--- title: "Heat Acclimatization Schedule: The NIOSH 20% Rule Explained" description: "Use NIOSH's 20% rule to ramp up new workers in the heat over 7 to 14 days. Get day-by-day tables for 8-hour and 10-hour shifts and the 50/60/80/100% plan." url: https://jmarsolutions.multisites.ai/guides/heat-acclimatization-schedule publisher: JMAR Solutions phone: +1-214-632-7780 quote: https://jmarsolutions.multisites.ai/contact --- Field guide # A heat acclimatization schedule for new and returning workers NIOSH 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. Updated November 5, 20266 min read ## 20% on day 1 New workers do no more than 20% of the usual heat exposure on their first day, then add 20% a day. ## 50/60/80/100 Experienced workers can step up through 50%, 60%, 80% and 100% on days 1 to 4. ## Cut time, not effort OSHA says to reduce the duration of work but not the intensity. ## Returning workers count OSHA says workers lose tolerance after leave, in winter and when the weather turns hot. ## What is heat acclimatization? Heat acclimatization is the body’s gradual adaptation to hot work. OSHA defines acclimatization as the body gradually adapting to tolerate higher levels of heat stress. NIOSH describes beneficial changes, such as increased sweating efficiency, after gradual exposure to a hot environment. OSHA’s page on new workers lists the changes. Acclimatized workers sweat at a higher rate, lose less salt in sweat, keep a lower body temperature and heart rate, and move more blood to the skin. Unacclimatized workers do none of this well. See the [acclimatization](https://jmarsolutions.multisites.ai/glossary/acclimatization) glossary entry for a short definition. ## Why does acclimatization matter so much for new workers? Because new workers are the ones who die. OSHA says “Almost half of heat-related deaths occur on a worker’s first day on the job or a worker’s first day back after an extended absence.” OSHA adds that over 70 percent of heat-related deaths occur during a worker’s first week. Fitness does not protect a new hire. OSHA says “Even young, seemingly “fit” workers can be susceptible to heat-related illness.” The risk also has a people side: new workers may push too hard, skip breaks or hide symptoms to avoid looking lazy. ## What is the 20 percent rule for new workers? Start at 20 percent of the normal time in the heat, then add 20 percent a day. OSHA and NIOSH recommend this for new workers. NIOSH says “No more than 20% of the usual duration of work in the heat on day 1,” with “No more than 20% increase on each additional day.” OSHA gives a worked example for an 8-hour day: new workers should work “no more than 1 hour and 40 minutes (20 percent of 8 hours) on their first day in the heat.” The rest of the day should be without heat stress. They should get at least one rest break during the heat period. Day Share of normal heat time Time on an 8-hour shift 1 20% 1 hour 40 minutes 2 40% 3 hours 12 minutes 3 60% 4 hours 48 minutes 4 80% 6 hours 24 minutes 5 100% 8 hours The 20 percent and the 1 hour 40 minutes figures come from NIOSH and OSHA. We did the math for days two through five. Many construction crews work 10-hour days. Apply the same percentages to the longer shift. This is our arithmetic on the NIOSH percentages: Day New workers (20% steps) Workers with experience (NIOSH steps) 1 2 hours (20%) 5 hours (50%) 2 4 hours (40%) 6 hours (60%) 3 6 hours (60%) 8 hours (80%) 4 8 hours (80%) 10 hours (100%) 5 10 hours (100%) Full schedule OSHA says new workers can spend the rest of the first-day shift without heat stress, so plan where the rest of each shift goes. OSHA says that by following the rule, new workers “will be working a full schedule by the end of their first week.” ## What schedule works for workers with heat experience? Experienced workers can ramp faster. NIOSH gives a four-day plan: 50 percent of the usual duration on day 1, 60 percent on day 2, 80 percent on day 3 and 100 percent on day 4. Use this plan only for people who have worked recently in the same kind of heat. Day New workers (NIOSH) Workers with previous experience (NIOSH) 1 No more than 20% No more than 50% 2 Add no more than 20% No more than 60% 3 Add no more