WBGT vs heat index: pick the right heat reading for your crew

What is the difference between WBGT and heat index?

WBGT counts four heat factors. The heat index counts two. OSHA says WBGT “incorporates temperature, humidity, sunlight, and air movement into a single measurement.” The heat index combines air temperature and relative humidity, measured in the shade, to show how hot it feels at rest.

OSHA’s Technical Manual chapter on heat stress “establishes that OSHA uses WBGT to determine if a heat hazard was present.” That is why safety managers care about the difference.

The next sections show why OSHA ranks WBGT first.

Why does OSHA prefer WBGT?

Because WBGT covers all four main environmental heat factors. OSHA says “Workplace environmental heat should be measured on-site using WBGT meters” and that “Use of heat index is a less desirable substitute.” OSHA also recommends WBGT in its main heat overview as the most accurate way to measure the environmental heat load on the body.

OSHA lists the four factors as air temperature, humidity, radiant heat from sunlight or artificial heat sources, and air movement. A plain thermometer reads only the first one.

The heat index leaves out sun, wind and radiant heat. OSHA says it “does not account for the effects of wind, sunlight, radiant heat sources, or workload.” The NWS adds that heat index values assume shade and light wind, so full sun can add up to 15°F. See our heat index chart guide for a worked example.

How does a WBGT meter work, and where do you put it?

A WBGT meter holds three thermometers. OSHA says WBGT devices contain “a dry bulb thermometer to measure the ambient air temperature,” “a natural wet bulb thermometer to measure the potential for evaporative cooling,” and “a black globe thermometer to measure radiant heat.” The meter combines them into one number.

Place the meter where the crew works. OSHA says the instrument “should be placed close to the work location.” If the work is in direct sunlight, the meter should be in the sun. Follow the maker’s instructions for setup, calibration and use.

1. Put the meter near the crew, not in the truck or the trailer.
2. Put it in sun if the crew works in sun.
3. Read it at set times, and again when conditions change.
4. Write down each reading, the time and the task.
5. Compare the reading to the limit for that workload.

How does OSHA classify workload?

OSHA sorts work into four levels: light, moderate, heavy and very heavy. It says “Workload can be classified as light, moderate, heavy, or very heavy.” The level sets which row of the limits table you read. When in doubt, assume the higher level.

OSHA’s table assumes a 70-kg (154-pound) worker and notes that workers who are overweight or obese might produce more metabolic heat on the same task.

What WBGT limits does OSHA publish?

OSHA publishes two tables adapted from NIOSH and ACGIH guidance. Table 2 gives limits by workload. Limits are lower for workers who are not acclimatized and for heavier work. OSHA calls the first table “a simplified version of recommendations from NIOSH and ACGIH.”

OSHA Table 2: effective WBGT limits (adapted from NIOSH guidelines, 2016)

Source: OSHA, Heat Hazard Recognition page, Table 2.

OSHA also gives a simplified Table 1. At an effective WBGT below 70°F, risk is low for both groups. From 70 to 77°F, strenuous work is possibly unsafe for unacclimatized workers. Above 77°F, risk is high for unacclimatized workers, and strenuous work is possibly unsafe for acclimatized workers.

Use the tables as guidance for your own plan. This guide is not legal advice. OSHA also offers a Heat Stress Calculator on its heat pages. Heavy and very heavy work, such as shoveling, roofing, stacking concrete or masonry, carry the highest risk, so crews doing those tasks need the lowest limits.

How do you use the tables, step by step?

Follow OSHA’s order: workload, measurement, clothing, acclimatization, then the tables. OSHA’s first step is to “estimate the workload as shown above.”

1. Estimate the workload: light, moderate, heavy or very heavy.
2. Measure environmental heat with a WBGT meter or a similar method.
3. Add the clothing adjustment factor to get the effective WBGT.
4. Decide if the worker is acclimatized. OSHA says to assume workers are unacclimatized if they have done the job for less than one to two weeks.
5. Check Table 1. If it shows possible danger, use Table 2 or OSHA’s Heat Stress Calculator.

Here is an example with made-up numbers, not a real reading. A crew shovels gravel, which is heavy work, in cloth coveralls, which add 0. The meter reads 24°C. For heavy work, OSHA’s Table 2 limit is 23°C for unacclimatized workers and 26°C for acclimatized workers. The reading is over the limit for new hires and under it for the veterans. New hires need shorter heat time and more rest, so see our heat acclimatization schedule.

How do clothing and PPE change the number?

They raise the effective WBGT. OSHA says “Effective WBGT equals measured WBGT plus any clothing adjustment factors.” Heavy protective gear traps heat. Add the factor to the measured reading before you compare it to the limit.

Source: OSHA, Heat Hazard Recognition page, adapted from NIOSH (2016).

Is the heat index still useful for a crew?

Yes, as a screen. OSHA says some employers “may find the Heat Index helpful as part of more comprehensive workplace hazard assessment.” The NIOSH/OSHA Heat App uses the heat index as a screening tool. It shows workload guidance, but it does not give WBGT.

OSHA is firm on the limit of that screen: “do not rely solely on the Heat Index reported by weather forecasts for your safety at work, as it may underestimate your actual risk.” Weather stations may sit far from your site, and OSHA says the potential error grows with distance from the weather station.

Good uses of the heat index:

• Planning tomorrow’s schedule from the forecast.
• Spotting advisory days that may bring NEP inspections.
• Giving crews a quick, shared feel for the day.

Weak uses of the heat index:

• Setting break length for heavy work in full sun.
• Clearing a trench, roof or indoor hot spot as safe.
• Deciding that workers are fine because the number looks low.

Where can you get a WBGT forecast?

The National Weather Service publishes a WBGT forecast and a guide. NWS says WBGT “can be used to inform activity modifications during exercise or outdoor work.” NWS lists OSHA’s protective measures for outdoor work, including acclimatization, shade, earlier or later schedules and work/rest schedules.

OSHA also offers tools to estimate WBGT from historical weather data. A forecast helps planning. An on-site meter tells you what the crew feels now. Our heat stress prevention pillar shows where measurement fits in a full program.

How does hydration fit with WBGT readings?

Higher WBGT means more sweat, and sweat carries out water and salt. OSHA’s guidance says to give water, rest and shade, and to offer electrolyte drinks on jobs that run longer than two hours. Rest breaks should be longer as heat stress rises.

JMAR Solutions’ Rapid Restoration Drops are a dietary supplement that replaces electrolytes lost in sweat and supports hydration. They are unflavored, with no sugar and no calories. Read about the hydration programs JMAR runs with safety managers, including onsite toolbox talks, training and coaching. Related terms: acclimatization and work/rest schedule.

If a worker shows confusion, slurred speech or collapse, call 911. That is a heat stroke emergency, whatever the WBGT reading says.

Next step

Want a heat routine your crew will use? Book a 15-minute demo with JMAR Solutions, or request a free sample.