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Heat Stress: Acclimatization


Acclimatization is crucial to working in hot conditions. Everyone can improve their heat tolerance through proper acclimatization.

Things you need to know:

  • Heat acclimatization is the improvement in heat tolerance that comes from gradually increasing the intensity or duration of work performed in a hot setting.
  • The best way to acclimatize yourself to the heat is to increase the workload gradually over a period of 1–2 weeks.
  • You begin to lose your acclimatization after about 1 week away from working in the heat.

What are the benefits of heat acclimatization?

  • Work is performed at a lower core body temperature and heart rate.
  • Heat exposure causes less strain to the heart and other vital organs.
  • Improved sweating (higher volume, earlier onset) cools the body more quickly.
    • Acclimatized workers need more water—not less—due to increased sweating.
    • "Toughening up" workers to require less water in heat stress conditions is not possible.
  • Sweat has decreased salt content with less electrolyte loss.
  • Workers increase their ability to comfortably perform physical tasks in the heat.


Tips for acclimatization

  • Best results will come from gradually increasing work time in hot conditions over a period of 7 to 14 days, and cooling off and fully rehydrating between shifts.
  • Typically, acclimatization requires at least two hours of heat exposure per day (which can be broken into two, 1-hour periods).
  • Full acclimatization requires physical work in the heat. Simply being in a hot place is not sufficient.
  • The body will acclimatize to the level of work demanded of it. Doing light or brief physical work in the heat will acclimatize you ONLY to light, brief work. More strenuous or longer tasks require more acclimatization.
  • Stay hydrated! Dehydration reduces the benefits of heat acclimatization.
  • Eating regular meals aids acclimatization. Food replaces electrolytes lost in sweat, especially during the first few days of acclimatization, when you lose the most salt in sweat.
  • Physical fitness aids acclimatization.


How quickly does the body LOSE heat tolerance after acclimatization?

  • Acclimatization can be maintained for a few days after heat exposure stops.
  • Absence from work in the heat for a week or more causes significant loss of acclimatization.
  • After 1 month away from work in the heat, most people’s heat tolerance will have returned to baseline.
  • Working for 1–2 days in cooler conditions or taking breaks in air conditioning will not hurt acclimatization.
NIOSH Acclimatization
Recommendations for New Workers 
1st day 20% usual work duration
2nd day 40% usual work duration
3rd day 60% usual work duration
4th day 80% usual work duration
5th day 100% usual work duration


Sample Acclimatization Schedule

  • Managers should have a formal acclimatization plan for employees working in the heat. Starting workers at full intensity when they are initially hired is not safe. Deaths from heat stress often occur during the first few days on the job.
  • An IDEAL acclimatization schedule would give workers 2 weeks to build up to full-intensity work in the heat. However, for many work sites this is not practical. For this reason, NIOSH provides a 1-week sample acclimatization schedule, which is better than no acclimatization and more likely to be used. Keep in mind that most workers will continue to see beneficial improvements for 2 weeks or longer after exposure starts.
  • Adjustments to an acclimatization schedule may be needed depending on the worksite situation and on individual factors.
    • Depending on age, physical fitness, medications, health conditions, and natural heat tolerance, some people may need longer to safely work long, physically intense shifts in the heat
  • Fully-acclimatized workers who experience a break of 1 week or more from working in the heat can regain heat tolerance faster than workers who are getting acclimatized for the first time.
  • Sudden shifts in season (for example, from traveling to a new area) or sudden increases in environmental temperatures may cause difficulties, even for acclimatized workers.


Case Study: Hazards of Not Acclimatizing Workers

construction worker wiping off sweatA 41-year-old construction worker was sawing boards to make concrete forms. The highest air temperature that day was 93 °F.
At 5 p.m., the worker collapsed in the parking lot. He was found 30 minutes later by another employee, who reported the situation to a supervisor. The receptionist called 911 while the supervisor administered emergency care to the collapsed worker. The worker’s body temperature was recorded as 107 °F by medical services, and was 108 °F when he was admitted to the hospital. The worker died the next day from heat stroke. At the time of the incident, the employee had been working for the company for one day. The company had no formal heat stress policy or acclimatization plan.

Lessons Learned
  • Heat casualties often occur with new or less experienced employees.
  • Employers should have heat stress policies, including acclimatization plans.