--- title: "Heat Index Work Rest Chart: How to Read It on a Jobsite" description: "Learn to read the NWS heat index chart (96F at 65% humidity feels like 121F), where it falls short for work/rest cycles, and what OSHA says to use instead." url: https://jmarsolutions.multisites.ai/guides/heat-index-work-rest-chart publisher: JMAR Solutions phone: +1-214-632-7780 quote: https://jmarsolutions.multisites.ai/contact --- Field guide # How to read a heat index chart for crew work and rest A heat index chart shows how hot it feels when humidity is added to air temperature. The NWS example: 96°F with 65% humidity feels like 121°F. The chart assumes shade and light wind, so full sun can add up to 15°F. OSHA calls the heat index a screening tool and prefers WBGT for work and rest decisions. Updated November 5, 20266 min read ## The NWS example 96°F air at 65% humidity gives a heat index of 121°F. ## Shade only The chart assumes shade and light wind. NWS says full sun can add up to 15°F. ## Screening, not a schedule OSHA calls the heat index a screening tool and prefers WBGT for work and rest decisions. ## Low numbers still hurt OSHA notes outdoor workers died of heat stroke when the day's highest heat index was only 86°F. ## 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](https://jmarsolutions.multisites.ai/glossary/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. Step NWS example Air temperature 96°F Relative humidity 65% Heat index (how hot it feels) 121°F Red area with no numbers Extreme danger 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](https://jmarsolutions.multisites.ai/glossary/wbgt). Neither is a heat index table. We will not invent break minutes for you. Check the [work/rest schedule](https://jmarsolutions.multisites.ai/glossary/work-rest-schedule) entry and OSHA’s heat stress calculator, and read our [WBGT vs heat index guide](https://jmarsolutions.multisites.ai/guides/wbgt-vs-heat-index) 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.” Factor In the heat index? Why it matters on a jobsite Air temperature Yes Sets the baseline Humidity Yes Slows sweat evaporation Sunlight No Direct sun adds up to 13.5°F to 15°F Wind No Moving air can cool a crew, or add danger when very hot and dry Radiant heat No Roofs, roads, furnaces and ovens add heat Workload No Heavy work makes the body produce more heat 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](https://jmarsolutions.multisites.ai/guides/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](https://jmarsolutions.multisites.ai/hydration-programs) page, or start with our [heat stress prevention pillar](https://jmarsolutions.multisites.ai/heat-stress-prevention). If a worker shows confusion, slurred speech or collapse, call 911. That is a [heat stroke](https://jmarsolutions.multisites.ai/glossary/heat-stroke) emergency, not a break problem. ## Next step Want a simple heat-day routine for your crews? [Book a 15-minute demo](https://calendly.com/jen-jmarsolutions/15-min-meeting) with JMAR Solutions, or ask for a [free sample](https://jmarsolutions.multisites.ai/free-sample) to try at your next toolbox talk. ## Frequently asked questions ### What does a heat index of 121 mean? It means the air feels like 121°F to a person at rest in the shade. The NWS example is 96°F air at 65% relative humidity. In full sun, NWS says the value can run up to 15°F higher. Treat it as an extreme danger signal for any crew. ### Does OSHA publish a heat index work rest chart? OSHA does not publish a minutes-per-hour table by heat index. OSHA's page says NIOSH and ACGIH give work and rest cycles by workload and WBGT. The NIOSH/OSHA Heat App uses heat index as a screening tool and shows workload guidance. ### Is the heat index or WBGT better for a jobsite? WBGT is better. OSHA says workplace heat should be measured on site with WBGT meters, and calls heat index a less desirable substitute. WBGT counts sun, wind and radiant heat. Heat index counts only temperature and humidity. ### At what heat index should work stop? No single heat index number is a safe cutoff. OSHA reports outdoor workers died of heat stroke when the day's maximum heat index was only 86°F. Use WBGT, workload, acclimatization and crew symptoms to set breaks. ### How much does sun add to the heat index? NWS says full sunshine can raise heat index values by up to 15°F. OSHA's heat page says direct sunlight can add up to 13.5°F. Both agree that a forecast heat index in the shade understates what a crew in the sun feels. ## Terms on this page - [Work/rest schedule](https://jmarsolutions.multisites.ai/glossary/work-rest-schedule) - [Acclimatization](https://jmarsolutions.multisites.ai/glossary/acclimatization) - [Buddy system](https://jmarsolutions.multisites.ai/glossary/buddy-system) - [Toolbox talk](https://jmarsolutions.multisites.ai/glossary/toolbox-talk) - [Heat stress](https://jmarsolutions.multisites.ai/glossary/heat-stress) - [Heat stroke](https://jmarsolutions.multisites.ai/glossary/heat-stroke) - [Heat index](https://jmarsolutions.multisites.ai/glossary/heat-index) - [Hydration](https://jmarsolutions.multisites.ai/glossary/hydration) - [HeatRisk](https://jmarsolutions.multisites.ai/glossary/heatrisk) - [NIOSH](https://jmarsolutions.multisites.ai/glossary/niosh) ## Sources 1. [NWS: Heat forecast tools](https://www.weather.gov/safety/heat-tools) 2. [NWS: Heat forecast tools](https://www.weather.gov/safety/heat-tools) 3. [NWS: Heat forecast