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Heat Stress: Overview


Understanding heat stress allows you to stay safe while working in hot environments.

Things you need to know:

  • Heat exposure can cause a range of effects on your body, from irritating rashes to heat stroke, which is often fatal.
  • Heat exposure can cause confusion and poor judgment—use the buddy system to monitor co-workers for symptoms of heat illness.
  • Fatigue, heat exposure on recent previous days, certain medications and health conditions, age over 60 years, protective gear such as respirators or chemical-resistant apparel, recent illness, and intense exertion each increase the risk of getting heat illness.
  • Drinking enough water is critical to preventing heat illness. Stay hydrated.
  • Heat tolerance—but not the need for water—can be improved through gradual exposure (also called acclimatization).

Types of Heat Illness

Vertical arrow color transitioning from yellow to red from top to bottom. Top labeled "less severe"; middle "severe"; bottom "often fatal."Heat rash/"prickly heat"

Cluster of red bumps, pimples, or small blisters, usually on neck, upper chest, groin, under breasts, and in elbow creases

Heat cramps

Muscle cramps, pain, or spasms in the abdomen, arms, or legs 

Heat syncope (fainting)

Dizziness, light-headedness, or brief loss of consciousness during prolonged standing or suddenly rising from a sitting or lying position

Heat exhaustion



Dizziness, weakness



Goose flesh or heavy sweating (Note: these symptoms may not always be present.)

Elevated body temperature, decreased urine output

Heat stroke

Confusion, altered mental status, slurred speech, loss of consciousness

Hot, dry skin or profuse sweating


Very high body temperature

Fatal if treatment delayed 


Factors that Increase Heat Illness Risk

  • High humidity
  • No wind or breeze
  • Lack of acclimatization
  • Dehydration
  • Physically demanding work
  • Protective gear, including non-breathable or minimally breathable clothing, respirators, or chemical-resistant apparel
  • History of previous heat illness
  • History of recent illness unrelated to heat (especially involving vomiting or diarrhea)
  • Recent alcohol use (within previous 24 hours)
  • Certain health conditions
  • Certain medications
  • Age over 60 years
two miners looking onwardWorkers need to look out for each other!
Use a buddy system!

Often it is a coworker who first notices signs of heat stress in another employee.


Points to Remember

 Hot environments can be hazardous!
Acclimatization is critical, and may need to be repeated!
  • Heat exhaustion is treatable, but it can progress to heat stroke quickly if not recognized and addressed properly.
  • Heat stress can affect alertness and judgment, which can lead to accidents and injuries.
  • Cases of severe heat illness do not always happen on the hottest days. They also occur in moderately hot or relatively cool conditions when performing heavy physical work.
  • Call 911 if heat stroke is suspected. Risk of death is higher with delayed treatment.
  • Heat acclimatization is the improvement in heat tolerance that comes from gradually increasing the duration or intensity of work performed in a hot setting.
  • Acclimatization is most effective if it takes place gradually over a period of 7 to 14 days.
  • You begin to lose your acclimatization after about one week away from work in the heat.
  • After 1 month away from work in the heat, most people will have lost nearly all heat acclimatization.
Stay hydrated!
Give your body time to cool off. Pay attention to work/rest schedules!
  • Hydration is one of the most important factors in avoiding heat illness.
  • Don’t rely on thirst to tell you when you are dehydrated—thirst lags behind dehydration by several hours.
  • Schedule regular water breaks when working in the heat.
  • Water is usually sufficient to maintain hydration.
  • Electrolytes can be replaced by eating regular meals.
  • Sports drinks can also help replace electrolytes, but are not usually necessary unless heavy sweating continues for more than 2 hours and eating meals or snacks is not an option.
  • You must take rest breaks periodically to allow your body to cool down. If possible, remove minimally breathable clothing or PPE during breaks.
  • Work/rest schedules are guidelines for how much physical work can be performed in the heat and how much rest is needed in order to prevent heat illness.
  • Know your personal limits and options for cooling at your worksite. Let a buddy know if you need to take a break to cool down.

Case Study: Heat Illness and Heavy Machinery

Loader turned on its side after accident.A 48-year-old employee was running a loader at an open pit mine in Arizona in mid-August when another employee noticed he was just sitting in the cab and not moving. A supervisor called the employee on the radio three times with no response. The supervisor went to the employee and discovered him to be confused and unresponsive.

More than just a health issue!

What might have happened if the employee had been driving or operating the loader when he became unresponsive?