According to reports by the Occupational Safety and Health Association (OSHA), almost half of heat-related deaths occur on a worker’s very first day on the job. Over 70 percent of heat-related deaths occur during a worker’s first week.

These tragedies can be avoided if employers take actions to protect new workers. The term “workers who are new to working in warm environments” includes the following groups:

  1. New, temporary, or existing employees who start new work activities in warm or hot environments, while wearing additional clothing (e.g., chemical protective clothing), or with increased physical activity.
  2. Workers returning to work environments with potential exposure to heat hazards after an absence of one week or more.
  3. Workers who continue working through seasonal changes when temperatures first begin to increase in the spring or early summer.
  4. Workers who work on days when the weather is significantly warmer than on previous days (i.e., heat wave).

In all examples above, the workers may not be used to the heat loads on that day. The above workers are at increased risk of heat-related illness because of physiological (related to body function and exertion) and/or behavioral factors.


The term “acclimatization” means that the body gradually adapts and tolerates higher levels of heat stress. Workers who are new to working in warm environments may not be acclimated to heat. Their bodies need time to adapt to working in hot conditions. 

Unacclimated workers do not sweat efficiently, whereas acclimated workers sweat at a higher rate, which helps dissipate heat through evaporative cooling. The sweat of acclimated workers also contains less salt, which prevents development of electrolyte imbalances and heat cramps.

When a worker is not acclimated to the heat, body temperature and heart rate increase more quickly when working, and blood flow is not optimized for heat dissipation. Acclimated workers maintain lower body temperature and heart rate, and their increased blood flow to the skin helps the body to lose heat through its surface. 

Other factors that are different from person to person (e.g., general physical fitness) may affect the acclimatization process.


To protect new workers from heat-related illness, employers should do the following:

  • Schedule new workers to work shorter amounts of time working in the heat, separated by breaks, in heat stress conditions.
  • Give new workers more frequent rest breaks.
  • Train new workers about heat stress, symptoms of heat-related illness, and the importance of rest and water.
  • Monitor new workers closely for any symptoms of heat-related illness.
  • Use a buddy system and don’t allow new workers to work alone.
  • If new workers talk about or show any symptoms, allow them to stop working. Initiate first aid. Never leave someone alone who is experiencing symptoms!

These increased precautions should last for 1 to 2 weeks. After that time, new workers should be acclimated to the heat and can safely work a normal schedule.

Note that new workers are not the only ones who might be unacclimated. Workers can lose their heat tolerance during an extended absence (e.g., vacation or sick leave). They can also lose heat acclimatization during the winter when temperatures are cooler. Existing workers are at increased risk of heat-related illness in these situations.

In the above situations, employers should allow workers to gain heat tolerance gradually. Use the same protection strategies that are used for new workers. Maintain the additional heat protections for at least one week. Unacclimated workers who feel fine on their first day in warm conditions might develop heat-related illness on a subsequent day.


New workers need time to acclimate unless they have previously worked in hot environments. To prevent heat-related illnesses, they should work shorter workdays in the heat during their first 1-2 weeks. OSHA recommends the “Rule of 20 Percent” for building heat tolerance:

  • 20 Percent First Day: New workers should work only 20 percent of the normal duration on their first day.
  • 20 Percent Each Additional Day: Increase work duration by 20 percent on subsequent days until the worker is performing a normal schedule.

For example, if the normal workday lasts 8 hours, then new workers should work no more than 1 hour and 40 minutes (20 percent of 8 hours) on their first day in the heat. They can spend the rest of the workday without heat stress. They should be given at least one rest break during the period when they are working.

By following the Rule of 20 Percent, new workers will be working a full schedule by the end of their first week. The Rule of 20 Percent should protect most workers who are physically fit and have no medical problems. Other workers may require more time to adapt to heat—up to 14 days in some cases. When in doubt, give workers more days to acclimate. As duration of work increases, workers will need more rest breaks to recover from the heat load.

To become acclimated to heat, workers should perform job tasks that are similar in intensity to their expected work. For example, if a new worker has been hired to lay bricks outdoors in hot weather, then he should lay bricks during his first week. Doing light work may not acclimate a worker to the demands of their job.

Remember, to help workers build heat tolerance, reduce the duration of the work but not the intensity of the work.


The Occupational Safety and Health Association (OSHA) is a division of the U.S. Department of Labor. For more, visit