workers — the two columns of the limit table, which ACGIH's TLV framework mirrors. At moderate work they sit near 82.4°F and 77°F effective WBGT respectively, dropping as workload rises.

What counts as light, moderate, or heavy workload?

Roughly: light is sitting or standing tasks with minimal walking; moderate is steady walking, pushing, or lifting at ordinary pace; heavy and very heavy are sustained digging, carrying, climbing, or fast-paced manual work. When a task straddles categories, OSHA's guidance says to assume the higher workload — the limits move a full risk band on that call.

Does OSHA require WBGT monitoring?

No current federal standard mandates a measurement method — but OSHA's hazard-recognition guidance recommends on-site WBGT as the accurate assessment, names heat index a less desirable substitute, and its inspectors evaluate heat programs against that guidance under the General Duty Clause. The enforceable-versus-recommended line is drawn in full in OSHA workplace heat requirements.

What does the NIOSH/OSHA Heat App actually use?

The heat index — and its own documentation labels it a screening tool that does not replace a WBGT-based hazard assessment, "the core tool used by occupational health professionals." The same is true of this site's heat stress calculator: temperature and humidity in, heat-index band and actions out, with WBGT explicitly out of its scope.

Why does heat illness happen below the heat-advisory threshold?

Because public advisories describe the general population at rest in shade, and workers are none of those things: they generate metabolic heat, stand in sun or beside radiant equipment, and wear clothing that traps heat. OSHA's guidance warns employers that heat illness risk exists below the advisory values — the entire reason occupational assessment uses workload-adjusted WBGT limits instead of waiting for the weather service.

Where should the WBGT meter be placed?

Close to the work location, exposed to what the workers are exposed to — in the sun if they work in the sun, at the hot aisle rather than the office, allowed its settling time per the manufacturer's instructions. A WBGT reading inherits the honesty of its placement; measuring the comfortable corner of the site produces a comfortable, useless number.

What symptoms should trigger action regardless of any reading?

All of them. Measurements set the plan; symptoms override it. A worker showing confusion, slurred speech, collapse, or seizure gets 911 and aggressive cooling no matter what any instrument says — the recognition ladder and first-aid steps are the whole of heat exhaustion vs heat stroke.

Further reading on this site

• OSHA workplace heat requirements — what is enforceable now versus proposed, and where measurement fits in the expected program.
• Heat exhaustion vs heat stroke — the recognition and 911 rules that override any reading.
• How to work safely in extreme heat — the field routine the measurements feed: water, rest, shade, acclimatization, and cooling gear.
• Heat stress calculator — the heat-index screening bands, kept honestly on the screening side of this page's line.
• Cooling gear — evaporative and phase-change equipment matched to the conditions the measurements reveal.
• Best cooling gear — the ranked buyer's guide, including which technology works in humidity and inside suits.
• Disposable coverall types explained — the suit fabrics behind the clothing adjustment factors.