There is a persistent misconception that OSHA has no heat rule, so heat is not regulated. That is wrong in a way that costs employers money.
OSHA has no comprehensive federal heat standard in force, but it enforces heat hazards under the General Duty Clause, Section 5(a)(1) of the OSH Act, which requires employers to furnish a workplace free from recognized hazards likely to cause death or serious physical harm. Heat is a recognized hazard with well-published control measures, which is exactly what the clause is built for. OSHA has issued citations and substantial penalties on that basis, and has run a National Emphasis Program on heat since 2022.
WBGT enters through the limits. The ACGIH threshold limit values that define what competent practice looks like are written in WBGT, and the proposed federal heat standard and existing state standards reference it as the more accurate measure for direct sun and heavy work.
How OSHA uses heat index and WBGT
OSHA's public-facing heat materials and its app historically lead with the heat index, for a practical reason: it is available from any weather source and requires no instrument. OSHA's own guidance is explicit that this is a screening convenience, and that the heat index understates risk in direct sunlight, where it advises adding roughly 15°F.
For anything more rigorous, WBGT is the measure. It is what ACGIH, NIOSH and ISO 7243 use, it is what appears in the proposed federal standard, and it is what an expert witness will use if a case ever turns on whether your program was adequate. See WBGT vs heat index for why they diverge.
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.
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 |
State standards that go further
Several states operate their own OSHA-approved programs with enforceable heat standards that are more specific than federal requirements, and they apply regardless of the federal rulemaking timeline.
California, Washington, Oregon, Colorado, Minnesota and Maryland all have heat illness prevention requirements of some form, typically specifying trigger temperatures, shade provision, water quantity, rest periods, acclimatization schedules and training. The triggers are often written in air temperature or heat index because those are simpler to enforce, but a WBGT-based program will comfortably satisfy them and will serve you better in heavy work or direct sun.
If you operate across state lines, build the program to the strictest standard you are subject to rather than maintaining several.
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.
Where programs actually fail
Citations and incident investigations keep landing on the same four things.
No acclimatization schedule
A large share of workplace heat deaths occur in a worker's first week. New hires and returning workers need a graded ramp and closer supervision, not a full shift on day one.
Water that is not where the work is
A cooler at the trailer is not water at the work area. If reaching it costs ten minutes, workers will not drink enough.
No records
Readings taken and not logged are indistinguishable from readings never taken, from the perspective of an inspector or a jury.
No rehearsed emergency response
Knowing the site address, how responders reach the work area, and who begins cooling immediately is the difference between a scare and a fatality.
Warning signs of heat illness
Heat stress shows up before it becomes an emergency. Anyone on site should speak up immediately on any of these:
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.
WBGT in OSHA mode
The app includes an OSHA mode with occupational thresholds rather than athletic ones, so supervisors get the number and the guidance that matches the work rather than having to translate from a sports chart.
- OSHA mode with occupational thresholds
- Live WBGT for the jobsite or any city
- Hourly and 14-day forecasts for shift planning
- A consistent number to record across crews
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
Does OSHA require WBGT monitoring?
No federal standard currently mandates WBGT specifically. OSHA enforces heat hazards under the General Duty Clause, and the ACGIH threshold limit values that define competent practice are written in WBGT. A WBGT-based program is the more defensible one.
What is the OSHA WBGT limit?
OSHA does not publish its own WBGT limits. The reference values are the ACGIH TLVs, which for acclimatized workers in continuous moderate work sit at 28.0°C, or 82.4°F WBGT, with lower action limits for unacclimatized workers.
Is there an OSHA heat standard?
OSHA has been developing a federal heat injury and illness prevention standard, and the proposed rule references WBGT. Until a final rule takes effect, heat is enforced under the General Duty Clause, plus state standards where they exist.
Does OSHA use heat index or WBGT?
Both, for different purposes. OSHA's public guidance and app use heat index as an accessible screening tool, with an instruction to add about 15°F in direct sun. WBGT is the measure used in the technical limits and in the proposed standard.
What should an employer do first?
Write a heat illness prevention plan covering measurement, trigger levels, water, shade, acclimatization, training and emergency response, then actually run it and keep records. The plan on the shelf is not the point; the records showing it operated are.
Related WBGT guides
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