Take the Stress Factor Out of the Heat Equation

June 1, 2010
There's one solution to heat stress: Control the environment through engineering, work practices and heat assessments.

Deadlines are tight, workloads are heavy and your workers are faced with a heat index of 105 F. How can you protect your employees from heat exposure while also ensuring that the work is accomplished on time?

Think of the solution in mathematical terms. By changing a factor in a mathematical equation, you change the end result. A similar approach can be used to control the heat. By changing the amount of heat a body gains from the environment — through engineering controls, work practices and heat assessments — you change an employee's heat-stress level, thereby removing it from the equation.

First, it's important to understand that although you may be confident in the capabilities of your employees, a body's response to heat exposure is different for each person. Hot work environments put a physical strain on employees, especially if they're unaccustomed to working in the heat. This can result in heat-induced illnesses, disorders and accidents. Many physical and environmental factors come into play when determining the heat tolerance of individual workers. Some of these factors are within your control and others are not.


Physical factors such as age, body fat, physical fitness, medications and medical conditions affect the level of heat stress a worker can handle. While it's true that these factors are beyond your control, a proactive approach toward monitoring employee health could prevent heat-related illness or death.

Keep an eye on employees and their physical appearance. If an employee appears to be struggling or showing signs of distress, taking steps to ensure the employee is okay may be reasonable. Medical exams may be warranted in some situations. However, in order for a medical exam or inquiry to be made of an employee, it must be job-related and consistent with business necessity per the Americans with Disabilities Act (ADA).

The need for an exam may be triggered by some evidence of problems related to job performance or safety, or an exam may be necessary to determine whether individuals in physically demanding jobs continue to be fit for duty. Generally, this means that you have a reasonable belief, based on objective evidence, that an employee is unable to perform an essential function or will pose a “direct threat” (as defined in the ADA) because of a medical condition.

Other factors that affect heat tolerance focus on environmental conditions: air temperature, humidity, radiant heat, conductive heat, clothing and PPE. Unlike physical factors, reducing or eliminating the impact of heat from the environment is within the employer's control. However, this may require modification of ventilation, air condition, screening, insulation or processes to take heat stress out of the equation.


One way to reduce or eliminate heat stress is to combat the heat with engineering controls. As workers perform job tasks, their bodies produce heat. The amount of heat produced during hard, steady work is much higher than when the work is intermittent or light. By ensuring a constant exchange of heat between a worker and the environment, a core body temperature of 98.6 F can be maintained, which is critical for the body to function properly.

Control the air condition — Heat usually transfers from a higher temperature object to a cooler one. The engineering approach that will enhance the transfer of heat away from your workers is limited to changing the air temperature and air movement. This can be accomplished through ventilation, air conditioning or spot cooling of an individual worker. Spot cooling can be a cost-effective approach, especially in large work areas.

Control radiant energy — When it comes to radiant heat, which is emitted from the sun and certain high-temperature manufacturing equipment such as ovens, the only engineering approach is to shield employees from the heat source. Barriers such as outside canopies, body cooling garments, heat protective clothing and furnace wall insulation are effective. For indoor manufacturing processes, radiant reflective shielding generally is the easiest to install and the least expensive, and it can reduce the heat load by as much as 85 percent, according to the National Institute for Occupational Safety and Health (NIOSH).

Control sweating — A body's natural response to excessive heat is sweating. Although sweating helps to cool the body, large losses of water can result in a rise in body temperature. That's why it's important to ensure your employees are hydrated.

However, when the air is hot and humidity is high, engineering controls that promote increased air movement or decreased humidity can be effective. Consider installing air conditioning or spot cooling equipment such as fans or blowers. This may be less expensive than increasing ventilation. Also, eliminating sources of water vapor that increase humidity in the environment is recommended.


Sometimes the use of engineering controls may be impossible or impractical to control heat stress, especially in seasonal heat waves. When this occurs, consider the following work practices:

  • Schedule maintenance and repair jobs in hot areas for cooler months.
  • Schedule hot jobs for the cooler part of the day.
  • Acclimatize workers by exposing them for progressively longer periods to hot work environments.
  • Reduce the physical demands.
  • Use relief workers or assign extra workers for physically demanding jobs.
  • Provide cool water or liquids to workers; avoid caffeine, alcohol or sugary drinks.
  • Provide rest periods in cool areas with water breaks.
  • Monitor workers who are at risk of heat stress.
  • Provide heat stress training.


Since a variety of factors contribute to heat stress, measuring the climate and assessing the type of work being performed are good practices. When considering the type of work, it is important to assess the amount of energy required. Classifying work into light, moderate or heavy categories can help in determining the heat load on your employees.

Air temperature — The simplest way to measure the air temperature is with a thermometer. Mercury-in-glass is preferred for hot conditions and alcohol-in-glass is preferred for cold conditions due to freezing points.

Environmental heat measurements must be made at or as close as possible to the work area where the employee is exposed. Conduct the assessments at least hourly during the hottest portion of each work shift, during the hottest months of the year and when a heat wave occurs or is predicted.

Humidity — The higher the humidity, the lower the evaporative heat loss and increased heat stress on workers. A psychrometer is the instrument most commonly used to measure relative humidity, because it provides a direct reading and has low sensitivity, especially at temperatures above 122 F.

Air speed — Wind, whether generated by body movements or air movement, is important in the heat exchange between the body and the environment. Wind velocity is measured with an anemometer. The two major types are vane- (swinging and rotating) and thermo-anemometers. However, since accurate results are difficult to achieve in work areas because of the large variability in air movement, the thermo-anemometers are more reliable, according to NIOSH. The downside is that they are not very sensitive to wind direction.

Radiation — Radiant heat sources can be artificially created from manufacturing equipment such as radiant ovens or they can be natural, as from the sun. The instrument that most commonly is used to measure the thermal load of solar and infrared radiation on the body is the black globe thermometer, according to NIOSH.

Black globe thermometers exchange heat with the environment by radiation and convection. As the thermometer stabilizes, it converts the globe temperature to radiant temperature for an accurate reading.

Metabolic heat — The energy expended on an activity, which is measured by metabolic heat, also is a major element in the heat-exchange balance between the body and the environment. The procedures for direct or indirect measurement of metabolic heat are limited to short-duration activities and require equipment for collecting and measuring the oxygen and carbon dioxide concentrations in breathed air by an employee, according to NIOSH. However, there are metabolic heat estimates that analyze energy expenditure or task analysis, though they are less accurate. Standards organizations are a resource for this information.

Although there are many approaches to taking the stress factor out of the heat equation, controlling the environment through engineering approaches, work practices and heat assessments is one solution to keeping your workers healthy and on the job.

Stefanie Williams is an associate editor with J. J. Keller & Associates Inc. Contact her at [email protected] or visit http://www.jjkeller.com.

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