Most workplace heat programs fail in the same place. The organisation buys an instrument, trains someone to use it, and considers heat handled. Then an incident happens and the investigation finds readings nobody recorded, a trigger level nobody defined, and a new hire on day three of a full shift.
The reading is the cheapest part of the program. What makes it worth anything is the structure around it: defined triggers, a named owner, an acclimatization schedule, and records.
Where to measure
A reading from the wrong place is worse than no reading, because it carries false authority.
- Measure at the work area, not the office, the trailer or the gate.
- Measure in the sun if the work is in the sun. A shaded instrument describes a shaded worker.
- Place the sensor at roughly chest height, clear of walls, vehicles, compressors, generators and anything else radiating or exhausting heat.
- Identify your hot spots and measure them separately. Roofs, trenches, enclosed spaces, areas near hot processes and anywhere reflecting off light-coloured concrete all run hotter than the site average.
- Re-measure on a fixed interval through the shift, commonly hourly and more often when conditions are moving.
Microclimates that beat the site reading
Trenches and enclosed areas
No wind and reflected radiant heat from the walls. These can run many degrees above an open-ground reading twenty feet away.
Roofs and reflective surfaces
Dark membrane absorbs and re-radiates; light concrete reflects solar radiation upward into the body from below.
Near hot processes
Furnaces, kilns, engines, asphalt plants and welding all add radiant load that no weather-based estimate can see.
Inside protective clothing
The environment around a worker in impermeable gear is not the environment the instrument is measuring. Use the clothing adjustment factors.
Setting trigger levels
Pick thresholds in advance and write down what happens at each. Deciding in the moment produces inconsistency, and inconsistency is what inspectors and plaintiffs look for.
| Adjusted WBGT | What happens |
|---|---|
| Under 25°C / 77°F | Standard practice. Water available at the work area. |
| 25 to 28°C / 77 to 82°F | Heat awareness briefing at start of shift. Supervisor monitoring. Shade available. |
| 28 to 30°C / 82 to 86°F | Formal work-rest cycles. Buddy system. Heavy work scheduled before noon. |
| 30 to 32°C / 86 to 90°F | Short work cycles with mandatory shaded rest. Non-essential heavy work postponed. Active cooling available. |
| Above 32°C / 90°F | Essential work only, with controls, continuous supervision and physiological monitoring. |
These are illustrative. Derive your own from the ACGIH TLVs for your actual workload and clothing; see the OSHA WBGT calculator.
The acclimatization schedule
A large share of workplace heat fatalities occur in a worker's first week. This is the highest-value control in the entire program and the one most often missing.
- New workers: no more than 20% of the normal workload in heat on day one, increasing by no more than 20% per day.
- Experienced workers returning after more than a week away: start at 50% on day one, then 60%, 80% and 100%.
- Treat any worker returning from illness, injury or vacation as needing a ramp, not a full shift.
- Supervise new workers closely during the ramp. The ramp only helps if someone notices when it is not being followed.
- A sudden heat wave de-acclimatizes everyone relative to the new conditions. Step back during the first few days of a significant jump.
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.
Records that matter
If an inspector or an attorney asks, these are the documents that answer.
- Every WBGT reading with date, time, location and who took it.
- The written plan with trigger levels, and the version history showing when it changed.
- Training records: who was trained, on what, when, and in what language.
- Acclimatization schedules for each new or returning worker.
- Any heat-related incident, including near misses and workers sent home, with what was done.
- Water and shade provision logs where your jurisdiction requires them.
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.
A consistent number across the whole crew
One instrument at the trailer gives you one reading for a site that may have several microclimates. The app puts the same calculation on every supervisor's phone, with OSHA mode thresholds and 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
What WBGT is unsafe for work?
It depends on workload and clothing. For acclimatized workers in continuous moderate work the ACGIH limit is 28.0°C, or 82.4°F. For heavy work it drops to 27.5°C even with rest cycles, and unacclimatized workers need roughly 2 to 3°C lower.
Where should WBGT be measured on a worksite?
At the work area, in the sun if the work is in the sun, at roughly chest height, clear of equipment and surfaces that radiate heat. Hot spots like trenches, roofs and areas near hot processes should be measured separately rather than assumed from a site average.
How often should readings be taken?
Hourly is a common baseline, more often when conditions are changing or when the reading is near a trigger level. Every reading should be logged with time and location; an unrecorded reading is not evidence of anything.
What is the most important part of a workplace heat program?
The acclimatization schedule. A large share of workplace heat fatalities happen in a worker's first week on the job, and a graded ramp with close supervision prevents more harm than any other single control.
Do we need a written heat plan?
Several states require one, and the General Duty Clause makes one effectively necessary everywhere. More practically, the plan is what lets a supervisor act consistently at three in the afternoon without having to make a judgment call.
Related WBGT guides
Know the real heat number
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