How long should you work between breaks in the heat?
It depends on WBGT and how hard the work is. At WBGT 28°C doing moderate work, the published tables put the limit at roughly 30 minutes of work and 30 minutes of rest per hour for an acclimatised worker in light clothing. Heavier work steps down sooner. Enter both above for the figure and the threshold above it.
What the table is doing
Heat stress is a balance. Work produces heat inside the body; the environment decides how fast that heat can
leave. When production outruns removal, core temperature climbs. Rest is how the balance gets settled: it
cuts production to near zero for part of each hour.
So the table has two axes. WBGT says how bad the environment is at removing heat, and workload says how much
heat you are making. Where they cross, you get the share of each hour that can be work.
Read the answer as a ceiling, not a target. Nothing about 30 minutes on and 30 minutes off makes a worker
safe if they arrived dehydrated, are on their second day in the heat, or are wearing a vapour-barrier suit.
The four things that make the real limit stricter
- Not acclimatised. NIOSH publishes a separate, lower set of limits for unacclimatised workers. In the
first week, treat the conditions as several degrees worse than measured. The
acclimatisation plan builder sets out the schedule.
- Clothing. Coveralls, and especially anything impermeable, add to the effective WBGT before you enter
the table.
- Dehydration or illness. The tables assume a worker starting the hour in normal fluid balance.
- Direct sun with no wind. Already in WBGT if you measured it properly — but if you estimated it, that
is exactly the case where an estimate reads low. The WBGT estimator flags it.
What this cannot be used for
This is a presentation of published guidance, and it is not a compliance determination. If you need a record
that stands up to an inspection, you need a measurement from a certified instrument and an assessment made
under whichever rules apply to you — the general duty clause and state plans in the US,
the 2025 regulation in Japan, the risk-assessment duty in the UK and
Australia. App or WBGT meter is the honest version of that comparison.
For the reasoning behind the schedule rather than the numbers,
work/rest schedules in heat goes through workload categories, break
design and what to do when the schedule says stop.
Are these OSHA's official work/rest requirements?+
No. OSHA has no numerical federal work/rest standard; its enforceable requirement is the general duty clause, with a specific heat rule still at proposal stage. The tables here follow the ACGIH threshold limit values and the NIOSH recommended alert limits, which is what most heat illness prevention plans and OSHA's own educational materials point at.
Do these numbers assume workers are acclimatised?+
Yes, and that matters a great deal. The limits shown are for acclimatised workers. NIOSH publishes lower limits for unacclimatised workers — roughly 2 to 4°C WBGT lower depending on workload — so a crew in its first week should be treated as though conditions were several degrees hotter.
What counts as moderate rather than heavy work?+
Roughly by metabolic rate. Light is sitting or standing work with light tools, around 200 W. Moderate is walking with moderate lifting or sustained hand and arm work, around 300 W. Heavy is pick and shovel work or carrying loads, around 400 W. Very heavy is sledgehammer work or climbing with loads, above 500 W.
Does clothing change the answer?+
Yes. Impermeable or encapsulating clothing blocks evaporation, and published practice is to add a clothing adjustment to the measured WBGT before entering the table — commonly a few degrees for coveralls and considerably more for vapour-barrier suits. This calculator does not apply one, so with that kind of clothing the true limit is stricter than it shows.
Can rest breaks be taken in the sun?+
A break that does not lower body temperature is not a rest break in the sense these tables mean. The recovery period assumes shade or air conditioning, with water available. Sitting on a kerb in full sun for fifteen minutes does not satisfy the schedule.
