Common logic says that workers who must wear protective apparel and equipment face a greater risk of heat-related illness as the temperature and humidity rise. Recognizing and dealing with the heat stress risk for protected workers can be a daunting task for even the best safety experts. An effective preventive program requires an understanding of how heat affects the human body, the chemistry of moisture transmission, and the physical properties of fabrics used in protective garments. Fortunately, leading garment manufacturers have studied the effects of heat stress on protected workers, and guidelines are available that can assist users in understanding and dealing with this widespread safety risk.

What's the Problem?

The problem with protective apparel is that it disrupts the body's normal cooling system and aggravates the conditions that cause heat stress.

A simple explanation is this: When faced with an unacceptably hot environment, the body reacts with two defense mechanisms: First, the capillaries near the skin's surface dilate and act as a "radiator" to exhaust excess heat away from the body (vaso dilation). If this does not provide enough cooling, then the individual starts to sweat, which further cools the body through evaporation.

Because of their inherent barrier properties, protective fabrics retard the passage of the hot, moist air released by vaso dilation and evaporation. This causes a buildup of humidity inside the closed garment. If this humidity cannot escape through the fabric fast enough, the cooling effect of sweating diminishes, the wearer's core temperature rises, and the risk of heat stress increases.

Moisture can move through protective fabrics in one of two ways, by diffusion or by airflow. Diffusion is measured by a yardstick known as moisture vapor transmission rate (MVTR). Until recently, this transmission rate was thought to represent the cooling capability of a fabric. However, latest studies have shown that a more meaningful measure is airflow, or the ease with which air can flow through the pores of a barrier fabric. These studies have also revealed that a worker's normal body movement creates enough airflow to reduce the humidity inside a protective suit.

Tests conducted at the University of South Florida showed that airflow - the "breathability" of the fabric - was the most significant factor influencing the heat stress performance of porous, limited-use protective apparel. Microporous film laminates, while displaying high moisture vapor transmission rates, were found deficient in airflow characteristics. Very porous and breathable SMS composite lacked sufficient barrier properties. To minimize the risk of heat stress, garments must provide a balance of barrier performance and airflow.

Guidelines for Minimizing Risks

Based on these findings, as well as the recommendations of OSHA and ACGIH, the following guidelines are suggested to minimize the risk of heat stress in the workplace:

1. Be aware that heat stress is common and can happen anywhere it is hot or humid, regardless of geography, climate, nature of work activity, or type of work clothing.

2. Develop a heat stress management plan and make it part of the overall site safety program.

3. Use engineering controls, change work practices and educate employees to recognize and evaluate the conditions that lead to heat stress.

4. When selecting protective apparel:

  • Remember that a fabric's airflow characteristics, not its MVTR, is the key to avoiding heat stress;

  • Don't trade-off barrier protection to get good airflow; instead, choose a fabric that offers a balanced combination of both characteristics.

For a copy of DuPont's studies on heat stress and more information on breathability and barrier properties of protective clothing, call 1-800-448-9835 or access the DuPont Tyvek(r) web site at

Tyvek and Tychem are DuPont registered trademarks.

Note: This information is based on technical data that DuPont believes to be reliable. This information is subject to revision as additional knowledge and experience are gained. DuPont makes no warranties, express or implied, including without limitation, no warranties of merchantability or fitness for a particular use. DuPont assumes no obligation or liability whatsoever in connection with any use of the information contained in this article. This information is not intended as a license to operate under or a recommendation to infringe any patent or technical information of DuPont or others covering any material or its use.