Protective Garment Breathability: Key to Preventing Heat Stress

Kimberly-Clark Away From Home Sector
Roswell, Georgia

Managers responsible for the safety and well-being of workers must be concerned with minimizing the risk of heat-related problems, such as heat stress. While protective garments do not necessarily cause heat-related disorders, they can contribute to the problem. Non-breathable garments restrict air circulation which can lessen the body’s ability to cool down through evaporation.

What is Heat Stress?
Heat stress is a potentially dangerous condition that occurs when the body is unable to regulate its temperature. The human body has a natural heat protection system built in. Normal body temperature is about 98.6 degrees Farenheit. To maintain that normal temperature, the body continuously reduces the heat it produces through increased flow of blood and perspiration. Blood vessels near the surface of the skin expand, allowing more blood to come to the skin’s surface. This allows heat generated within the body to be released into the air. When increased blood flow is not sufficient for cooling the body, perspiration begins. The brain signals the sweat glands to release fluids. As sweat evaporates, the body is cooled.

Vigorous activity in hot areas can cause the body’s temperature to rise dangerously. As the body tries to cool itself, other body functions may be neglected, and heat stress disorders may begin. For example, when more blood flows to the skin for cooling, less blood is available for the brain, internal organs and working muscles. Excessive perspiration can cause the body to lose large quantities of fluid and salt. And when high heat is accompanied by high humidity, perspiration doesn’t evaporate, and the body isn’t cooled. In extreme circumstances, the temperature-regulating system can fail and sweating stops completely.

Results of heat stress can range from tiredness, irritability, loss of concentration and coordination, headache, muscle cramps, and prickly heat, to nausea, dizziness, fainting or even death (under certain circumstances).

Preventing Heat Stress
One of the best ways to reduce heat stress is to minimize heat in the workplace. However, there are some work environments where heat production is difficult to control, such as when furnaces or sources of steam or hot water are present in the work area or when the workplace itself is outdoors and exposed to varying warm weather conditions.

To avoid heat stress, it’s important to allow the body to adjust to heat naturally and gradually. Most people acclimate to warmer temperatures in four to seven days. On the first day of work in a hot environment, the body temperature, pulse rate, and general discomfort will be higher. With each succeeding daily exposure, all of these responses will gradually decrease, while the sweat rate will increase. When the body becomes acclimated to the heat, the worker will find it possible to perform work with less strain and distress. When workers are away from their hot environment for a week or more, they need to start the acclimation process again.

In hot environments, the body loses up to three gallons of fluid a day. Drinking cool water every 15 to 20 minutes helps to combat the effects of heat stress, even when workers don’t feel thirsty. Moving around and stretching when working in hot temperatures can improve circulation and decrease the risk of fainting. And short, frequent rest breaks in an air-conditioned or well-ventilated room are more effective than longer infrequent rest breaks.

Protective Clothing
Protective clothing can limit the length of time that work can be performed in moderate to warm environments because of restriction of normal heat dissipation resulting in heat storage in the body. Strength, alertness and accuracy can be affected. One way to increase worker productivity when wearing protective clothing is to reduce heat stress by improving protective clothing design and using fabrics which decrease heat storage by permitting sweat evaporation, or reducing insulation.

In choosing the most appropriate protective apparel, safety professionals need to consider both protection and comfort. Begin by identifying the particulates, liquids and other hazardous substances that will be present in the worksite and the hazards associated with those substances. In general, the nature of the hazard will steer you to the appropriate garment material. Be sure to consider the breathability and comfort quality of the fabric, which are both critical factors when trying to prevent heat stress. You don’t want workers ripping or cutting their garments to improve air circulation to keep cooler.

Common sense says that when workers are more comfortable, they’re more productive. Today, technological advances in fabric development mean that safety professionals no longer need to trade off between comfort and protection. Nonwoven fabric constructions featuring outer layers made with spunbond polypropylene provide extra strength and cloth-like comfort. Meltblown middle layers composed of an intricate matrix of microfibers that act like a filter keep out many fine particulates and water-based liquids. Since these meltblown middle layers are breathable, both air and sweat vapor can pass through the garment to keep the skin cool, thus reducing the risk of heat stress in hot environments. As an alternative, a microporous film middle layer provides repellency to many non-hazardous liquids, even when under pressure, such as when kneeling or flexing. This microporous film layer also provides resistance to many dry particulates, while allowing moisture and vapor to pass through for added comfort.

