What the schedule is balancing
Working bodies produce heat — a few hundred watts of it, most of which has to leave through sweat evaporating. The environment sets how fast that can happen. When production outpaces removal, core temperature rises, and it keeps rising until something changes.
Rest is the lever that changes production. Thirty minutes of rest in an hour roughly halves the heat generated in that hour, and gives the body a window to catch up.
That is why every published table has two axes: WBGT for the environment, workload for the production.
Classifying workload honestly
This is where schedules go wrong in practice, because everyone underestimates their own crew's workload.
| Category | Approx. rate | Examples |
|---|---|---|
| Light | ~200 W | Sitting or standing work, light hand tools, driving, inspection |
| Moderate | ~300 W | Walking with moderate lifting, sustained hand and arm work, pushing a wheelbarrow on the level |
| Heavy | ~400 W | Pick and shovel work, carrying loads, sustained heavy lifting, climbing stairs with tools |
| Very heavy | 500 W and above | Sledgehammer work, climbing with loads, intense sustained effort |
Two rules of thumb that help:
- Categorise the hardest sustained task, not the day's average. A crew that spends two hours of an eight hour day on heavy work is a heavy-work crew for those two hours.
- If someone cannot hold a conversation while doing it, it is at least heavy.
The three adjustments the table does not make for you
- Acclimatisation. Use the unacclimatised limits for anyone in their first week, back from a week or more away, or facing the first hot spell of the year. Heat acclimatization covers how long it takes and how fast it is lost.
- Clothing. Add to WBGT before you look it up. Coveralls a few degrees; anything impermeable, a lot more.
- Direct sun with no air movement. Already inside a measured WBGT — but if the WBGT was estimated from weather data, that is precisely the case where the estimate reads low, by up to 2°C. The estimator flags it.
Designing the break
A schedule is a number; a break is a place. If the break area does not lower body temperature, the schedule is being followed on paper only.
- Shade that is actually shaded. A tarpaulin over hot ground with no airflow is not much of an improvement.
- Water at the break, and before it. Waiting for thirst means starting each hour behind.
- Somewhere to sit. Standing in shade is still work for the cardiovascular system.
- Long enough that it works. Cooling is not instant; short frequent breaks beat one long one, but very short breaks do very little.
- Someone counting. Self-managed breaks in heat get shorter as the day goes on, exactly when they should be getting longer.
Run the work/rest calculator for the ratio, and the construction scenario for how the whole day fits together. In Japan the same decision is made against a statutory number rather than a recommendation — the Japanese page on outdoor work covers that version.