Effects of heat stress on workers

Answered by Scott Gammons, vice president of Adroit Medical Systems Inc.; and Rich Shafer, president of Shafer Enterprises LLC/Cool Shirt.net.

The negative effects of heat stress on human performance are well-documented. Private research and government tests have consistently proven that an ambient temperature exceeding a person's normal skin temperature (90-95° F at the surface) causes mental fatigue and physical exhaustion.

The reason is simple: The skin is the human body's primary cooling system. In fact, 65 percent of the heat generated by our bodies exhausts, or radiates, out of our skin. An additional 25 percent evaporates through sweat glands in our skin. However, this remarkably efficient cooling system is also a double-edged sword. The same skin that helps keep us cool is nearly defenseless against external heat sources greater than our own body temperature. As the body releases heat, it also allows heat in. Thus, any environmental condition or circumstance that tips the natural balance between a warm body and a cooler surrounding will quickly and dramatically stress a body's ability to function at peak performance.

A NASA study concluded that when the temperature is 95° F for an extended period, people can make 60 mistakes per hour – without realizing it. When the ambient temperature reaches 95° F, almost half of the blood moves to the skin to produce moisture (in the form of perspiration) to naturally cool the body. The heart is pumping up to 150 beats per minute with less volume to get the blood to the skin. That means the rest of the organs, including the brain and muscles, are only operating on half the blood they normally need. This interferes with cognitive thinking skills and can provoke emotions such as anger – and even combative behavior.

Any environment or circumstance that blocks the body's natural ability to release internal heat energy – including heavy outdoor work, extreme working conditions, and any activity involving protective clothing or heavy gear – can cause heat stress. Similarly, any environment or circumstance that exposes the body to heat sources higher than 90-95° F will quickly overwhelm a person physically and mentally. This can happen at low temperatures over time or instantly under extreme conditions.

In conclusion, mental errors by workers in ambient temperatures higher than 95° F are caused by the inability of the body to release internal heat energy and the eventual overheating and dehydration of the body. These two factors are responsible for a cascading series of events that cause a person to be both physically uncomfortable and fatigued, along with diminished thinking skills.