Physiological Responses to Protective Clothing
In 1995, the Human Performance Laboratory at the University of Alabama undertook a study to measure the impact on worker productivity of two types of liquid/dry-particle barrier, vapor permeable suits (Garment A—a one-piece hooded disposable protective clothing constructed of a three-layer polypropylene composite nonwoven of spunbond fibers/meltblown fibers/spunbond, or SMS fabric, and Garment B—a one-piece hooded disposable protective clothing constructed of polyethylene flash-spun fibers). Sixteen males, aged 21-30 years, performed six work bouts each, in WBGTs (Wet Bulb Globe Temperature—the weighted mean of dry, wet and globe temperatures which is a popular metric for describing hot environments because it incorporates measures of dry bulb temperature, relative humidity, and radiant heat as well as air motion) of 18, 22 and 26 degrees Celsius, wearing each suit once in each environment, in a repeated measure design with blocked (on environment) counterbalanced order to avoid any effects of order of suit exposure. Full face mask, gloves and over-shoes were worn throughout. Long trousers and a long-sleeve shirt made of a 50/50 polyester/cotton blend, typical of industrial work clothing, were worn under the protective suits.

The work bouts consisted of a walk/arm curl combination at a time-weighted work rate of 1.0 L/min (300 kcal/hr), a typical work rate for moderate industrial work. Thermal, moisture and perceived exertion ratings (RPE) were recorded every 10 minutes. After wearing both suits in all environments, subjects completed a questionnaire regarding suit comfort, including coolness, freedom of movement, comfort, touch preference and speed of cool-down.

In 26 degrees Celsius, 12 of 16 subjects successfully completed 120 minutes of work in Garment A (the one-piece hooded disposable protective clothing constructed of polypropylene SMS fabric), whereas only 9 of 16 completed 120 minutes in Garment B (the one-piece hooded disposble protective clothing constructed of polyethylene flash-spun fibers). (See Figure 1). A lower heart rate (p<0.10) was seen in Garment A relative to Garment B in the 26 degrees Celsius WBGT at minutes 50, 60, 70, 90, 100, 110 and 120 and after 10, 20 and 30 minutes of rest.

In addition, the thermal rating was lower (5.6 v. 6.6., i.e. more comfortable) for Garment A than for Garment B at 120 minutes. The rating for comfort (thermal and moisture) and RPE (rating of perceived exertion—a measure of the subject’s overall perception of how hard he is working at a given point in the test) showed a trend to be consistently lower for Garment A than for Garment B.

A significant difference was seen between Garment A and Garment B for the mean slope of the change in (delta) Tre (rectal temperature—an indicator of body core tempature, the most crucial measure of heat strain) with respect to time in the 22 degrees (p=0.003) and 26 degree (p=0.02) WBGT. There was also a trend toward higher delta mean skin temperature in Garment B than Garment A, with some of the intermediate mean skin temperatures being significantly lower in Garment A compared to Garment B.
Fourteen of fifteen subjects reported feeling cooler in the the one-piece hooded disposable protective clothing constructed of polypropylene SMS fabric (Garment A) versus the one-piece hooded disposable protective clothing constructed of polyethylene flash-spun fibers (Garment B) (one reported no difference), and 13 of 15 felt they could work longer in Garment A compared to Garment B (two reported no difference).

These results indicate minimal difference between garments at 18 degrees Celsius, but for prolonged work in a WBGT of 22 or 26 degrees, it was concluded that the one-piece hooded disposable protective clothing constructed of polypropylene SMS fabric (Garment A) would result in a lower body core temperature and heart rate, allowing for a longer safe work period.

If the mean data are used to project the results for longer time periods, after six hours of work, body core temperatures in Garment A should be about 1 degree Celsius lower than in Garment B for work at 22 or 26 degrees Celsius WBGT. Projected work times to a body core temperature of 38.5 degrees C using the same procedure would be about 55 percent longer in Garment A relative to Garment B in the 22 and 26 degree Celsius environments. (See Figure 2.)

• The investigators concluded that:
  • In the one-piece hooded disposable protective clothing constructed of polypropylene SMS fabric (Garment A), workers experience less fatigue compared to the one-piece hooded disposable protective clothing constructed of polyethylene flash-spun fibers (Garment B) in the same environments and doing the same work.
  • In Garment A, workers experience less heat strain compared to Garment B in the same environments and doing the same work.
  • In Garment A, workers should be able to do more work for the same heat strain and fatigue.

Of course, the primary responsibility of protective clothing is to protect the wearer—the critical factor in selecting proper apparel. Safety professionals must properly train workers in wearing apparel appropriately. Keep in mind that the information outlined above does not cover all considerations that must be made when selecting the right protective apparel.

The author would like to thank the National Institute for Occupational Safety and Health (NIOSH) for providing information for this article.

For additional information on product safety and usage, contact Kimberly-Clark at
1-800-835-8351 for a product brochure. Kimberly-Clark makes no warranty with respect to the suitability of any garment or fabric for specific applications.