than 20% No more than 80% 4 Add no more than 20% No more than 100% Days 5 to 14 Keep adding up to full time; closely supervise through day 14 Full schedule NIOSH says the whole process takes 7 to 14 days. It adds that “Workers who are not physically fit need more time to fully acclimatize.” OSHA says the 20 percent rule should protect most workers who are physically fit and have no medical problems, and some need up to 14 days. When in doubt, give workers more days. ## Who else needs a ramp-up besides new hires? Anyone whose heat tolerance has dropped. OSHA lists four groups: new, temporary or existing workers who start new work in warm or hot places, workers returning after an absence of one week or more, workers working through spring and early summer as heat first rises, and workers on days much warmer than the days before. OSHA says workers can lose heat tolerance during vacation, sick leave and winter. For them, use the same steps as for new workers and keep the added protections for at least one week. OSHA’s planning page adds that temporary workers may need closer supervision, and that people back from leave of more than two weeks may be at higher risk. Our [acclimatizing new hires in Texas](https://jmarsolutions.multisites.ai/blog/acclimatizing-new-hires-texas-summer) post puts this into a summer start plan. ## What should the work look like during the ramp-up? Keep the work real but short. OSHA says to “reduce the duration of the work but not the intensity of the work.” A new brick layer should lay bricks during the first week, because light work may not prepare the body for the job. During the ramp-up: 1. Give longer, more frequent rest breaks. NIOSH says new and unacclimatized workers need longer, more frequent rest periods. 2. Use a [buddy system](https://jmarsolutions.multisites.ai/glossary/buddy-system). Do not let new workers work alone. 3. Teach the signs of heat illness and the importance of rest and water. Use a [work/rest schedule](https://jmarsolutions.multisites.ai/glossary/work-rest-schedule) for the heat hours. 4. Let any worker with symptoms stop work, start first aid and never leave them alone. 5. Watch for the lag effect. OSHA warns that unacclimatized workers who feel fine on day one can get sick on a later day. ## What mistakes slow down a safe ramp-up? Most mistakes come from pressure to look tough. OSHA says “The first days of a new job or a new work activity can contain unique psychological and behavioral pressures.” New workers may push too hard to prove themselves, skip breaks so no one thinks they are lazy, or not know who to tell when they feel bad. Watch for these on your site: - A new hire who refuses a break. Make the break part of the schedule, so no one has to ask. - A crew that races to keep up with a veteran. Pair new hires with a calm buddy. - A supervisor who skips the ramp-up in a rush. Put the schedule in the daily plan. - A returning worker who is treated like a veteran after a long absence. - No one on site knows the 911 steps. Keep a simple log: worker name, day number, hours in the heat, breaks taken and symptoms reported. It takes a minute and shows the plan ran. ## Does air conditioning or a few days off undo acclimatization? Not quickly. NIOSH says “Acclimatization can be maintained for a few days of non-heat exposure.” It also says “Taking breaks in air conditioning will not affect acclimatization.” So cool break areas are safe for the plan. Longer breaks matter. After an absence of a week or more, treat the worker as unacclimatized and restart a ramp-up. OSHA says to assume workers are unacclimatized if they have been doing the job for less than one to two weeks. ## How do hydration and electrolytes fit into acclimatization? Water stays on every day of the ramp-up. Acclimatized workers lose less salt in sweat, but they still sweat, and new workers have not adapted yet. OSHA says water cannot replace electrolytes and recommends electrolyte drinks on jobs longer than two hours. JMAR Solutions’ Rapid Restoration Drops are a dietary supplement that replaces electrolytes lost in sweat and supports hydration. They do not replace a ramp-up schedule, rest or shade. JMAR’s [hydration