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A Hot Topic: Heat Stress

By Joan Mantini, Chief Editor

Outdoor workers exposed to hot and humid conditions can be at risk of heat-related illness. The risk of becomes greater as the weather gets hotter and more humid. The combination of both air temperature and humidity affect how hot outdoor workers feel in hot-weather conditions.

Although OSHA does not have a specific standard that covers working in hot environments, under the OSHA Act, employers have a duty to protect workers from recognized, serious hazards in the workplace, including heat-related hazards.

Employers need to take into consideration the “heat index,” which is a single value that takes both temperature and humidity into account. The higher the heat index, the hotter the weather feels. The heat index is considered a better measure than air temperature alone for estimating the risk to workers from environmental heat sources.

NOAA issues extreme-heat advisories to indicate when excessive, extended heat will occur. The advisories are based mainly on predicted heat index values:

  • Excessive Heat Outlook: issued when the potential exists for extended excessive heat (heat index of 105-110°F) over the next 3-7 days. This is a good time to check on supplies, such as extra water coolers, and refresh worker training.
  • Excessive Heat Watch: issued when excessive heat could occur within the next 24-72 hours, but the timing is uncertain.
  • Excessive Heat Warning: issued when the heat index will be high enough to be life-threatening in the next 24 hours. This warning indicates that the excessive heat is imminent or has a very high probability of occurring.
  • Excessive Heat Advisory: similar to an Excessive Heat Warning but less serious. This is issued when the heat index could be uncomfortable or inconvenient but is not life-threatening, if precautions are taken.


Extra measures, including implementing precautions at the appropriate risk level, are necessary for reducing the risk of heat stress for employees working outdoors in extreme heat. The employer’s response at the four risk levels is the subject of the remainder of OSHA’s guidelines. The steps employers should take in response to an elevated heat index are the same type of steps that they would follow to address other hazards in the workplace:

  • Develop an illness-prevention plan for outdoor work, based on the heat index.
  • Train your workers on how to recognize and prevent heat-related illness. Train workers about safe work practices before heat index levels go up. Workers should be prepared to recognize the signs and symptoms of heat-related illness; how to prevent it; and what to do if someone is demonstrating symptoms.
  • Track the worksite heat index daily; communicate it and the required precautions to workers. Knowing how hot it will be during scheduled work activities can help to determine which preventive measures should be taken in preparation.
  • Implement your plan; review and revise it throughout the summer.

It is suggested that workers are trained before hot outdoor work begins. Training can be more effective if it is matched to job tasks and conditions and is reviewed and reinforced throughout hot weather conditions. The following OSHA-suggested training topics might be addressed in one session or in a series of shorter sessions:

  • Risk factors for heat-related illness
  • Different types of heat-related illness, including how to recognize common signs and symptoms
  • Heat-related illness prevention procedures
  • Importance of drinking small quantities of water often
  • Importance of acclimatization; how it is developed; and how your worksite procedures address it
  • Importance of immediately reporting signs or symptoms of heat-related illness to the supervisor
  • Procedures for responding to possible heat-related illness
  • Procedures to follow when contacting emergency medical services
  • Procedures to ensure that clear and precise directions to the worksite will be provided to emergency medical services


By developing an acclimatization plan, employers can protect their workers from heat stress.  According to the Center for Disease Control and Prevention (CDC), “Acclimatization is the result of beneficial physiological adaptations (e.g., increased sweating efficiency and stabilization of the circulation) that occur after gradual increased exposure to a hot environment.” The CDC gives three tips to assist with this plan:

  1. Gradually increase the time spent in hot environmental conditions over a 7-14 day period.
  2. For new workers, the schedule should be no more than 20% exposure to heat on day one and an increase of no more than 20% exposure on each additional day.
  3. For worker who have had previous experience with the job, the acclimatization should be no more than:
Day 1 Day 2 Day 3 Day 4








The CDC also suggests a buddy system be set up to check on workers routinely—to make sure they are making use of readily available water and shade, and that they do not have heat-related symptoms. Workers should also be encouraged to wear clothing that is breathable, light-colored and loose-fitting, and although in some working environments personal protective equipment is necessary, it is also important to be aware that it may increase the risk of heat stress.


Workers should be encouraged to drink hydrating fluids, such as water, throughout the day. The CDC recommends that, during moderate activity in moderately hot conditions, workers should drink about 1 cup every 15-20 minutes. However, staying hydrated before work is equally important. Being hydrated when you start your workday makes it easier to keep hydrated, and replenishing after work is equally important. According to the CDC, “Hydrating after work is even more important, if you work in the heat on a regular basis. Chronic dehydration increases the risk for a number of medical conditions, such as kidney stones.”

Indeed, water will almost always maintain hydration levels, when consumed properly, while working in a hot and/or humid environment. Other electrolyte-enhanced beverages may also prove to be beneficial; however, if proper nutrition through a well-balanced diet it maintained, these types of drinks are not necessary.


Using the Head Index: A guide for Employers was created to help employers and worksite supervisors prepare and implement hot weather plans. Employers and employees can determine when extra precautions are needed at a worksite, with the goal to protect workers from environmental contributions to heat-related illness. You can find more about information about heat stress at Using the Heat Index: A Guide for Employers. WMHS

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