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Adjusting to Work in the Heat: Why Acclimatization Matters

Categories: Agriculture, Forestry, and Fishing, Construction, Heat Stress, Outdoor Work

Heat stress infographic

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Acclimatization is important in keeping your workforce safe and well as temperatures rise. This natural adaptation to the heat takes time, and from a management perspective, it may require careful planning.

Make acclimatization part of your plan

A good heat illness prevention plan takes into account the need for more breaks, a cool place to rest, the availability of fluids, and the careful allotment of time for a worker to become fully adjusted or acclimatized to the heat. It will need to be flexible based on the intensity of the heat, the level of humidity, the workers’ experience on the job, and the workers’ physical fitness.

Time to adapt

  • New workers on an 8-hour shift should spend only about an hour and a half in the heat on their first day. Their exposure time should increase slowly—no more than a 20% increase per day.
  • Experienced workers on an 8-hour shift can spend up to 4 hours in the heat on the first hot day. The next day they can likely manage 5 hours in the heat and about 6.5 hours third day. By the fourth day, most healthy workers should be able to tolerate 8 hours in the heat assuming they are well-hydrated and have appropriate rest breaks throughout their shift.


Cover of HHE documentSee our recently-published health hazard evaluation on extreme heat in a national park in California.

Fluids are necessary to acclimatization. In fact, failure to replace the water lost in sweat will slow or even prevent the development of the physiologic adaptations to heat. Workers need to drink small amounts of water throughout the day so that they never become thirsty. For moderately intense work in moderate heat, this equates to approximately 1 cup every 15 to 20 minutes. Supervisors should encourage and remind workers to take water breaks. In some cases, when workers are experiencing heavy sweating, drinks containing electrolytes might be warranted.


In the heat, workers need more rest than they would in cooler environments. Provide a cooler place out of the direct sunlight where they can take their scheduled breaks. A shaded or air-conditioned rest area would be best. Air conditioning will not affect acclimatization. Remind and encourage workers that they need to take advantage of these rest breaks.

How does the body adapt?

When workers are initially exposed to hot work environments, they may readily show signs of distress and discomfort. Their core temperatures and heart rates increase. They may experience headaches, nausea, and other symptoms of heat-related illness. The brain’s thermoregulator detects the increases in skin, muscle, and organ temperature and ignites the body’s cooling mechanisms—primarily sweating and vasodilation of the skin’s blood vessels. As the body is exposed to the heat and is given proper recovery time, it begins to adapt. Total sweat production increases, and sweating begins at a lower skin temperature.

How should workers maintain these benefits?

A few days out of the heat won’t ruin a worker’s acclimatization, but absence from work in the heat for a week or more can result in a significant loss of those helpful adaptations. Upon return, the worker may combat acute dehydration, illness, or fatigue so supervisors need to make adjustments so the worker has time to re-acclimatize. This can take 2 to 3 days when returning to a hot job.

Planning for the heat requires extra care and effort. But a good heat illness prevention plan that allows workers to properly acclimatize, will reduce the risk of heat-related illness and death. For more information, visit the NIOSH Topic Page on Heat Stress.

Brenda Jacklitsch, MS

Ms. Jacklitsch is a health scientist with the NIOSH Education and Information Division.

Public Comments

Comments listed below are posted by individuals not associated with CDC, unless otherwise stated. These comments do not represent the official views of CDC, and CDC does not guarantee that any information posted by individuals on this site is correct, and disclaims any liability for any loss or damage resulting from reliance on any such information. Read more about our comment policy ».

  1. May 15, 2016 at 5:57 pm ET  -   JORGE VENEGAS

    Buenos Días:

    Por favor me pudieran orientar sobre información en español que tenga OSHA para exposiciones a calor. existe el estándar recomendado en español?



    English translation:

    Good morning,

    Could you please orient me towards any materials in Spanish that OSHA may have about exposures to heat? Do any standards exist in Spanish?

    Thank you,

    Link to this comment

  2. May 27, 2016 at 11:03 pm ET  -   BRADLEY J TANKERSLEY

    Hi, I work at a polyurethane parts manufacturing company. The ovens we use range in heat around 200 to 300 degrees and the metal molds are about the same temp. I regularly move and manually lift the molds to and from the ovens. I’m looking for information pertaining to diet and nutrition for a person who works in these extreme conditions.
    Thank you for your time,

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    • AUTHOR COMMENT June 8, 2016 at 10:57 am ET  -   Brenda Jacklitsch

      Thank you for your comment. One resource you might consider is the National Academies’ Institute of Medicine report from 1993, “NUTRITIONAL NEEDS IN HOT ENVIRONMENTS: Applications for Military Personnel in Field Operations.” A pdf copy can be downloaded for free at:

      A healthy diet is important when working in a hot environment like what you have described. Healthy, physically fit individuals have a reduced incidence of heat injury and illness. In addition, heat acclimatization is better maintained by those that are physically fit. It is important to be careful to not restrict the salt in your diet when sweating heavily, as you want to maintain your electrolyte balance. We do not recommend the use of salt tablets, but in cases where heavy sweating is present, sports drinks may be more beneficial than water. In section, the NIOSH Criteria for a Recommended Standard: Exposure to Heat and Hot Environment mentions that a high protein diet might increase the urine output needed for nitrogen removal and increased water intake requirements. We suggest meeting with a dietician familiar with high heat, high exertion environments (workplaces or even sports-related). A plan that leads to overall better physical fitness will be beneficial. Be careful if any dietary supplements are recommended, as some (e.g., creatine and Hydroxycut) may actually put you at greater risk for heat-related illnesses, like rhabdomyolysis.

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