Published in Facility Safety Management, April 2007
A Hot Topic by Mark Smith and Cheryl Tomsic

Preventing Heat Stress

Let’s face it; any job becomes more difficult when it’s hot! Add protective clothing, heavy exertion a little humidity and performing basic tasks can become downright dangerous. Heavy exertion in the heat is second only to major blood loss in the stress it places on the body. A person performing moderate work in an ambient temperature above eighty degrees wearing protective clothing can reach critical temperatures in as little as 20 minutes. Once core body temperature rises, physical and mental functions become impaired and potential risk from heat related injury increases dramatically. And, with each heat related injury the body becomes more susceptible to re-injury.  For those who treat workers with heat stress and related injuries everyday, you know the high cost in time and money such incidents can incur.  According to the National Weather Service Forecast Office in Shreveport, Louisiana, between 1975 and 2004 heat accounted for 38% of all weather fatalities.   Heat illness is such a serious issue; California OSHA recently passed a Heat Illness Prevention Emergency Regulation that applies to all outdoor places of employment when environmental risk factors for heat illness are present.   

The good news is ALL heat related injuries are preventable.  But it’s important to know the signs and conditions that can lead to heat illness.  Environmental risk factors to consider include the wet bulb globe temperature (WGBT) index, which takes into account air temperature, relative humidity, and wind velocity.  In addition it’s important to know workload, protective clothing and personal protective equipment worn by employees and time to complete the task to get a comprehensive picture of potential risk.  In general, it should be assumed that there is a significant risk of heat illness for employees working in Heat Index 80 Degrees or above.  In fact the risk of injury can double with an increase of just 3 degrees in the heat index above 80. Keep in mind that Heat Index assumes people are wearing light summer clothing.  When workers use heavier clothing or some types of personal protective equipment, the body’s ability to lose heat is compromised and the risk of heat illness becomes significant at a lower Heat Index.  Chemical Protective clothing such as HAZMAT suits can put a worker at risk of heat illness even when temperatures are mild.  The importance of prevention cannot be overstated and it is up to the employer to develop effective procedures for identifying, evaluating and controlling the risk factors in their workplaces. Evaluate each situation to determine heat illness risk.  Develop an assessment template to determine risk from day to day and task to task accounting for all risk factors including worker fitness.

The Body's Temperature Regulating Process.
The human body functions best in a narrow range between 96.8 degrees F and 100.4 degrees F.  Working in thermally stressful conditions heat illness symptoms of,  exhaustion, mental fatigue, and cramping can easily escalate into much more serious conditions of nausea, vomiting, loss of coordination, consciousness and into heat stroke. The body uses two different mechanisms for cooling.  First, blood is diffused to various portions of the body including the hands, feet, face and skin for conductive and convective cooling. Heart rate and stroke volume are increased to maintain pressure in the circulatory system. If sufficient cooling does not occur, the body then begins to sweat to facilitate evaporation.  If the environment is equal to or greater than the body temperature, or work rate is greater than the ability to lose heat, core body temperature will continue to rise as the body stores excess heat

Too Hot to Handle
As temperature and heart rate increase, more and more energy is used to dissipate heat using limited and precious resources such as water. Fluid loss from sweating further compromises the body’s ability to dissipate heat leading to dehydration.  At this point, the body's core temperature rises above 100.4 degrees F and this can lead to a number of heat related illnesses including loss of endurance, cramping, loss of cognitive functions, fatigue, heat stroke and even death.

Heat Illnesses:  Know the Signs!

  • Headaches
  • Muscle Cramping
  • Nausea
  • Vomiting
  • Fainting
  • Loss of Coordination

If not recognized and dealt with immediately these symptoms can escalate into more serious problems associated with Heat Stroke including

  • Confusion
  • Seizures
  • Body Temperature of 104 Degrees or more


Prevention is the key! 
A good heat stress prevention plan includes daily assessment of conditions, work tasks, work loads, convenient break locations out of the heat, adequate supplies of water and education of all workers to recognize the signs of heat stress in themselves and their fellow workers.

  • Alternate Work Rest periods
  • Provide adequate amounts of water  (as much as 1qt per worker per hour)
  • Allow new hires and workers returning from an absence of two weeks or longer to acclimatize
  • Provide mechanical solutions where possible, such as shade, air movement, spot coolers or evaporators

Just Cool It - and Fast!

If preventive measures aren’t adequate and a worker experiences symptoms of heat stress it is time to do everything possible to lower their body temperature. This can include consumption of cool fluids, move to a shaded area, cooling the hands and feet.

If heat stroke is suspected you need to call 911 and seek medical treatment IMMEDIATELY!

What's New In Body Cooling?

Until recently, preventive cooling was limited to frequent breaks, cool beverages, the shedding of heavy clothing and the application of any number of topical cooling methods, from cool mist fans to ice vests.  While these methods make you feel better, science is now recognizing the benefits of cooling the body's core temperature.   Fortunately for everyone, new methods are now available allowing this to be done safely, quickly and non-invasively.

Cooling the body core temperature simply means that we are lowering the temperature of the core organs.    This provides many benefits.   First, newer methods of cooling the body's core allow for the acceleration of the body's natural heat dissipation capacity and have been shown to cool up to five times faster than conventional skin surface methods.    This can be extremely important in situations where someone is approaching heat stroke.  If used as a preventative measure, core cooling is shown to slow the body’s heating up process considerably.    Furthermore, laboratory studies have shown significant endurance and performance increases in subjects who practiced these core cooling methods.

What’s the big difference?

The topical cooling methods most widely used can sure make you feel cooler, and fast.   This is because our body’s sensors are located at the skin’s surface.   Unfortunately, applying something cold to the skin’s surface will cause a vasoconstriction, or a closing of the blood vessels.   This pretty much shuts down the skins ability to dissipate the heat by reducing blood flow and could actually result in a higher core temperature.

Cooling the body’s core is quite a bit different.   To begin with, current core cooling methods utilize a vacuum seal that prevents that vasoconstriction mentioned earlier.  While humans don’t have internal sensors that allow us to feel the cooling of the body’s core, those who have tried it report feelings of increased mental clarity, increased energy, reduced mental and physical fatigue and a feeling of having started the day over.  

Craig Heller, Ph.D., a Stanford researcher who has done extensive research on body core cooling explains, “While traditional cooling methods make the victim feel cooler on the outside, they are not effective in extracting heat form the body’s core organs, In fact, laboratory testing demonstrates that these methods may make matters worse.  When cold is applied to the body’s surface, it can trigger constriction of the skin’s blood vessels which interferes with cooling.”

Stay Cool, Stay Safe  

The goal is always to maintain a safe and healthy work environment.   This can best be done by paying attention to ourselves and our co-workers, knowing how to recognize a heat problem or potential heat problem, and acting quickly to respond to heat issues before they escalate into something serious. 

 Additional information is available at the OSHA website

Mark Smith and Cheryl Tomsic work for AVAcore Technologies, a leader in the human thermoregulation field, located in Ann Arbor, Michigan.