Finding the Comfort Zone

Beating the outdoor heat requires forethought and awareness.

As the dog days of summer approach, it is only logical for workplace concerns to turn toward the danger of thermal stress, specifically heat stress. Not only is the great outdoors a potential hot spot to watch out for; artificial environments that range from freezers to ovens and beyond can also be danger zones when dealing with thermal stress. Still, work must go on. Because not entering these environments is not a viable option for most businesses, proper procedures become the best defense for both managers and employees.

Human beings have five zones of thermal stress. Right in the middle is the comfort zone. On either side of this comfort zone lie discomfort zones for both heat and cold stress. These discomfort zones are just that: uncomfortable. There is no real risk of serious disorders resulting from such exposure, yet people will feel either hot or cold and the result may be a decrease in productivity or quality of work.


Choosing the right clothing and having a basic understanding of how heat stress develops are the best tools to keep everyone cool.

It is in the outer zones that workers are exposed to the most serious threats. In these health risk zones, work capacity and quality are severely limited, and the likelihood of disorders related to heat stress and cold stress increases by leaps and bounds. Here is where good management and sound safety procedures can make all the difference this summer.

Three Influencing Factors

Thermal stress is decided by three factors. First and foremost, the environment plays a major role in the level of heat stress. It's elementary that working in the hot sun--even with proper protection--only will make you hotter. In fact, it is in this observation that the other two factors of thermal stress become obvious: work demands and clothing. While climate alone is often used to gauge the degree of thermal stress, details of temperature, wind chill, and relative humidity are not the only relevant factors in the determination of thermal stress level.

Work demand often is measured most effectively by examining the role of metabolic rate in heat balance. In heat stress, according to the authoritative reference book "Fundamentals of Industrial Hygiene," metabolic rate can add 10 to 100 times more heat to the body than radiation (heat flow from solid bodies of different temperature) and convection (the exchange of heat between the skin and the surrounding air) combined. At the same time, three variables associated with clothing greatly alter thermal balance. They are insulation, permeability, and ventilation.

Insulation is a measure of the resistance to heat flow by radiation, convection, and conduction (heat flow from two solid bodies in contact). The more insulation, the lower is the rate of heat flow. During heat stress, insulation reduces heat flow by radiation and convection.

Permeability, which influences thermal stress by adjusting the amount of evaporative cooling that occurs, is measured by examining the resistance to water vapor movement through the clothing. Permeability generally will decrease as insulation increases. Unfortunately, in many cases the risk of heat stress will be sacrificed to the risks of dangerous skin-to-chemical contact, or vice versa. New trends in protective clothing do indicate a rising wave of fabrics that can do a better job of killing two birds with one stone: providing skin protection and effective water vapor transmission.

Ventilation is a crucial part of the clothing factor. Without proper ventilation, ambient air cannot move through the fabric or even through actual garment openings. Garments that do enable the movement of air will enhance cooling via evaporation, while garments with less air movement enabled will tend to stay warmer. This is why, at least in cold stress situations, most clothing is outfitted with drawstring hoods, sleeves, and waists. The wearer is free to loosen up and let some trapped heat escape. It is not so easy for those suffering with a hot climate.


Heart rate is a factor in heat stress and reveals what is going on inside an overheated body.

Signs of Heat Stress

Someone suffering from heat stress experiences an increase in body temperature, a faster heart rate, and excessive sweating. An increased metabolism means an increase in the body's core temperature. Because heat exchange with the environment occurs on the skin, blood circulating through the core is sent to the skin where the blood is cooled before returning to be rewarmed at the core.

Heart rate is a factor in heat stress and reveals what is going on inside an overheated body. The total blood through the heart is proportional to the metabolic rate and inversely proportional to the temperature difference between the body core and the skin. As "Fundamentals of Industrial Hygiene" explains, "as work demands and metabolic rate increase, cardiac output increases, as seen in the heart rate."

Sweat rate is another means to measure heat strain. The greater the heat stress, the greater the rate and volume of sweat. In extreme cases of heat stroke, sweating ceases. The skin will be hot and dry.

Most of the signs of heat stress are fairly obvious. But what about the signs that a work environment is in need of evaluation? How do managers know when their workers are routinely working in a potentially dangerous situation? Answering these questions will help in assessing an operation.

  • Is the environment hot, in your opinion? Do you find it uncomfortable?
  • Are the work demands objectively high?
  • Is it possible that required protective clothing might increase the chance of heat stress?
  • How is the workers' morale? Their absenteeism? Do they seem to be behaving safely and rationally?
  • Is there a history of fatigue, weakness, rashes, headaches, or high body temperature in or around the area being evaluated?
  • Is the average worker suffering from high body temperature, excessive heart rate, or sweat loss?

If any of these questions was answered "yes" or made you think about the condition of your work environments, it is time to conduct a heat stress evaluation. Once a person is dehydrated or suffering from heat exhaustion, the time for prevention and prediction is over.

Controlling Heat Stress

Once a determination of potential or actual heat stress has been made, there are several steps to take in the control of heat stress.

Training is one of the best tools in heat stress management. All employees, not just those in high heat-exposed positions, should be trained in at least the basics about heat strain. Being able to recognize telltale signs in yourself and others leads to better treatment. Those working in clearly heat-related positions should receive advanced and specialized training in the area of heat stress management.

Exposure to heat stress must end swiftly and surely. If ignored, these signs can blossom into much more serious symptoms. Managers must be able and willing to terminate a situation with the potential for heat stress, regardless of deadlines or work schedules.

Heat stress hygiene practices are a way of managing the risks. As much as six quarts of water can be lost from the body daily in the form of sweat, the equivalent of about 13 pounds. Replace this fluid by drinking cold water, lemonade, iced tea, or commercial fluid-replacement drinks. Drinking such fluids in small quantities should occur as frequently as possible throughout the day.

A well-balanced diet is crucial to the body's ability to combat all illness, not just heat stress. Make sure employees who are exposed to heat do not eat a large meal during work breaks. Doing so increases both circulatory load and metabolic rate. Adequate sleep is also very important in fighting heat stress.

Apparel and Cooling Systems

When selecting personal protective clothing in the fight against heat stress, there are many options available. Personal cooling systems are offered that are considerably more extensive than more traditional methods. A circulating air system consists of circulating air under the clothing and around the torso. Air is delivered by either a high-pressure air line and a pressure reducer or by a portable blower. Evaporation is increased as well as convective cooling, making the worker more comfortable. Circulating water systems are another option, with cool water being circulated through tubes and channels around the body. One type almost covers the entire body, while others only cover portions of the chest and back. There are portable versions for this method as well.


A rising wave of protective fabrics does both jobs, providing skin protection and effective water vapor transmission.

Ice garments, commonly called ice vests, remove body heat by way of conduction from the skin to packets of ice. With the typical vest weighing 5 kilograms, or about 11 pounds, these provide good mobility as well as considerable cooling potential.

Reflective clothing also combats heat stress. While personal cooling systems are designed to absorb body heat, reflective clothing does just that--reflects the heat. Reflective garments can reduce sweat evaporation, however, and actually make levels of heat stress increase if the clothing is not suitable to both the job and the heat source.

Putting It All Together

Keeping in mind all of the variables that go into the management of heat stress, managers and workers should be able to anticipate the worst before it happens. Choosing the right protective and cooling clothing, being able to recognize heat stress symptoms and potential danger zones, and having a basic understanding of how heat stress develops are the best tools to keep everyone cool.

Beating the heat can be done, but not without forethought and awareness.

This article originally appeared in the February 2008 issue of Occupational Health & Safety.

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