Scenario

Heat safety on a construction site

What does a heat plan on a construction site consist of?

Water, rest, shade, acclimatisation, training and someone watching. In practice it is a sequence: check the forecast at the start of the shift, front-load the heavy work into the morning, run the published work/rest ratio for the WBGT and workload, and treat anyone in their first week as though the day were several degrees hotter.

The day, in order

A heat plan that lives in a binder does nothing. This is the sequence that actually gets used.

Before the shift — look at the forecast, not the sky. The peak matters more than this minute, and it arrives mid-to-late afternoon. If the afternoon is going into the Danger band, the schedule has to change before anyone picks up a tool.
Front-load the heavy work. Concrete pours, digging, anything in coveralls: morning. The published limits step down by workload, so moving the heavy task into the cool part of the day buys more than any amount of encouragement.
Name the new starters out loud. Anyone in their first week, back from a week away, or new to this climate. They work to the unacclimatised limits — several degrees stricter — and someone specific is watching them. The plan builder has the schedule.
Set the water and shade before it is needed. Water within 30 seconds of the work face. Shade that is genuinely shaded and sitting-height.
Run the ratio. WBGT and workload into the calculator, and the break structure follows from that rather than from how everyone is feeling.
Re-check at the peak. Mid-afternoon, when the estimate should be replaced by a measurement if there is one to be had.
End the day deliberately. Multi-day spells accumulate. A crew on day four of a heatwave starts from a worse position than on day one, even at the same temperature.

The triggers, on one card

Published limits for acclimatised workers in light clothing. Subtract several degrees for anyone unacclimatised, and more again for impermeable clothing.
Workload	First break needed	Work should stop
Light	WBGT 30°C · 86°F	WBGT 32.2°C · 90°F
Moderate	WBGT 26.7°C · 80°F	WBGT 31.1°C · 88°F
Heavy	WBGT 25°C · 77°F	WBGT 30°C · 86°F
Very heavy	WBGT 23°C · 73°F	WBGT 28.5°C · 83°F

The two failure modes

Sites do not usually fail because they lack a policy. They fail in one of two specific ways.

Nobody is watching. The person in trouble is the last to report it, and in a culture where asking for a break is a status cost, they will not. Heat monitoring is a named job or it does not happen. NIOSH pairs its acclimatisation schedule with supervision for exactly this reason.

The new starter falls through the gap. The acclimatisation schedule belongs to the employer, but on a site with three subcontractors and an agency, the person on their first day may have no employer present who knows it is their first day. That is the single most consistent finding in the fatality data, and it is an organisational problem rather than a physiological one.

What the estimate is and is not good for

An estimated WBGT is a good answer to "does today need the meter out". It is not a compliance measurement, because the radiation term is modelled rather than measured, and it reads low in exactly the hot, dry, still conditions a site is worst in. App or WBGT meter is the honest comparison, and OSHA and heat covers what is actually enforceable.

Common questions

What WBGT should stop work on a site?

It depends on the workload and on acclimatisation, which is why no single number works. For acclimatised workers in light clothing the published limits reach "stop" around 32°C for light work, 31°C for moderate, 30°C for heavy and 28.5°C for very heavy. Unacclimatised crews are several degrees lower, and impermeable clothing lower again.

Who is most at risk on a site?

New starters, in their first few days — consistently, across the fatality data. Then anyone back from a week or more away, anyone working in a respirator or coveralls, anyone over 60 or with a cardiac or metabolic condition, and anyone whose pay depends on output. Agency and subcontracted workers are over-represented, partly because acclimatisation schedules rarely follow them between sites.

How much water is enough?

Roughly a cup every 15 to 20 minutes during heavy work in heat, which is about a litre an hour — and it has to be within reach, because a tap at the far end of the site is a tap nobody uses. Over a long shift, something with sodium in it matters as well, since heavy sweating loses salt.

Does the site need a written plan?

In several US states, yes, explicitly. Federally the general duty clause does not name a document, but the absence of one is hard to defend after an incident. Practically, a written plan is what makes the sequence survive the foreman being off that week.

What do you do when someone shows signs of heat illness?

Stop them working and cool them immediately. If they are confused, slurring, unsteady or unconscious, treat it as heat stroke and call emergency services — do not drive them yourself and do not give fluids to someone who is not fully alert. Cool aggressively while waiting.

Sources

Published 30 July 2026