Heat Index vs WBGT: Which One Should Run Your Heat Program?

The short answer

If you only ever look at one number, look at WBGT for the work area and heat index for the forecast. They are not competitors; they sit at different points in the day. The forecast is how you plan the week. The meter is how you know what the crew is standing in.

What the heat index actually measures

The heat index is a National Weather Service product. It combines air temperature and relative humidity into an apparent temperature using the Rothfusz regression, with two correction terms for very dry and very humid air. You can run it yourself with the heat index calculator.

The critical detail is the assumption underneath it: heat index values are published for shady, light-wind conditions. That is the environment the number describes. NWS states that full sunshine can raise heat index values by up to 15 °F; OSHA's heat hazard page cites up to 13.5 °F. Both are upper bounds you add by hand — neither is a measurement of the sun falling on your crew.

What heat index is genuinely good at: it is free, it is available tomorrow morning before anyone leaves the yard, and it is the metric everyone already understands. That makes it the right tool for planning — for deciding on Sunday night whether Tuesday's pour starts at 5 a.m.

What WBGT measures that the heat index cannot

A WBGT meter carries three sensors, and each one is answering a different question:

• Natural wet-bulb (T_nwb). A wetted sensor left in the moving air of the work area. It measures how well sweat can evaporate — so humidity and air movement both land in this reading. It carries 70% of the weight in both equations.
• Black globe (T_g). A hollow copper sphere with a matte black finish. It reads radiant energy: direct sun, and the heat thrown off machinery, furnaces and hot structures.
• Dry-bulb (T_db). Plain shaded air temperature, shielded from radiant heat. The one number the heat index also uses.

OSHA's Technical Manual publishes both equations:

Put your three readings into the WBGT calculator and you get the number for that spot. Note what the equations imply: the globe reading only has to run a few degrees above the dry bulb before WBGT and heat index start describing genuinely different worlds. That gap — sun on a black membrane roof, an oven line, a poured deck — is the whole reason OSHA prefers the meter.

Taking a WBGT reading that means something

• Measure at the work area, or as close as you can get. A reading from the trailer describes the trailer.
• Let the globe stabilize. It takes time to come to equilibrium after being moved into or out of the sun; read it too early and the number is low.
• Measure at least hourly during the hottest part of the shift, during the hottest months, and whenever a heat wave hits or is forecast. OSHA cites the 2016 NIOSH criteria document for that interval.
• Average across a moving crew. OSHA's time-weighted equation, WBGT_avg = Σ(WBGT_n × t_n) / Σt_n, exists exactly because people walk from sun to shade all day. Average over 60 minutes for continuous exposure, 60 to 120 minutes for intermittent work at different heat levels.
• Record it. After an incident, the reading you took that morning is evidence; a memory of it is not.

What OSHA's proposed rule does with the two metrics

There is no final federal heat standard. OSHA published a proposed Heat Injury and Illness Prevention rule on August 30, 2024; the informal hearing ran June 16 to July 2, 2025, and the post-hearing comment period closed October 30, 2025. Until a final rule exists, heat is cited under the General Duty Clause, Section 5(a)(1) of the OSH Act. The full picture of what applies today is in our guide to OSHA heat rules for employers.

What the proposal does with metrics is worth reading closely, because it settles the argument in a way most summaries skip. Each trigger is defined twice:

Three consequences fall straight out of that text. First, the employer chooses the metric, and the choice determines which trigger values apply. Second, if the employer does not identify a metric in the plan or does not monitor, the heat index values apply by default — the fallback is the easy metric, not the accurate one. Third, the WBGT route does not hand you a single number: the NIOSH alert and exposure limits vary with workload and with whether workers are acclimatized, so picking WBGT means committing to categorize the work as well as measure the air.

There is also a monitoring exemption in the proposal that is quietly practical: an employer can simply assume a work area is at or above both triggers and skip the measuring, as long as it provides every control that goes with both. For a small roofing crew in Phoenix in July, that may be less paperwork than a monitoring program, and it is the more protective answer.

Choosing the metric your program runs on

The honest way to decide is to ask what your site does to the air.

Both routes end in the same place: a written plan that says what gets measured, who measures it, what the trigger values are, and what happens when they are crossed. That plan is the document an inspector asks for after someone goes down — build yours with the heat illness prevention plan generator.

Turning a WBGT into a decision

A WBGT of 88 °F is not safe or unsafe until you add two things, and OSHA publishes both on its heat hazard recognition page, adapted from NIOSH guidelines (2016).

First the clothing adjustment factor, added to the measured WBGT to give an effective WBGT: 0 for work clothing and cloth coveralls, 0.9 °F for SMS coveralls, 1.8 °F for polyolefin coveralls, 5.4 °F for double-layer cloth clothing, and 19.8 °F for limited-use vapor-barrier coveralls. That last figure is worth sitting with — a PPE decision alone can add nearly 20 °F to the heat a worker is effectively standing in.

Then the workload limits, compared against that effective value.

Two readings of that table change how a crew plans. The gap between an unacclimatized and an acclimatized worker is worth 3.6 to 7.2 °F of effective WBGT — same day, same job, different body. And very heavy work crosses its unacclimatized action limit at 69.8 °F, which does not sound like a dangerous morning at all.

What stays out of reach is the next step: a work/rest cycle in minutes per hour. Those come from ACGIH threshold limit value documentation, and OSHA's own Technical Manual states that the ACGIH heat stress information “is copyrighted by ACGIH and is not publicly available.” So your plan can name the metric, the clothing factor and the limit from public sources — and has to name its own source for any cycle it prints.

The heat index has no equivalent table at all. There is no published heat index limit by workload, which is the deeper reason OSHA calls it a screening tool: it can tell you the day is dangerous, but it cannot tell you the job is.

Bottom line

• Heat index = temperature + humidity, in the shade, with a light breeze. Free, forecastable, and blind to the sun.
• WBGT = three sensors at the work area, weighted 0.7 / 0.2 / 0.1 outdoors and 0.7 / 0.3 indoors. It sees radiant heat and air movement, which is why OSHA calls the meter the most accurate tool.
• There is no conversion between them, and OSHA has no final heat standard today.
• In the proposed rule the employer picks the metric — and the heat index triggers of 80 °F and 90 °F apply by default to an employer that picks nothing.
• WBGT has published limits; the heat index does not. Add the clothing factor, then compare with OSHA's workload table — down to 69.8 °F for unacclimatized very heavy work.
• Deaths have happened at a heat index of 86 °F. Whichever number you run on, the number is the start of the conversation, not the end of it.