Occupational heat screening is not a single lookup. Unlike an athletic chart, where the reading maps straight onto a flag, the occupational limit depends on how hard the work is, how much of each hour is spent doing it, what the worker is wearing, and whether they are acclimatized.
That is four inputs before the reading matters, and skipping them produces a number that looks rigorous and means nothing.
The screening sequence
- Measure or compute WBGT at the work area with the WBGT calculator or an on-site instrument.
- Add the clothing adjustment factor for what the crew is actually wearing.
- Classify the workload honestly as light, moderate, heavy or very heavy.
- Decide whether the crew is acclimatized. Anyone in their first week, or returning after more than a week away, is not.
- Compare the adjusted WBGT against the TLV for acclimatized workers, or the lower action limit for unacclimatized ones.
- If the adjusted reading exceeds the limit, reduce the work fraction of each hour until it does not, or apply engineering and administrative controls.
Clothing adjustment factors
Clothing that blocks evaporation raises the effective WBGT. Add the adjustment to your measured reading before comparing it with a limit.
| Clothing | Add to WBGT |
|---|---|
| Work clothes, long sleeves and trousers | 0°C |
| Cloth coveralls | 0 to 2°C |
| Double-layer woven clothing | 3°C |
| SMS polypropylene coveralls | 0.5°C |
| Polyolefin coveralls | 1°C |
| Limited-use vapor-barrier coveralls | 11°C |
That last row is not a typo. An encapsulating vapor-barrier suit shuts down evaporative cooling almost completely, and WBGT screening stops being adequate at that point. Physiological monitoring takes over.
What counts as light, moderate and heavy work
The workload category moves the threshold by several degrees, so classifying it honestly matters more than getting the reading to a decimal place.
| Category | Roughly | Examples |
|---|---|---|
| Light | Up to 200 W | Sitting or standing to control machines, light hand or arm work, driving |
| Moderate | 200 to 350 W | Sustained hand and arm work, walking with moderate lifting, pushing a wheelbarrow |
| Heavy | 350 to 500 W | Shoveling, sledgehammer work, carrying heavy loads, continuous digging |
| Very heavy | Over 500 W | Intense sustained effort, axe work, climbing stairs or ladders with a load |
ACGIH screening limits
The ACGIH threshold limit values are the numbers industrial hygienists actually screen against. They are set by workload and by how much of each hour is spent working, and they assume acclimatized workers in light summer clothing.
| Work per hour | Light | Moderate | Heavy | Very heavy |
|---|---|---|---|---|
| 75 to 100% | 31.0°C | 28.0°C | — | — |
| 50 to 75% | 31.0°C | 29.0°C | 27.5°C | — |
| 25 to 50% | 32.0°C | 30.0°C | 29.0°C | 28.0°C |
| 0 to 25% | 32.5°C | 31.5°C | 30.5°C | 30.0°C |
For unacclimatized workers, the action limit sits roughly 2 to 3°C lower across the board. In Fahrenheit, the moderate-work continuous limit of 28.0°C is 82.4°F, which is a long way below the 90°F that people intuitively think of as dangerous.
A worked screening
A roofing crew in cloth coveralls on an 84°F WBGT afternoon.
| Step | Value |
|---|---|
| Measured WBGT | 84.0°F / 28.9°C |
| Clothing adjustment, cloth coveralls | +2.0°C |
| Adjusted WBGT | 30.9°C |
| Workload | Heavy |
| TLV, acclimatized, 50 to 75% work | 27.5°C — exceeded |
| TLV, acclimatized, 25 to 50% work | 29.0°C — exceeded |
| TLV, acclimatized, 0 to 25% work | 30.5°C — exceeded |
| Conclusion | Screening limit exceeded at every work fraction. Heavy work is not appropriate; reduce workload, reschedule to cooler hours, or move to physiological monitoring with controls in place. |
This is the result that surprises people. An 84°F WBGT reading sounds unremarkable, and for heavy work in coveralls it is over the line at every work-rest ratio the table offers.
When screening is not enough
The TLVs are a screening tool, not a limit on exposure in the sense of a chemical PEL. Exceeding them does not mean work must stop; it means screening has failed to demonstrate safety and you need to go further.
Going further means either a detailed analysis, such as the ISO 7933 predicted heat strain model, or physiological monitoring of the individual workers: heart rate recovery, core temperature where feasible, body mass change across the shift, and direct observation. Combine that with engineering and administrative controls: shade and cooling, reflective barriers, scheduling heavy work before noon, mechanizing the heaviest tasks, and rotating crews.
WBGT screening also breaks down entirely with vapor-barrier clothing, where evaporation is blocked almost completely. There, physiological monitoring is the only defensible approach.
What a heat illness prevention plan contains
Whether or not a specific standard applies to you, this is the structure regulators, insurers and plaintiffs' attorneys all expect to see.
- Measurement. Who takes the reading, where, how often, and how it is recorded.
- Trigger levels. What changes at each threshold, written down in advance rather than decided in the moment.
- Water. Cool potable water available at the work area, in quantity, with a target of roughly one cup every 15 to 20 minutes.
- Shade and rest. Shade available at or before the trigger, large enough for everyone on break at once, and rest that is actually taken.
- Acclimatization. A graded schedule for new workers and anyone returning after more than a week away, with closer supervision during it.
- Training. Symptoms, reporting, first aid and the emergency plan, for workers and supervisors, in a language they understand.
- Emergency response. Who calls, what the site address is, how responders reach the work area, and who starts cooling in the meantime.
When in doubt, cool first and call for help. Collapse or confusion in the heat should be treated as possible exertional heat stroke: begin cold-water immersion immediately and call 911. Cooling comes before transport.
The reading, on every supervisor's phone
A single instrument at the trailer gives you one number for a site that may span several microclimates. The app puts a consistent WBGT and OSHA mode guidance on every phone, with a forecast for scheduling the heavy work early.
- Real-time WBGT for your location or any city
- Hourly and 14-day WBGT forecasts to plan ahead
- Color-coded flag levels with mode-specific guidance
- Humidity, wind, sun and cloud cover behind every reading
WBGT is estimated from weather data with the peer-reviewed Liljegren model. Where a policy requires on-site measurement, use the app for planning and as a cross-check alongside your instrument.
Common questions
How do you calculate OSHA heat exposure?
Measure WBGT at the work area, add a clothing adjustment factor, classify the workload, then compare against the ACGIH TLV for that workload and work fraction, using the lower action limit if workers are unacclimatized.
What is the WBGT limit for heavy work?
For acclimatized workers in light clothing, the ACGIH TLV for heavy work is 27.5°C at 50 to 75% work per hour and 29.0°C at 25 to 50%. Heavy continuous work has no TLV at all, because it is not considered sustainable.
What is the difference between the TLV and the action limit?
The TLV applies to acclimatized workers; the action limit applies to unacclimatized workers and sits roughly 2 to 3°C lower. Since most heat fatalities occur in a worker's first week, the action limit is often the one that matters.
Do I add the clothing factor before or after comparing to the limit?
Before. Add the adjustment to the measured WBGT, then compare the adjusted value against the unmodified limit. Adjusting the limit instead is a common error that produces the wrong answer.
What if the reading exceeds the TLV?
Screening has failed, which is not the same as work being prohibited. Either reduce exposure through controls and work-rest scheduling until the adjusted reading sits under the limit, or move to a detailed analysis and physiological monitoring.
Related WBGT guides
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