How Heat Stress Affects Performance

June 1, 2010
Even a 2 percent dehydration level caused by heat stress can dramatically reduce a worker's reaction times and ability to focus.

NIOSH notes in its publication, Occupational Exposure to Hot Environments (1986), that although workers can acclimatize themselves to different levels of heat, each worker has an upper limit for heat stress beyond which that worker can become a heat casualty.

It's accepted that businesses like foundries, heavy machine manufacturing, ship building and a variety of others have areas that subject employees to heat stress. In temperatures as low as 80 F, the human body compensates for heat levels in the inner core by pumping blood to the skin for cooling. When combined with the fact that most people (an estimated 80 percent of the U.S. population) start the day in a dehydrated state, heat stress is a major contributing factor in preventable accidents and work-related injury.

Many types of businesses encounter daily activity that can cause heat stress in the people that work for them, and they do not even know it. “We have always done things this way” is a quote that is all too familiar when asked why preventive measures were not taken to prevent heat stress in the workplace.


Sixty-five percent of the body's heat is released through radiation. This occurs when ambient air temperature is lower than the body's skin temperature. Radiation is the movement of heat energy from a warmer object to a cooler object, such as when heat radiates from the sun to the earth.

Convection accounts for approximately 10 percent of heat loss. Convection is the transfer of heat energy from a warmer object or space to a cooler object or space through differences in density and the action of gravity.

Approximately 23 percent of heat loss is due to evaporation of perspiration from the skin. Evaporation is the cooling of a surface through the process of a liquid changing to a vapor and leaving that surface. Conduction will add another 2 percent to the heat loss total. Conduction is the transfer of heat energy from a warmer object to a cooler object through direct contact.

When the ambient temperature of the surrounding air is 95 F or higher, radiation, convection and conduction stop working. Evaporation is all that is left to cool the body. Protective clothing used by welders, firefighters, racers and hazmat workers will make the heat situation even worse.

A performance study by NASA using telegraph key operators showed that in temperatures of 80 F, the operator will make five errors an hour and 19 mistakes after 3 hours. At 90 F, the operators made nine mistakes per hour and 27 after 3 hours. At 95 F, the mistakes went to 60 in 1 hour and 138 in 3 hours. Although errors made by telegraph key operators may not be critical, this same hot environment will produce a proportional amount of errors regardless of the task.

When a person is in a hot environment, up to 48 percent of the blood is pumped by the heart to the skin for cooling. The first effect is to release heat, but water also is released through perspiration. If an individual loses 2 percent of body weight due to perspiring, that person is considered to be in a heat-exhausted state. A study by Wasterlund and Chaseling1 placed forest workers in a controlled environment, where one group was properly hydrated and the other group was dehydrated to an extent of 1 percent of body weight loss. The test included the time taken to debark and stack 2.4 cubic meters of plywood. They found a 12 percent decrease in productivity from the dehydrated group.

Another study by Gopinthan et al2 focused on mental performance and the effects of dehydration on the decision-making process and could be related to an increase in work-related accidents. The study concluded that with 2 percent of body weight loss, visual motor tracking, short term memory, attention and arithmetic efficiency all were impaired. In the extreme, the study notes that a 23 percent reduction in reaction time occurred with a 4 percent body fluid loss.


At the ambient temperature of 95 F, the body can no longer keep up with its internal heat generation levels and the inner core temperature begins to rise. The only mechanism to release body heat from the inner core is for up to 48 percent of the body's blood to be pumped to the skin to create perspiration.

This creates two problems. The first is blood loss to the organs, muscles and brain. The second problem is dehydration. When the brain, muscles and major organs are receiving half of the blood they normally receive, the heart must work much harder to try to deliver the same volume of blood to those organs to keep them nourished by beating up to 150 times a minute. When you factor in a thickening of the blood due to fluid loss (dehydration), you begin to understand why heart attacks are a major byproduct of heat stress.

When an employee performs heavy physical work, fluid intake may not overcome the effects of sweat output. Employees who perform duties in fully encapsulated protective clothing may have increased sweat rates of 2.25 liters per hour.

Other studies link job-related accidents to orthostatic intolerance. Carter et al3 established that with a 3 percent dehydration state due to heat exposure, subjects experienced a significant reduction in cerebral blood flow velocity when changing from a seated to a standing position, which can cause workers to lose consciousness.

Warning signs of heat exhaustion include heavy perspiration, fatigue and weakness, muscle and body ache, headache, nausea, rapid heartbeat, confusion, loss of consciousness and vomiting with or without loss of consciousness.

This may lead one to believe that taking fluids to hydrate the body is enough to prevent heat stress. However, it can take as much as 24 hours for the body to absorb enough fluid to fully rehydrate.

Work may need to be curtailed while fluid is replaced, or the dehydration rate must be slowed by using personal cooling methods such as misting fans, ice vests or active cooling products that pump cooled fluid through tubing or a bladder sewn to a garment that the employee wears under the protective clothing.


Godek, Bartolozzi, et al4, have shown that fluid intake alone does not reduce core body temperature. Action must be taken to allow the worker to cool in addition to taking fluid. The inner core temperature will continue to rise for up to 30 minutes after work is stopped, unless other means are used to cool the blood that has been pumped to the skin for cooling.

While conduction accounts for only 2 percent of heat loss under normal circumstances, the OSHA Technical Manual (Section III: Chapter 4) talks about how active cooling products using water are useful in preventing heat stress by using conduction to enhance the body's capacity to cool. In fact, it has been demonstrated that water is 28 times faster in cooling a subject than cooled air. These products slow the rate at which the core body temperature rises by using conduction to greatly increase the body's capacity to cool the blood that is pumped to the skin during times of elevated core temperatures. In turn, this slows the fluid loss caused by sweating. By using shirts and vests that incorporate active cooling on about 40 percent of the body surface, the danger of heat stress greatly can be reduced.

Education of employees is the most critical element in reducing heat stress related accidents in the workplace. When workers and supervisors do not take into account the effects that heat stress can have on the body, dangerous events can take place. Reduction in cognitive function, attention span and visual motor tracking all can lead to mistakes that could have tragic consequences.

Policies that allow workers to use products that help prevent heat stress in the workplace will greatly reduce heat stress related illness and injury. Employee training and company policies must help the employees decide how they can protect themselves from heat stress.

Bruce Baker and John LaDue are with Shafer Enterprises LLC/Cool Shirt .net. Shafer Enterprises develops and manufactures thermoregulation and temperature therapy products for industry, medical, military and sport activities. More information on personal cooling can be found on their Web site at If you have any questions, the authors can be reached at 800-345-3176.


  1. Wasterlund DS, Chaseling J, Burstrom L: “The Effect of Fluid Consumption on the Forest Workers' Performance Strategy.” Appl Ergon 35:29-36, 2004.

  2. Gopinathan PM, Pichan G, Sharma VM: “Role of Dehydration in Heat Stress-Induced Variations in Mental Performance.” Arch Environ Health 43:15-17, 1988.

  3. Carter R 3rd, Cheuvront SN, Vernieuw CR, Sawka MN: “Hypohydration and Prior Heat Stress Exacerbates Decreases in Cerebral Blood Flow Velocity During Standing.” J Appl Physiol 101:1744-1750, 2006.

  4. Godek S, Bartolozzi A, Burkholder R, Sugarman E, Dorshimer G: “Core Temperature and Percentage of Dehydration in Professional Football Linemen and Backs During Preseason practice.” J Athl Train 41(1):8-17, 2006.

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