What is Acclimatization?

According to the National Institute for Occupational Safety and Health (NIOSH), “Acclimatization is the beneficial physiological adaptations that occur during repeated exposure to a hot environment. These adaptations include improved sweating efficiency, stabilization of circulation, and the ability to perform work with lower core temperature and heart rate."

In simpler terms, acclimatization is the body’s process of becoming accustomed to a new climate or to new conditions—in this case, it refers to the process of a worker’s body adjusting to working in the heat and building up their heat tolerance.

Acclimatization is the body’s process of gradually adapting to and tolerating higher levels of heat stress. This natural process helps workers to better maintain a lower core body temperature and lower heart rate as they work in the heat.

This process is critical for preventing heat stress and other more serious heat-related illnesses like heat stroke. Having an established acclimatization schedule is critical for new workers who are not accustomed to exerting themselves in the heat.

What industries may need to acclimatize workers?

Acclimatization is not a process for outdoor workers only. There are numerous indoor occupations that may also require this process to keep workers safe and productive.

Outdoors

  • Agriculture
  • Construction—particularly road work and roof work
  • Landscaping
  • Mail and package delivery
  • Oil and gas well operations

Indoors

  • Bakeries, kitchens, and laundries i.e. workplaces with indoor heat-generating appliances
  • Electrical utilities, especially boiler rooms
  • Fire service
  • Iron and steel mills and foundries
  • Manufacturing applications with hot, local heat sources like furnaces
  • Warehousing

What Are the Steps for Acclimatization?

NIOSH outlines structured acclimatization protocols to reduce the risk of heat-related illnesses for workers exposed to hot environments. NIOSH recommends employers follow this acclimatization schedule:

1. For New Workers (No Previous Heat Exposure)

  • Schedule: Start with no more than 20% of the usual work duration in the heat on day one. (20% first day)
  • Time Increase: Increase time in the hot environment by no more than 20% per day until the full duration is reached. (20% each additional day)
  • Example: If the routine workday is 8 hours, then a new employee should not work more than 1 hour and 40 minutes on their first day, which is 20% of 8 hours. A worker should have at least one rest break during the working period. With this schedule, a new worker will work a full schedule by the end of the first week. However, workers with medical problems or a lower physical fitness level may need the full 14 days to adapt.

2. For Experienced Workers (Returning after Absence of ≥4 Days)

  • Schedule: Start with 50% of the usual work duration on day one.
  • Time Increase: Progress to 60% on day two, 80% on day three, and 100% on day four.

3. General Guidelines

  • Full acclimatization typically takes 7–14 days.
  • Acclimatization can be lost after one week of no heat exposure but is regained more quickly.
  • Short-term breaks in heat exposure (e.g., weekends) generally have minimal impact.

What are the Benefits of Acclimatization for Workers?

Acclimatization significantly improves workers’ health and performance under heat stress. This process drastically helps reduce the possibility of heat-related illnesses while also improving workers’ productivity in hot environments.

Benefits include the following:

  • Lower core body temperature during work
  • Lower heart rate and improved cardiovascular stability
  • Increased sweating efficiency, leading to better thermoregulation
  • Fewer heat-related illnesses, including heat exhaustion or heat stroke
  • More blood flows to the skin, which offers more efficient cooling through the skin
  • Improved comfort, endurance, and productivity in hot conditions
  • The body is more efficient at getting rid of heat via a slower heart rate and slower body temperature increase
  • Sweat contains less salt loss which provides better hydration, improved electrolyte balance, and a lesser likelihood of heat cramps

Statistics Related to Acclimatization

While there are limited statistics solely focused on acclimatization in occupational settings, there are several studies and surveillance data sets that highlight the risks of inadequate acclimatization and its association with heat-related illnesses and fatalities. Here’s a summary of key findings and relevant statistics:

1. NIOSH/OSHA Review of Heat-Related Fatalities

A 2014 OSHA review of heat-related fatalities showed that:

  • Most outdoor fatalities, 50% to 70%, occur in the first few days of working in warm or hot environments because the body needs to build a tolerance to the heat gradually over time.

2. Timeframe of Fatalities

  • 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.
  • Most heat-related deaths occurred within the first 3 days of working in a hot environment.
  • Over 70 percent of heat-related deaths occur during a worker's first week.

3. Lack of Acclimatization as a Leading Cause

  • NIOSH identified that lack of acclimatization is the most common risk factor for heat illness in incident investigations involving hot environments.

4. BLS Data on Heat-Related Illness

  • According to the Bureau of Labor Statistics (BLS):
    • In 2020, there were 798 cases of occupational heat-related illness causing days away from work. This is likely an undercount, as many heat-related symptoms go unreported or are misclassified.

Acclimatization Guidance for Employers

Employers play a crucial role in implementing acclimatization programs to protect new workers and those returning after extended periods. New workers should work shorter periods in the heat, separated by more frequent rest breaks. A buddy system should also be implemented so new workers can be monitored for signs or symptoms of heat stress.

Increased precautions for new works should be implemented for 1-2 weeks. After that period, new works should be safely acclimatized to the heat to work a regular schedule.

Lack of acclimatization can be a problem for experienced workers in the following scenarios:

  • Returning to warmer work environments after an absence of one week or more
  • After winter months when temperatures are much cooler, the body may need to be re-acclimatized as temperatures warm.
  • When temperatures begin to rise in the spring or early summer
  • When weather is significantly warmer than on previous days, such as during a heat wave or heat dome

Employers should consider the following guidance to maintain worker safety and productivity.

1. Develop a Formal Acclimatization Plan

  • Include protocols for both new hires and returning workers.
  • Ensure plans are part of the company’s heat stress prevention program.

2. Monitor Workers Closely

  • Observe for signs of heat illness, especially during the acclimatization period.
  • Encourage buddy systems and supervision for early symptom recognition.

3. Schedule Work Appropriately

  • Modify work/rest cycles, especially during the first week.
  • Avoid scheduling intense tasks during the hottest part of the day.

4. Provide Training

  • Educate workers and supervisors on:
    • The importance of acclimatization
    • Symptoms of heat-related illnesses
    • Emergency procedures

5. Provide Water and Rest Breaks

  • Ensure access to cool drinking water and shaded rest areas.
  • Encourage frequent breaks during the acclimatization period.

6. Adjust PPE and Clothing Requirements

  • Where possible, modify or rotate tasks to reduce heat retention from protective clothing.

Acclimatization is a critical process across numerous industries for both indoor and outdoor work. It not only applies to new workers, but also to those with years of experience of working in the heat. Factors such as weight loss, weight gain, and the development of new medical conditions can also affect workers heat tolerance. All “hot work” employees should be properly trained on the signs and symptoms of heat stress to stay safe day in and day out.