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Preventing and Responding to Heat Illness

Best Practises - Your Written Program

I. Effective Work Practices

5. Acclimatization

Cal/OSHA investigations (Study 1) showed that in 2005:

  • 46% of the reported cases of heat illness occurred on the employee's first day on the job.
  • 80% of the reported cases of heat illness occurred within the first four days of employment.

Acclimatization means temporary adaptation of the body to work in the heat that occurs gradually when a person is exposed to it. Acclimatization peaks in most people within four to fourteen days of regular work for at least two hours per day in the heat. In fully acclimatized individuals, sweating starts faster and the sweat carries less salt and other minerals out of the body. As a result, by sweating more efficiently the body cools down faster. Also there is less demand on the heart and cardiovascular system. For the reasons given above, being fully acclimatized can allow workers to continue working in warm or hot conditions and decreases the risk of heat illness and unsafe acts.

In general, physically fit individuals become acclimatized about 50% faster than those individuals who are not physically fit. Overweight individuals may retain more body heat and therefore may be more prone to developing heat illness.


A heat wave is a sudden and temporary rise of temperature above the seasonal average. During a heat wave even previously acclimatized employees are at risk for heat illness because the body has not had enough time to adjust to a sudden, abnormally high temperature or other extreme conditions.

Smart Tips

To minimize the risk of heat illness, encourage employees to report to their supervisors if they have:

  • Returned to work after an absence
  • Recently been working in cool climate and are just now starting work in a warm or hot climate
  • Had a change in their work activities, locations or conditions

Any of the above mentioned circumstances may mean that employees are not acclimatized to working in warm or hot environments, and are at greater risk for heat illness.

Make Sure Employees Are Trained, Monitored and Tracked Closely

Remember that employees who are not acclimatized to working in the heat are at greater risk for developing heat illness. Un-acclimatized employees should not work alone. It is important to:

  • Train employees and supervisors on the importance of recognizing and reporting heat illness symptoms
  • Have supervisors and coworkers use a "buddy system" to watch each other closely for discomfort or signs of heat illness
  • Set-up a system to account for the whereabouts of your crew at appropriate intervals throughout the work shift and at the end of the work shift (e.g., keep a log of employees on your work crews including their names, supervisors, work locations, and hours worked on a given day, etc.)

Smart Tips

Options for Acclimatizing Employees to Work in Warm or Hot Environments include One or More of the Following:

Have Employees Pace Themselves

  • If they are not accustomed to working in warm or hot environments, they should start work slowly and pick up the pace gradually.

Reduce Physical Demands

  • Assign employees to less physically demanding tasks during their first 14 days of working in a warm or hot environment.
  • Schedule and provide frequent breaks. Supply sufficient amounts of drinking water

Start an Acclimatization Program

  • These programs have employees work for progressively longer periods in warm or hot conditions where they are at risk for heat illness.
  • One such program is suggested by NIOSH (1986):
    For employees who have previous experience at worksites where they are at risk for heat illness
    Day # Hours Worked in Hot Work Environment

    For employees new to worksites where they are at risk for heat illness

    Day # Hours Worked in Hot Work Environment
    11.6 (96 minutes)
    23.2 (192 minutes)
    34.8 (288 minutes)
    46.4 (384 minutes)
    58.0 (480 minutes)


The acclimatization schedule by NIOSH, as shown above, is only an example program. Depending on the environmental and personal risk factors present, you may need to extend the time employees spend in your acclimatization program. This means that for some employees it may take up to 14 days of working in the heat for them to become fully acclimatized. Throughout this time period you should gradually increase employee's time working in the heat.

Replace Salts and Minerals

For un-acclimatized employees, or those on a restricted salt diet, additional salting of food with the approval of their physician may be required to replace the salt and minerals lost in sweating. Acclimatized employees generally loose relatively little salt in their sweat, therefore salt and mineral supplements are normally not required.