How Heat Stress Affects Performance

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.

HOW THE BODY RELEASES HEAT

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.

WHEN THE BODY CAN'T KEEP UP

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.