programs](https://jmarsolutions.multisites.ai/hydration-programs) include onsite toolbox talks and coaching for new-hire heat starts. For a full program, see our [heat stress program guide](https://jmarsolutions.multisites.ai/guides/heat-stress-program-for-employers) and the [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. ## Next step Want to roll out a new-hire heat plan this season? [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) for your first toolbox talk. ## Frequently asked questions ### What is heat acclimatization? It is the body's gradual adaptation to heat. NIOSH says workers sweat more efficiently after gradual increased exposure to a hot environment. OSHA says acclimatized workers sweat more, lose less salt in sweat, and keep a lower body temperature and heart rate when they work. ### What is the 20 percent rule? OSHA and NIOSH recommend it for new workers. On day one, work no more than 20 percent of the normal time in the heat. Add no more than 20 percent each day after. On an 8-hour day, that is 1 hour 40 minutes on day one. ### How long does heat acclimatization take? NIOSH says to gradually increase time in hot conditions over 7 to 14 days and to closely supervise new employees for the first 14 days or until they are fully acclimatized. OSHA says some workers need up to 14 days, and more time is fine when in doubt. ### Do returning workers need to acclimatize again? Yes. OSHA says workers can lose heat tolerance during an extended absence and during winter. They are at higher risk after an absence of one week or more, when spring heat starts, and when the weather turns much warmer than the days before. ### Can a worker acclimatize by doing light work? Not well. OSHA says new workers should do job tasks similar in intensity to their expected work, such as laying bricks for a brick layer. Reduce how long they work in the heat, not how hard they work. ## 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) - [Electrolyte](https://jmarsolutions.multisites.ai/glossary/electrolyte) - [Heat stress](https://jmarsolutions.multisites.ai/glossary/heat-stress) - [Heat stroke](https://jmarsolutions.multisites.ai/glossary/heat-stroke) - [Hydration](https://jmarsolutions.multisites.ai/glossary/hydration) - [NIOSH](https://jmarsolutions.multisites.ai/glossary/niosh) ## Sources 1. [CDC/NIOSH: Heat stress workplace recommendations](https://www.cdc.gov/niosh/heat-stress/recommendations/index.html) 2. [CDC/NIOSH: Heat stress workplace recommendations](https://www.cdc.gov/niosh/heat-stress/recommendations/index.html) 3. [CDC/NIOSH: Heat stress workplace recommendations](https://www.cdc.gov/niosh/heat-stress/recommendations/index.html) 4. [CDC/NIOSH: Heat stress workplace recommendations](https://www.cdc.gov/niosh/heat-stress/recommendations/index.html) 5. [CDC/NIOSH: Heat stress workplace recommendations](https://www.cdc.gov/niosh/heat-stress/recommendations/index.html) 6. [CDC/NIOSH: Heat stress workplace recommendations](https://www.cdc.gov/niosh/heat-stress/recommendations/index.html) 7. [CDC/NIOSH: Heat stress workplace recommendations](https://www.cdc.gov/niosh/heat-stress/recommendations/index.html) 8. [CDC/NIOSH: Heat stress workplace recommendations](https://www.cdc.gov/niosh/heat-stress/recommendations/index.html) 9. [CDC/NIOSH: Heat stress workplace recommendations](https://www.cdc.gov/niosh/heat-stress/recommendations/index.html) 10. [CDC/NIOSH: Heat stress workplace recommendations](https://www.cdc.gov/niosh/heat-stress/recommendations/index.html) 11. [CDC/NIOSH: Heat stress workplace recommendations](https://www.cdc.gov/niosh/heat-stress/recommendations/index.html) 12. [CDC/NIOSH: Heat stress workplace recommendations](https://www.cdc.gov/niosh/heat-stress/recommendations/index.html) 13. [CDC/NIOSH: Heat stress workplace recommendations](https://www.cdc.gov/niosh/heat-stress/recommendations/index.html) 14. [OSHA: Protecting new workers from heat](https://www.osha.gov/heat-exposure/protecting-new-workers) 15. [OSHA: Protecting new workers from heat](https://www.osha.gov/heat-exposure/protecting-new-workers) 16. [OSHA: Protecting new workers from heat](https://www.osha.gov/heat-exposure/protecting-new-workers) 17. [OSHA: Protecting new workers from