heat stress

Acclimatization

New employees are at the highest risk for heat illness. 

Acclimatization is crucial to working in hot conditions.

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 performed 

in a hot setting gradually over a period of 

1–2 weeks.

• You begin to lose your acclimatization 

after about 1 week away from working in 

the heat.

Photo by NIOSH

What are the benefits of heat acclimatization?

• Heat exposure causes less strain to the heart and other vital organs.

• Sweating improves (higher volume, earlier onset), which cools the

body more quickly. Acclimatized workers need more water—not less—

due to increased sweating.

• 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.

• Pushing to the point of heat exhaustion will hurt, not help, your heat tolerance.

• Typically, acclimatization requires at least two hours of heat exposure per day (which can be broken into

two, 1-hour periods).

• The body will acclimatize to the level of work demanded of it. Simply being in a hot place is not sufficient.

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 will be maintained for a few days after heat exposure stops, but will begin to be lost after

about 1 week away from working in the heat.

• 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.
heat stress Acclimatization

Sample Acclimatization Schedule

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

NIOSH Acclimatization Recommendations 

for Workers with Previous Experience* with 

the Same Job 

1st day 50% usual work duration

2nd day 60% usual work duration

3rd day 80% usual work duration

4th day 100% usual work duration

*Workers returning from an absence

• Managers should have a formal acclimatization plan for 

employees working in the heat. Starting new employees at 

full intensity is not safe. 

• Adjustments to the acclimatization schedule may be 

needed depending on the worksite’s situation and on 

individual factors.

• Most workers should be able to safely handle a full 

workload after 4 days of gradual increase, even though 

they will usually not be fully acclimatized yet. Most 

people will continue to see beneficial improvements in 

heat tolerance for up to 2 weeks after exposure starts.

• Sudden shifts in work intensity or sudden increases in 

environmental temperature can increase the risk for heat 

illness even for acclimatized workers.

Case Study: Hazards of Not Acclimatizing Workers 

A 41-year-old construction worker was sawing boards in 93 °F 

heat. At 5 p.m., the worker collapsed in the parking lot. He was 

found by another employee. His body temperature was recorded 

at 108 °F when he was admitted to the hospital. He died the next 

day. 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.

• Deaths from heat stress often occur during the first few days 

on the job.

• Employers should have heat stress policies, and should implement acclimatization plans. 

Photo by ©Thinkstock

DEPARTMENT OF HEALTH AND HUMAN SERVICES

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health

DHHS (NIOSH) Publication No. 2017-124