In outdoor or indoor work conditions, heat stress results from the body being unable to get rid of excess heat and cool itself – whether from physical labor, the work environment or the clothing being worn. Just like an overheated vehicle engine, the body can get overheated too, and this heat stress may cause workers to feel sick or disoriented and put workers at risk for related occupational injuries. If not treated immediately, heat stress can cause the body to begin to shut down and may eventually lead to death.

Environmental acclimatization, rest breaks in the shade and proper hydration are a few proven best practices that help prevent heat stress. The clothing worn by workers can also play a role in either contributing to or reducing risk for heat illness. It’s well-known that bulky or non-breathable protective clothing and equipment puts workers at greater risk for heat-related illness. When appropriately dressed, most people feel comfortable in air temperatures ranging from 68-80°F, assuming relative humidity isn’t higher than 60 percent. Average human body temperature is 98.6°F, and that temperature can safely rise or fall within a single degree throughout the day due to physical activity or emotional state. When suffering from heat exhaustion, body temperature may rise from 101°F (38.3°C) to 104°F (40°C), at which point the body loses the ability to cool itself.

Performance Differences in Synthetic Fabrics and Natural Fibers

Many employers are beginning to focus on lighter weight, breathable products to help workers combat heat stress. Not only does heat stress mitigation improve worker health and safety, but it can improve productivity as well. By reducing the distraction of physical discomfort, workers can focus on the task at hand.

Taking the lead from athletic apparel companies, many are turning to synthetic fibers and fabrics that help wick away moisture, and this clothing is now common in the workplace clothing and PPE marketplace. Companies claim moisture-wicking fabric is essential for workers who must perform in extreme temperatures, just as it is for athletes participating in extreme sports. Synthetic sports apparel is often advertised to keep athletes cooler, drier and more comfortable. Many of today’s high-performance moisture-wicking fabrics are a blend of hydrophilic (water-loving) and hydrophobic (water-fearing) fibers. Together, they have a “pull and push” effect, pulling sweat away from the skin and pushing it evenly across a surface, such as a T-shirt, to help with evaporation.

However, for decades, the mantra in occupational health and safety circles has been lightweight, breathable, natural fibers. NIOSH has traditionally encouraged construction workers to wear breathable fabrics like cotton and wool and to avoid wearing non-breathable synthetic fabrics. A review of fourteen studies of clothing worn during exercise in hot environments found that while a few studies showed synthetic fabrics and blends were more comfortable when exercising, most studies found no difference in comfort or how fabrics controlled body temperature. So while athletic apparel technology may help the wearer feel more comfortable, research has not identified it as an effective control for heat stress.

Proponents of synthetic fibers point out that when cotton and other natural fibers get saturated with sweat, they lose their thermoregulatory effect and become heavy and uncomfortable to wear. On the other hand, apparel companies selling synthetic fabrics refuse to release their studies and data to support claims that these fabrics help regulate body temperature. Where does that leave us?

Preventing heat stress in workers is both imperative and achievable. Clothing should maximize the body’s ability to cool off and regulate its temperature in a variety of ways.

Suggestions for high-performance workwear include:

• Breathability is key. Heat trapped against the skin causes temperatures to rise, contributing to heat stress. Breathable clothing allows hot air to escape through the garment, minimizing the rise in skin temperature and keeping the wearer cooler.
• Moisture-wicking should be considered. Excessive perspiration leads to skin irritation and heat rash. Moisture management apparel can keep workers dry by pulling moisture away from the skin and pushing it over a larger area so it can dry quickly. Fast drying also helps mitigate heat stress through evaporative cooling.
• Fit and garment features can help combat heat stress without sacrificing protection. Looser-fitting garments allow for more breathability and moisture release, while features like venting and gussets also improve airflow.
• Avoid heavy garments, as they add to a workers’ overall heat load.

While water, rest and shade are the key measures for heat stress prevention, workwear and PPE can also play a role. Layering with lightweight, moisture-wicking and breathable garments increases the body’s ability to stay comfortable, without sacrificing protection.

[Walter Jones is the LHSFNA’s Director of Occupational Safety & Health.]