Heat stress injuries are predictable—and preventable—with planning

Every summer, headlines report crews collapsing on road projects, utility workers suffering cramps after long shifts, and warehouse pickers being shuttled to clinics with heat exhaustion. These incidents are not acts of nature. They stem from mismatches between environmental heat, worker workload, and recovery time. Occupational hygienists have decades of data showing how quickly core body temperatures climb when metabolic heat production combines with hot, humid air. The challenge is translating that science into a shift plan a foreperson can actually implement. This planner bridges that gap. It asks for the variables supervisors can observe—dry-bulb temperature, humidity, solar load, air speed, and task intensity—and converts them into wet bulb globe temperature (WBGT), the index most agencies use to manage heat stress. From there, it recommends work-rest ratios and hydration volumes grounded in American Conference of Governmental Industrial Hygienists (ACGIH) guidance, generating a schedule you can download for toolbox talks or compliance documentation.

Heat stress management requires more than rules of thumb. Saying “drink water and take breaks” is meaningless if the workload still drives core temperatures into the danger zone. By quantifying the environment and the metabolic demand of the job, you reveal the invisible heat load on the body. The planner helps you answer practical questions: How many minutes per hour can linemen spend in direct sun pulling cable before they must rotate? What happens to the schedule when humidity spikes after a midday thunderstorm? If a subcontractor brings in unacclimatized workers, how much more recovery time do they need? The calculator puts numbers behind those questions so the safety plan can adapt in real time.

How the WBGT and work-rest formulas tie together

The backbone of the tool is the wet bulb globe temperature equation. WBGT combines air temperature, humidity, radiant heat, and airflow into a single value that correlates with the body’s ability to shed heat. Because few job sites have WBGT sensors, the planner estimates the index from more common measurements using a combination of wet bulb approximations and globe temperature adjustments. The MathML expression below captures the estimation used in the calculation for outdoor conditions with solar load.

$\mathrm{WBGT}=0.7·T_w+0.2·T_g+0.1·T_a$

In the equation, T_w is the wet bulb temperature computed from air temperature, humidity, and air velocity using Stull’s approximation; T_g represents globe temperature adjusted for solar load; and T_a is the dry-bulb air temperature. Indoors, where radiation loads are lower, the calculator increases the weight on air temperature instead. Once the WBGT estimate is in hand, it is compared against the work-rest guidance for the declared metabolic rate and acclimatization status. Each guidance band corresponds to a fraction of an hour spent working versus recovering. The tool defaults to a 60-minute cycle because many crews already plan around hourly toolbox talks or equipment checks, but you can change the cycle to 45 or 90 minutes to match your operation.

The recommended work fraction is the key output. A 50 percent recommendation means crews should spend 30 minutes in the heat followed by 30 minutes of rest in a cooler area with ventilation or air-conditioning. The schedule generator converts that ratio into a shift-long plan with start times, work blocks, and rest blocks. Because hydration is also vital, the tool multiplies the base hydration rate for the task category by the crew size and the fraction of an hour spent working. That way the water column in the table reflects the actual fluid volume required in each cycle, not a generic “drink more water” reminder.

Worked example: paving crew on a humid afternoon

Imagine a paving contractor resurfacing an urban arterial in midsummer. At noon, a supervisor measures 95°F (35°C) air temperature with 55 percent humidity. The crew operates in full sun, with only a light breeze of 0.8 m/s created by passing traffic. Each worker is pushing around 350 watts of metabolic load while raking asphalt and feeding the paver. Six laborers make up the team, all acclimatized after several weeks on the project. Plugging those numbers into the planner produces an estimated WBGT of about 30.2°C (86.4°F). With that WBGT and the heavy workload category, the recommended work fraction for acclimatized workers is 50 percent. The summary advises 30 minutes of work followed by 30 minutes of recovery each hour, and about 3 liters of water per hour for the crew—roughly one half-liter bottle per worker every cycle.

The schedule table translates those ratios into actual times. If the shift starts at 7:00 a.m., the sixth cycle begins at 12:00, exactly when the heat load peaks. Crew leaders can post the downloaded CSV in the trailer, assign floaters to cover rest periods, and coordinate with the asphalt plant to avoid supply gaps during break windows. If humidity climbs later in the day, supervisors can rerun the planner with updated values; the resulting work fraction may drop to 25 percent, signaling that only 15 minutes of work per hour is safe unless additional controls like shade tents or misting fans are added.

Using the comparison table to choose controls

Heat mitigation is not limited to slowing down. Engineering controls—shade, ventilation, cooling vests—and administrative controls—shift staggering, buddy systems—change the inputs to the planner. The table below illustrates how different interventions affect the WBGT estimate and work fraction for the same paving crew scenario.

Impact of controls on recommended work-rest ratios

Control strategy	Adjusted input	Estimated WBGT (°C)	Work fraction
Baseline (no controls)	Temp 35°C, RH 55%, sun	30.2	50%
Shade canopy over staging	Switch to partial sun	28.9	75%
Misting fans at rest area	Air velocity 1.5 m/s	29.4	50%
Cooling vests + shade	Partial sun, effective load 280 W	28.9	100%

Switching from full sun to partial shade lowers the WBGT enough to allow a 75 percent work fraction, effectively giving back 15 minutes per hour. Combining shade with reduced metabolic load—perhaps by using powered screeds or alternating tasks—can return the crew to continuous work at safe core temperatures. The planner makes these trade-offs explicit, enabling data-driven conversations with project managers about investing in shade structures, ventilation fans, or additional personnel to rotate through the hottest tasks.

Reading the summary metrics

The summary box at the top of the results is designed to give supervisors the handful of numbers they need to brief a crew. The estimated WBGT appears in both Celsius and Fahrenheit because agencies around the world use different units. The intensity classification reminds you which metabolic rate band you selected—light work such as inspection, moderate work such as carpentry, or heavy work such as rebar tying. The crew water requirement translates hydration recommendations into actual liters so you can order enough coolers. When conditions are so extreme that the work fraction hits zero, the summary warns you to halt operations; OSHA’s General Duty Clause expects employers to act on these warnings. Think of the summary as a dashboard that you revisit whenever the weather or workload shifts.

Integrating the schedule with site operations

Generating a schedule is only the first step. Implementing it requires coordination with forepersons, subcontractors, and the logistics team. Use the CSV export to load the schedule into your project management software or to print badges that specify each crew’s work and recovery windows. Stagger crews with different start times to avoid everyone entering the rest area simultaneously. If the schedule forces equipment downtime, negotiate with suppliers or clients upfront so productivity metrics reflect the reality of heat management. Many firms now document these schedules to show regulators and insurers that they took proactive steps, which can mitigate penalties if an incident occurs. The planner’s data-driven output makes that documentation straightforward.

Limitations and assumptions

No field calculator can replace professional industrial hygiene assessments. The WBGT estimation here relies on empirical formulas that assume average solar loads and do not account for radiant heat from hot surfaces like freshly poured asphalt or industrial furnaces. The metabolic rate categories are coarse; if you need precise energy expenditure measurements, conduct time-motion studies or use wearable sensors. Hydration guidance assumes workers are otherwise healthy and that water is readily available. It does not address electrolyte replacement or personal medical restrictions. Finally, the planner outputs schedules based on steady-state conditions. Real shifts experience fluctuations—clouds, wind shifts, task changes—that may require more frequent recalculation. Use the tool as a conservative baseline, then layer in site-specific monitoring, buddy checks, and medical surveillance to keep every worker safe during extreme heat.