OSHA Heat Work/Rest Chart: WBGT Thresholds and Acclimatization

The OSHA heat work/rest chart tells you how many minutes of each hour your crew should work and how many they should rest, based on three inputs: a Wet Bulb Globe Temperature (WBGT) reading taken at the worksite, how physically hard the job is, and whether the workers are acclimatized to heat. Get those three right and the schedule falls out of the chart. Get any one wrong and the schedule you’re following isn’t the one that protects your people.

Start With a WBGT Reading, Not the Weather App

A regular thermometer or a phone’s heat index captures air temperature and humidity but ignores radiant heat from asphalt, machinery, or direct sun, and it ignores wind. A 95°F reading in a breezy shaded spot describes a very different working environment than 95°F next to a furnace with still air. WBGT is designed to close that gap. It combines a dry bulb thermometer for air temperature, a natural wet bulb thermometer that reflects humidity and wind through evaporation, and a black globe thermometer that picks up radiant heat.¹

The formula changes depending on sun exposure. Outdoors with a solar load, WBGT weights the wet bulb reading at 70%, the globe temperature at 20%, and the dry bulb at 10%. Indoors or in full shade, the dry bulb drops out and the weights become 70% wet bulb and 30% globe.² Outdoor readings in sun usually run higher than indoor readings at the same air temperature because of the radiation component.

Portable WBGT monitors run from about $35 for basic models up to several hundred dollars for professional instruments with data logging. Place the monitor where the work is actually happening, at roughly chest height. Indoor sites with ovens, boilers, or laundry equipment need readings taken near those sources, not in a cooler corner of the room.

If you don’t own a WBGT monitor, the National Emphasis Program on heat treats an 80°F heat index as the threshold for a “heat priority day” that raises the risk of an OSHA inspection.³ Heat index is easier to look up but less precise than a worksite WBGT.

Classify the Workload Honestly

The chart uses different WBGT limits for different levels of physical exertion. Misjudging the workload is one of the easiest ways to end up on the wrong line of the chart, because tasks that feel routine to a conditioned worker can push an unacclimatized one into danger. OSHA and NIOSH group work into three tiers:

Light work: sitting, standing, driving, light assembly, operating controls.
Moderate work: walking at normal pace, pushing, pulling, or sustained hand and arm work such as hammering or sawing.
Heavy work: digging, shoveling, carrying heavy loads, climbing with a load, or any task that leaves workers visibly out of breath within minutes.

When an hour contains a mix of tasks, classify by the most demanding, not the average. A roofer who spends 40 minutes carrying materials and 20 minutes measuring is doing heavy work, not moderate.

The Action Limits That Trigger Work/Rest Cycling

The heart of the chart is a set of WBGT thresholds adapted from NIOSH guidance. Below the applicable limit, workers can generally work continuously with standard monitoring and hydration. At or above it, the schedule needs to shift toward rest. OSHA publishes the limits in two columns, one for unacclimatized workers (the “action limit”) and one for acclimatized workers (the “threshold limit value”):¹

Light work: 82.4°F WBGT unacclimatized; 86°F acclimatized.
Moderate work: 77°F WBGT unacclimatized; 82.4°F acclimatized.
Heavy work: 73.4°F WBGT unacclimatized; 78.8°F acclimatized.

Those numbers surprise most employers. An unacclimatized worker doing heavy labor hits the action limit at 73.4°F WBGT, on a day the air can feel mild. That is why standard thermometers keep sending crews out under conditions that a proper reading would flag.

Once the WBGT crosses the applicable limit, the chart moves through four tiers of work/rest ratio within each hour: continuous work below the limit, then 75/25, 50/50, and 25/75 as conditions worsen. Each tier has its own WBGT ceiling for each workload category, and OSHA’s published work/rest chart supplies the minute-by-minute breakdowns for each combination of risk level, workload, and acclimatization status.⁴ The rule underneath all of it is simple: hotter conditions and harder work mean shorter work periods and longer rests.

Adjust the WBGT for Clothing Before You Read the Chart

Protective clothing traps heat against the body and blocks the evaporation that sweat depends on. Before comparing your reading to any of the limits above, add the appropriate clothing adjustment to the measured WBGT. This step gets skipped constantly, and it can turn what looks like a safe schedule into a dangerous one. OSHA lists these adjustments:¹

Standard work clothing or cloth coveralls: no adjustment.
SMS (synthetic) coveralls: add 0.9°F.
Polyolefin coveralls: add 1.8°F.
Double-layer cloth clothing: add 5.4°F.
Vapor-barrier coveralls: add 19.8°F.

