How to read a heat index chart for crew work and rest

What is a heat index chart?

A heat index chart shows how hot the air feels when humidity is added to temperature. The National Weather Service (NWS) says the heat index “is a measure of how hot it really feels when relative humidity is factored in with the actual air temperature.” You find your air temperature on one side and relative humidity on the other.

Crews call it a work rest chart because supervisors try to set break lengths from it. The chart itself gives a feel-like temperature and a danger zone. It does not give break minutes. Later sections show what to use for break timing.

How do you read the chart? A worked example

Find the air temperature, move to the humidity, and read the number where they meet. NWS gives one example on its heat tools page.

NWS puts it this way: “if the air temperature is 96°F and the relative humidity is 65%, the heat index (how hot it feels) is 121°F.” NWS also says the red area without numbers indicates extreme danger. You can use the chart image or NWS’s online heat index calculator for any pair of values.

We do not reprint the full chart here. Use the NWS chart so your numbers match the source.

Why does humidity matter so much?

Humidity slows the one cooling system a worker has. OSHA says “High relative humidity makes it difficult for the body to cool itself through sweating.” Sweat cools you only when it evaporates. In wet air it drips off instead, so the body keeps its heat.

OSHA explains that body heat is the balance of heat gained, from work and from outside, and heat lost, mostly through sweat evaporation. At the same air temperature, a humid day takes more from a crew than a dry one. This is why the chart reads higher as humidity rises.

Does full sun change the heat index?

Yes. The chart assumes shade and light wind, so a crew in direct sun feels more heat than the number says. NWS says “exposure to full sunshine can increase heat index values by up to 15°F.” Strong winds with very hot, dry air can also be dangerous.

OSHA’s heat hazard page gives a nearby figure: “Direct sunlight can increase Heat Index by up to 13.5°F (7.5°C).” The two agencies state the limit differently. Use the higher number when you plan for a crew on a roof, a slab or a road.

Can the heat index set work and rest cycles?

Not by itself. OSHA calls the heat index a screening tool. OSHA says the NIOSH/OSHA Heat App “uses the Heat Index, a screening tool” and does not replace a WBGT-based hazard assessment. OSHA also says “do not rely solely on the Heat Index reported by weather forecasts for your safety at work.”

OSHA’s water, rest and shade page says NIOSH and ACGIH give work and rest cycles as a function of workload and WBGT. Neither is a heat index table. We will not invent break minutes for you. Check the work/rest schedule entry and OSHA’s heat stress calculator, and read our WBGT vs heat index guide for the numbers OSHA does publish.

The rules that do apply on any heat index day are plain:

• OSHA says rest breaks should get longer and more frequent as heat stress rises.
• OSHA says “In hot conditions, skipping breaks is not safe!”
• OSHA says heavy and very heavy work “carry the highest risk of heat-related illness.”

How does workload change what the number means?

Hard work adds heat from inside the body. OSHA says two heat sources add up. Environmental heat comes from the surroundings. Metabolic heat is “generated by the body” and “related to workload.” To judge total heat stress, employers must assess both.

OSHA sorts work into light, moderate, heavy and very heavy. Driving a car or standing watch is light. Hammering nails or using hand tools is moderate. Shoveling, roofing, carrying loads and welding are heavy. Climbing ladders, stacking concrete and firefighting are very heavy. OSHA says heavy and very heavy work carry the highest risk of heat-related illness.

So the same weather asks more of a crew shoveling gravel than of a worker driving a truck. When in doubt, assume the higher workload.

What does the heat index leave out?

Four things that change how hot a crew feels. The heat index uses only temperature and humidity. OSHA says it “does not account for the effects of wind, sunlight, radiant heat sources, or workload.”

Weather reports also come from a station that may sit far from your site. They cannot see a trench, a hot roof or the inside of a building. OSHA’s hazard page names those cases and says to measure on site.

Why can a low heat index still be dangerous?

Because workload and acclimatization matter as much as the weather. OSHA reports that “Outdoor workers have died of heat stroke when the day’s maximum Heat Index was only 86°F.” OSHA also says less severe heat illness “can happen at even lower Heat Index values.”

New workers face the most risk. OSHA says to assume workers are unacclimatized if they have been on the job for less than one to two weeks. A new hire in a warm spell can get sick at a number that a seasoned crew handles. Our heat acclimatization schedule shows how to ramp up.

When is the heat index still useful?

Use it as a quick screen and a planning cue. A forecast heat index tells your lead what the day may bring before anyone arrives. NWS also publishes HeatRisk, a daily color and number for up to seven days ahead. NWS notes HeatRisk “is an experimental product and is not supported 24/7.”

Use these cues together:

1. Check the forecast heat index and NWS heat advisories the evening before.
2. Move strenuous work earlier or later in the day when you can. NWS lists this among OSHA’s protective measures.
3. Use work/rest schedules and relief workers when needed. NWS lists both as OSHA protective measures for outdoor work.
4. Confirm conditions on site with a WBGT meter when you have one.
5. Watch the crew. Symptoms count more than any chart.

What should a supervisor cover at the morning huddle?

Keep it to five points and say them out loud:

1. Today’s forecast heat index and any NWS heat advisory.
2. Who works in the sun, who works indoors and who wears heavy PPE.
3. When breaks happen and where the shade and water are.
4. Who is new or back from leave, and who is paired with them.
5. The 911 plan and who makes the call.

What should a crew do on a high heat index day?

Raise the controls and watch people closely. Start with Water. Rest. Shade. Add breaks as the day heats up, put new workers on a ramp-up schedule, and use a buddy system. Know your 911 steps before anyone gets sick.

Hydration is part of the plan. JMAR Solutions’ Rapid Restoration Drops are a dietary supplement that replaces electrolytes lost in sweat and supports hydration. They are unflavored, with no sugar and no calories. Learn how a program fits your crew on the hydration programs page, or start with our heat stress prevention pillar. If a worker shows confusion, slurred speech or collapse, call 911. That is a heat stroke emergency, not a break problem.

Next step

Want a simple heat-day routine for your crews? Book a 15-minute demo with JMAR Solutions, or ask for a free sample to try at your next toolbox talk.