2.2 WBGT: measuring the heat the crew is in

1 Three sensors, three questions

A WBGT meter carries three thermometers. Each answers a different question about the same air.

The natural wet bulb is the one people get wrong. It is not a sling psychrometer whipped around by hand, and it is not the "wet bulb temperature" from a weather site. It is a wetted sensor sitting in whatever air movement the work area actually has, because air movement is part of what it is measuring.

Some meters use a waterless wet-bulb sensor instead. The Technical Manual describes it as a tested and validated alternative that models the wet-bulb value from dry-bulb temperature, globe temperature, relative humidity and air flow sensors. Either instrument is acceptable; the equations below do not change.

2 The two equations

There are two, and which one applies depends on whether there is a solar load.

The weights tell you what the metric believes. Outdoors, the natural wet bulb carries 70 percent, the globe 20 percent and the dry bulb only 10 percent. Evaporation is most of the story; plain air temperature is a minor term. Indoors the dry bulb drops out entirely, because with no radiant load the globe and the dry bulb read the same — and the globe stays in the equation to catch non-solar radiant sources like furnaces, ovens and boilers.

Work an example. Natural wet bulb 78 °F, globe 110 °F, dry bulb 92 °F outdoors: 0.7 × 78 + 0.2 × 110 + 0.1 × 92 = 54.6 + 22.0 + 9.2 = 85.8 °F WBGT. The WBGT calculator on this site runs the same arithmetic in either unit system, including the indoor form.

Because both equations are weighted averages of temperatures, they behave the same in °F and in °C — you convert the inputs or the answer, not the weights.

3 What WBGT sees that the heat index cannot

Compare the two metrics on the same day. Air at 92 °F and moderate humidity gives a heat index in the low nineties, in shade, with light wind. Now put a black globe on the same roof deck and it reads 110 °F, because the membrane below and the sun above are both radiating. WBGT catches an 18-degree gap that the heat index has no input for.

It works in the other direction too. A strong breeze lowers the natural wet-bulb reading, because evaporation improves; the heat index cannot register that either, since it assumes light wind. WBGT is the only one of the two that responds to the specific spot the crew is standing in.

4 Taking the reading, and averaging it

A number from the wrong place or the wrong moment is worse than no number, because it looks like data. OSHA's Technical Manual gives the practice.

1. 1
   Measure at the work area, or as close as possible to it. When a worker moves between areas with different conditions, or conditions vary a lot within the area, take representative measurements for each condition.
2. 2
   Measure at least hourly during the hottest portion of the shift, during the hottest months of the year, and whenever a heat wave occurs or is predicted — OSHA cites the 2016 NIOSH criteria document for this.
3. 3
   For continuous all-day or several-hour exposures, use the average WBGT over a 60-minute period.
4. 4
   For intermittent exposure or exposure at different heat levels, average over 60 to 120 minutes depending on the exposure duration.

The averaging equation is a plain time-weighted average: WBGT_avg = [(WBGT₁ × t₁) + (WBGT₂ × t₂) + … + (WBGT_n × t_n)] / (t₁ + t₂ + … + t_n), with times in minutes. A worker who spends 40 minutes of each hour on a sunny deck at 88 °F WBGT and 20 minutes in a shaded staging area at 78 °F WBGT is at (88 × 40 + 78 × 20) / 60 = 84.7 °F WBGT for that hour.

5 When there is no meter

If a WBGT meter or historical WBGT records are not available, OSHA's Technical Manual points to a calculator built by Argonne National Laboratory that estimates WBGT from weather data — air temperature, humidity, wind speed, barometric pressure and an estimate of solar irradiance from cloud cover — using a published model. It is the documented route for reconstructing conditions after an incident, when nobody was measuring at the time.

Understand what that estimate can and cannot do. It works from weather at a location; it does not know that your crew was on the shaded north face, or in the wind shadow of a building, or next to a running generator. OSHA's own guidance is that a measurement taken at the site is the accurate method and the estimate is the fallback.

6 From a number to a decision: clothing and workload

A WBGT reading is not a verdict on its own. Two more things turn it into one, and OSHA publishes both on its heat hazard recognition page, adapted from NIOSH guidelines (2016).

First, clothing. A clothing adjustment factor is added to the measured WBGT to give an effective WBGT, because what a worker wears changes how much of that heat they can shed.

Read that last line again. Putting a worker in vapor-barrier coveralls adds nearly 20 °F to the heat they are effectively working in — the difference between a warm morning and a dangerous one, produced entirely by the PPE decision. This is the arithmetic behind the proposed rule's requirement to evaluate the heat stress that vapor-impermeable clothing creates.

Second, workload. The limits are lower for harder work, and lower again for workers who are not acclimatized.

Two things jump out. The gap between an unacclimatized and an acclimatized worker is worth 3.6 to 7.2 °F of effective WBGT — the same day, the same job, a different body. And a very heavy job crosses its unacclimatized action limit at 69.8 °F effective WBGT, a number that sounds like a pleasant morning. OSHA classifies shoveling, roofing, welding and drilling concrete as heavy work, and intense digging, sledgehammer use and masonry as very heavy.

Crossing a limit is a signal to act on the work — cool the area, cut the workload, move the schedule, watch people harder — not a permitted exposure time. What this course does not print is a work/rest cycle in minutes per hour: those come from ACGIH threshold limit value documentation, which OSHA's own Technical Manual describes as copyrighted and not publicly available. If your program uses cycles, they belong in your plan with their source named. The WBGT calculator applies the clothing factor and compares your reading with the table above.