heat](https://www.osha.gov/heat-exposure/protecting-new-workers) 18. [OSHA: Protecting new workers from heat](https://www.osha.gov/heat-exposure/protecting-new-workers) 19. [OSHA: Protecting new workers from heat](https://www.osha.gov/heat-exposure/protecting-new-workers) 20. [OSHA: Protecting new workers from heat](https://www.osha.gov/heat-exposure/protecting-new-workers) 21. [OSHA: Protecting new workers from heat](https://www.osha.gov/heat-exposure/protecting-new-workers) 22. [OSHA: Protecting new workers from heat](https://www.osha.gov/heat-exposure/protecting-new-workers) 23. [OSHA: Protecting new workers from heat](https://www.osha.gov/heat-exposure/protecting-new-workers) 24. [OSHA: Protecting new workers from heat](https://www.osha.gov/heat-exposure/protecting-new-workers) 25. [OSHA: Protecting new workers from heat](https://www.osha.gov/heat-exposure/protecting-new-workers) 26. [OSHA: Protecting new workers from heat](https://www.osha.gov/heat-exposure/protecting-new-workers) 27. [OSHA: Protecting new workers from heat](https://www.osha.gov/heat-exposure/protecting-new-workers) 28. [OSHA: Protecting new workers from heat](https://www.osha.gov/heat-exposure/protecting-new-workers) 29. [OSHA: Heat planning and supervision](https://www.osha.gov/heat-exposure/planning) 30. [OSHA: Heat planning and supervision](https://www.osha.gov/heat-exposure/planning) 31. [OSHA: Protecting new workers from heat](https://www.osha.gov/heat-exposure/protecting-new-workers) 32. [OSHA: Protecting new workers from heat](https://www.osha.gov/heat-exposure/protecting-new-workers) 33. [OSHA: Protecting new workers from heat](https://www.osha.gov/heat-exposure/protecting-new-workers) 34. [OSHA: Protecting new workers from heat](https://www.osha.gov/heat-exposure/protecting-new-workers) 35. [OSHA: Protecting new workers from heat](https://www.osha.gov/heat-exposure/protecting-new-workers) 36. [OSHA: Heat hazard recognition](https://www.osha.gov/heat-exposure/hazards) 37. [OSHA: Water. Rest. Shade.](https://www.osha.gov/heat-exposure/water-rest-shade) 38. [OSHA: Water. Rest. Shade.](https://www.osha.gov/heat-exposure/water-rest-shade) 39. [OSHA: Protecting new workers from heat](https://www.osha.gov/heat-exposure/protecting-new-workers) 40. [OSHA: Water. Rest. Shade.](https://www.osha.gov/heat-exposure/water-rest-shade) 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 - [Acclimatizing New Hires for a Texas Summer StartNew workers should do 20 percent of a normal heat shift on day one, then add 20 percent each day. OSHA and NIOSH recommend this rule of 20 percent. Experienced workers new to a site start at 50 percent. Almost half of heat-related deaths happen on a worker's first day.](https://jmarsolutions.multisites.ai/blog/acclimatizing-new-hires-texas-summer) - [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) - [Build a heat stress program in 8 stepsA heat stress program has eight parts: name a daily lead, measure heat on site, set advisory-day triggers, give water, rest and shade, acclimatize workers, train everyone, plan first aid and 911, then review. OSHA says employers should write the plan down. This follows OSHA and NIOSH guidance, not a final federal rule.](https://jmarsolutions.multisites.ai/guides/heat-stress-program-for-employers) - [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) - [Heat illness: signs, first aid and when to call 911Heat stroke is an emergency. Call 911, stay with the worker, move them to shade and cool them fast. NIOSH says body temperature can reach 106°F within 10 to 15 minutes. Heat exhaustion needs medical evaluation. Heat cramps, heat rash, fainting and rhabdo each have their own signs and steps. Know them before the shift.](https://jmarsolutions.multisites.ai/heat-illness) - [A jobsite hydration plan that crews followNIOSH says workers doing moderate activity in the heat for under 2 hours should drink 1 cup (8 oz) of water every 15 to 20 minutes. If sweating lasts several hours, NIOSH says to drink sports drinks with balanced electrolytes. Fluid intake generally should not exceed 6 cups per hour.](https://jmarsolutions.multisites.ai/guides/jobsite-hydration-plan) ![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)