tools](https://www.weather.gov/safety/heat-tools) 4. [NWS: Heat forecast tools](https://www.weather.gov/safety/heat-tools) 5. [NWS: Heat forecast tools](https://www.weather.gov/safety/heat-tools) 6. [OSHA: Heat hazard recognition](https://www.osha.gov/heat-exposure/hazards) 7. [OSHA: Heat hazard recognition](https://www.osha.gov/heat-exposure/hazards) 8. [OSHA: Heat hazard recognition](https://www.osha.gov/heat-exposure/hazards) 9. [OSHA: Heat hazard recognition](https://www.osha.gov/heat-exposure/hazards) 10. [OSHA: Heat hazard recognition](https://www.osha.gov/heat-exposure/hazards) 11. [OSHA: Heat hazard recognition](https://www.osha.gov/heat-exposure/hazards) 12. [OSHA: Heat hazard recognition](https://www.osha.gov/heat-exposure/hazards) 13. [OSHA: Heat hazard recognition](https://www.osha.gov/heat-exposure/hazards) 14. [OSHA: Water. Rest. Shade.](https://www.osha.gov/heat-exposure/water-rest-shade) 15. [OSHA: Heat hazard recognition](https://www.osha.gov/heat-exposure/hazards) 16. [OSHA: Heat overview, working in outdoor and indoor heat environments](https://www.osha.gov/heat-exposure) 17. [NWS: Heat forecast tools](https://www.weather.gov/safety/heat-tools) 18. [NWS: Heat forecast tools](https://www.weather.gov/safety/heat-tools) 19. [NWS: Heat forecast tools](https://www.weather.gov/safety/heat-tools) 20. [NWS: Heat forecast tools](https://www.weather.gov/safety/heat-tools) 21. [NWS: Heat forecast tools](https://www.weather.gov/safety/heat-tools) 22. [NWS: Heat forecast tools](https://www.weather.gov/safety/heat-tools) 23. [OSHA: Heat hazard recognition](https://www.osha.gov/heat-exposure/hazards) 24. [OSHA: Water. Rest. Shade.](https://www.osha.gov/heat-exposure/water-rest-shade) 25. [OSHA: Water. Rest. Shade.](https://www.osha.gov/heat-exposure/water-rest-shade) 26. [OSHA: Heat hazard recognition](https://www.osha.gov/heat-exposure/hazards) 27. [OSHA: Heat hazard recognition](https://www.osha.gov/heat-exposure/hazards) 28. [OSHA: Heat overview, working in outdoor and indoor heat environments](https://www.osha.gov/heat-exposure) 29. [OSHA: Heat hazard recognition](https://www.osha.gov/heat-exposure/hazards) 30. [OSHA: Heat hazard recognition](https://www.osha.gov/heat-exposure/hazards) 31. [OSHA: Heat hazard recognition](https://www.osha.gov/heat-exposure/hazards) Checked against the sources above on November 5, 2026. Rules and guidance change, so check the linked page before you act. This page is general information, not legal or medical advice. ## Keep reading - [Heat stress prevention for high-risk crewsHeat stress prevention starts with water, rest and shade, then adds acclimatization for new workers, supervisor training and a written plan. NIOSH says to give workers cool water near the work area and to build up heat exposure over 7 to 14 days. Electrolytes support hydration when a crew sweats for hours.](https://jmarsolutions.multisites.ai/heat-stress-prevention) - [WBGT vs heat index: pick the right heat reading for your crewWBGT is the better heat reading for crews. It combines temperature, humidity, sunlight and air movement. The heat index uses only temperature and humidity, measured in the shade. OSHA recommends WBGT meters, calls the heat index a less desirable substitute, and says OSHA uses WBGT to decide whether a heat hazard was present.](https://jmarsolutions.multisites.ai/guides/wbgt-vs-heat-index) - [A heat acclimatization schedule for new and returning workersNIOSH says to build heat tolerance over 7 to 14 days. New workers do no more than 20% of the usual time in the heat on day 1, then add no more than 20% each day. Experienced workers can do 50%, 60%, 80%, then 100% on days 1 through 4. Cut the time, not the work intensity.](https://jmarsolutions.multisites.ai/guides/heat-acclimatization-schedule) - [What OSHA expects on heat illness prevention todayAs of November 2026, there is no final federal OSHA heat standard. The proposed rule is still in rulemaking. OSHA enforces heat hazards today under the General Duty Clause and its revised Heat National Emphasis Program, effective April 10, 2026. Employers should write a heat plan, train crews, give water, rest and shade, and acclimatize new workers.](https://jmarsolutions.multisites.ai/guides/osha-heat-illness-prevention) - [OSHA heat rules: what applies to your crew todayAs of November 2026, OSHA has no final federal heat standard. A heat rule is proposed, not in force. OSHA enforces heat hazards today under the General Duty Clause and its Heat National Emphasis Program, revised April 10, 2026. Texas has no state heat standard, and HB 2127 ended local water-break rules.](https://jmarsolutions.multisites.ai/osha-heat-rules) - [Texas heat rules for employers, as of November 2026As of November 2026, Texas has no state heat standard, and there is no final federal OSHA heat standard. Texas HB 2127 ended local water-break rules in Austin and Dallas. Federal OSHA still enforces heat hazards under the General Duty Clause and a revised Heat National Emphasis Program from April 10, 2026.](https://jmarsolutions.multisites.ai/guides/texas-heat-rules-for-employers) ![Ironworkers silhouetted on a steel frame against a hot orange sunset](https://jmarsolutions.multisites.ai/_astro/sunset-steel.IhAMDca0_6wk53.webp) ## See it on your crew in 15 minutes Jennifer walks your safety team through the drops, the protocol and the numbers on a short live call. [Book a 15-minute demo](https://calendly.com/jen-jmarsolutions/15-min-meeting)