When the heat is on

April 6, 2009
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When employees are exposed to hot temperatures, maintaining a safe work environment can be more challenging than you think. Extreme temperatures affect the body’s natural ability to handle heat and can lead to heat-related illness. Since everyone reacts differently, it is difficult to predict who will be affected by heat and who will not — and at what temperatures.

Encouraging employees to drink fluids and take frequent rest breaks is important, but when the heat is on, it helps to know that engineering controls such as shade, ventilation, air cooling, and shielding from radiant heat sources can play a key role in protecting employees.

Consider the risk factors
When evaluating a hot work environment, there are many personal and environmental risk factors to consider. Hot work environments put a physical strain on employees, especially if they’re not accustomed to working in the heat. In addition, a person’s sensitivity to heat can be affected by physical factors such as age; weight; degree of physical fitness; metabolism; medications; water consumption; caffeine; and medical conditions.

Environmental factors also play a powerful role. Some of these factors include air temperature; humidity; radiant heat; conductive heat sources; clothing; and personal protective equipment.

Balance the heat
There’s more to measuring heat exposure than simply sampling the air temperature. The amount of metabolic energy required to perform a task is a key factor in the body’s ability to handle heat. Tasks are classified as follows:
  • Light hand work: writing
  • Heavy hand work: typing
  • Heavy work with one arm: hammering nails
  • Light work with two arms: filing metal, planing wood, or raking
  • Moderate work with the body: cleaning the floor
  • Heavy work with the body: railroad track laying, digging, or debarking trees.
For the human body to function properly, it is necessary to maintain a core body temperature of 98.6. This requires a constant exchange of heat between a person’s body and the environment. This heat exchange can be affected by air temperature, humidity, skin temperature, air speed, radiant temperature, and the type, amount, and characteristics of the clothing being worn.

Based on NIOSH dry bulb temperature measurements (which measure ambient air with a thermometer) most people feel comfortable at 71.6°F - 77.9°F while performing light work. But as work intensity increases, the air temperature needs to decrease to maintain a comfortable temperature.

For work environments where high humidity and low air speed are factors, NIOSH determined that 86°F is the maximum temperature for light work before performance is affected. For moderate levels of physical work, 82.4°F is the upper limit. Based on wet bulb temperature measurements (which use an instrument to measure humidity), temperatures that reach beyond these threshold values could result in decreased employee performance, increased accidents and heat illness.

Convective heat control
How can you prevent employee exposure to excessive heat and the resultant heat-induced disorders? One way is to modify the air temperature and air movement across the skin. But the amount of heat a body loses also depends on how quickly the air moves over the skin.

If the air temperature is above skin temperature (95°F) consider:
  • Reducing air temperature by bringing in outside air or using air conditioning.
  • Reducing air movement across the skin to permit sweat to evaporate freely.
  • Having employees wear the customary, onelayer of work clothing. If air temperature is below skin temperature (95°F) consider:
  • Increasing air movement across the skin through ventilation.
  • Reducing the amount of clothing.
Although spot cooling, such as fans or blowers, is an effective approach to controlling body heat, it may interfere with the effectiveness of your ventilating system.

Radiant heat control
Radiant heat comes from the sun and from sources such as high-temperature manufacturing equipment (e.g., radiant ovens). The only engineering approach that can control radiant heat is shielding employees from the source. Examples of shields include metallic reflecting screens, heat reflective clothing and furnace wall insulation. Controlling radiant heat can be accomplished by:
  • Relocating, insulating, or cooling the heat source.
  • Placing line-of-sight radiant reflective shielding between the heat source and employees.
  • Changing how much heat is emitted from the hot surface by applying a coating over the material.
When considering these alternatives, radiant reflective shielding is usually the easiest and most inexpensive solution. Shielding can reduce the heat load by as much as 80 to 85 percent, according to NIOSH.

Evaporative heat control
Heat is lost from the body when sweat evaporates from the skin’s surface. The use of engineering controls can help this natural process. Although fans and blowers are often used, air conditioning equipment is usually required. Be sure to eliminate additional sources of water vapor such as steam leaks from valves and lines and evaporation of water from wet floors. This can reduce overall vapor pressure in the air and increase evaporation of sweat from the skin’s surface.

Use alternative controls
If engineering controls may be impossible or impractical or where the level of environmental heat stress may be unpredictable, consider other solutions to reduce heat stress. There are five types of preventative practices you could use:
  • Limit or modify employee exposure to the heat.
  • Reduce the level of energy employees must exert to complete a job.
  • Acclimate employees to the heat.
  • Train employees in safety and health procedures for work in hot environments, including how to recognize warning signs of heat illness.
  • Screen employees for heat intolerance.

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