That last figure is not a typo. A worker in vapor-barrier gear on a 65°F WBGT day is effectively working at 84.8°F WBGT, which already exceeds the action limit for moderate and heavy work. Anyone in chemical-resistant suits or non-breathable PPE typically needs aggressive work/rest scheduling even on days that feel comfortable to the rest of the crew.⁵

Which Column Applies: Acclimatized or Not

Acclimatization decides which column of the chart you’re reading, and the gap between columns is wide. An acclimatized worker doing moderate labor can work continuously up to 82.4°F WBGT; an unacclimatized worker hits the action limit at 77°F. That 5.4-degree spread reflects a real physiological difference in how the body handles heat load.¹

A worker is unacclimatized if they are new to the job or returning after an absence of a week or more. That includes vacation, medical leave, and even a stretch of cool weather that broke the pattern of regular heat exposure.⁶ Acclimatization can also fade over winter.

Building Up New Workers

NIOSH recommends brand-new workers start at no more than 20% of normal heat exposure on day one, adding no more than 20% each following day, reaching 100% by day five.⁷ For a crew that normally works eight hours in heat, that means roughly 1.5 hours of heat exposure on day one, 3 hours on day two, and so on, with the rest of the shift spent in cooler tasks or on additional breaks.

Bringing Returning Workers Back

Someone who has held the job before but has been away can use a shorter four-day schedule: 50% on day one, 60% on day two, 80% on day three, and full duty on day four.⁷ They keep some physiological adaptation, but sending them straight back to full exposure after a week off is where a significant share of heat injuries occur.

Make the Rest Period Actually Cool Them Down

The chart’s rest minutes only work if the rest location is genuinely cooler than the work area. Standing in full sun next to the job barely drops core body temperature, and the next work period starts with the worker already heat-loaded. OSHA recommends shade, an air-conditioned vehicle or building, a tent, or an outdoor area with fans and misting. Indoor workers should rest in a cool space away from ovens, furnaces, and other heat sources.⁸

Distance matters too. If the recovery area is a ten-minute walk away, a 30-minute rest loses a third of itself in transit. Water and shade should be near the work, easy to reach, and known to the crew before the shift starts. When no cool rest area is available, OSHA advises extending break duration to compensate for the slower recovery.⁸

Hydration During the Schedule

Workers in heat should drink about one cup (8 ounces) of cool water every 15 to 20 minutes, roughly a quart per hour. OSHA emphasizes drinking on a schedule rather than waiting for thirst, since thirst arrives after dehydration has already begun affecting performance and heat tolerance. Water should be cooler than 60°F, and total intake should not exceed 48 ounces per hour to avoid hyponatremia.⁴ For routine heat exposure, plain water combined with normal meals and snacks replaces the salt lost through sweat. Electrolyte drinks are generally unnecessary unless a worker develops heat cramps, in which case sports drinks every 15 to 20 minutes can help resolve the cramping.⁹

Why the Chart Matters for Compliance

Federal OSHA does not currently have a standalone heat illness prevention standard. A proposed rule has been in rulemaking since August 2024 and would formally recognize either heat index or WBGT, with an initial trigger at 80°F heat index (or the NIOSH Recommended Alert Limit in WBGT) and a high heat trigger at 90°F heat index (or the NIOSH Recommended Exposure Limit in WBGT).¹⁰ Until that rule is final, OSHA cites heat cases under the General Duty Clause, which requires employers to keep workplaces free from recognized hazards likely to cause death or serious physical harm.¹¹ The National Emphasis Program directs enforcement at construction, agriculture, and general industry operations near radiant heat sources, and triggers area-office attention on any day the local heat index is expected to reach 80°F.³ Seven states run their own mandatory heat standards with specific temperature triggers, shade requirements, and water rules that go beyond the federal baseline.

The chart is not a legal safe harbor on its own, but following it consistently, with the clothing adjustment applied and acclimatization tracked, is a documented, defensible way of demonstrating that the crew was scheduled against a recognized method for controlling heat exposure. One last boundary: the chart handles scheduling, not medical response. If a worker shows signs of heat stroke, that is a 911 call and rapid cooling, not a schedule adjustment.¹²

1
Occupational Safety and Health Administration. Heat Hazard Recognition
2
Occupational Safety and Health Administration. OSHA Technical Manual – Section III Chapter 4 – Heat Stress
3
Occupational Safety and Health Administration. National Emphasis Program – Outdoor and Indoor Heat-Related Hazards (CPL 03-00-024)
4
Occupational Safety and Health Administration. How to Use the OSHA Heat Work/Rest Chart
5
Centers for Disease Control and Prevention. PPE Heat Burden
6
Occupational Safety and Health Administration. Protecting New Workers
7
Centers for Disease Control and Prevention. Acclimatization
8
Occupational Safety and Health Administration. Water. Rest. Shade.
9
Occupational Safety and Health Administration. Protecting Workers from Heat Illness
10
Occupational Safety and Health Administration. Heat Injury and Illness Prevention in Outdoor and Indoor Work Settings – Proposed Rule Text
11
Occupational Safety and Health Administration. Heat – Standards
12
Occupational Safety and Health Administration. Heat-Related Illnesses and First Aid