For maximum protection against fires, the body must be covered as completely as possible with flame-resistant protective clothing. This requires long pants and long sleeves, making for hot work in warm and humid climates.
Understandably, industrial workers may be concerned about potential heat stress when wearing protective apparel.
There are three factors at work: external conditions, which include temperature, humidity, and wind; weight of garment and type of clothing; and type and level of activity.
Factors that contribute to heat stress:
Any activity causes our bodies to generate heat. This heat must be lost or dissipated through our skin and clothing for the body to maintain its proper temperature balance. If the body cannot lose all the heat it is building up, then heat stress will occur. There are two primary ways our bodies lose heat:
1. Dry heat transfer — when the surrounding temperature is lower than that of our skin. Since our internal body temperature is over 90°, dry heat transfer takes place most of the time.
2. Evaporative heat transfer — sweating and sweat evaporation. Sweating leaves a layer of moisture on our skin surface; when this water evaporates, our body is cooled. The balance between the heat our bodies create by activity and the heat that is lost through dry and evaporative heat transfer determines how hot or cool we feel.
Weight matters more than fabric type.
When it comes to the clothing itself, fabric weight has more influence than fabric type in determining when and if heat stress occurs in hot, humid environments.
Manufacturers of FR clothing will typically list the weight of the fabric at ounces per square yard (oz/yd2). This translates into the overall weight of the garment.
For example, garments made from DuPont™ Nomex® fiber and DuPont™ Protera® fabric are considered very lightweight. Ounce for ounce, they provide the lowest possible weight at the highest level of protection. At a weight of 4.5 oz/yd2, an actual garment made of DuPont™ Nomex® IIIA may weigh as little as 1.33 pounds.
In contrast, even the lightest-weight commercially available FRT cotton coveralls are made from fabrics with a specified weight of 7 oz/yd2, while some commonly used FRT cotton fabrics for coveralls have a specified weight of 9 oz/yd2. These heavier fabric weights mean that FRT cotton garments are typically a full pound or more heavier than garments of Nomex®.
Actual fabric weight differs from specified weight.
Air permeability also a key to reducing heat stress
Air permeability is the ability of a fabric to let air pass through the material. Because most FRT offerings are treated or covered with flame retardant treatment, these garments are not only heavier, but they also have low air permeability. In contrast, garments made with DuPont™ Nomex® and DuPont™ Protera® offer up to twice the air permeability than FR-treated fabric.
Overall, heat, humidity, and activity level are the primary factors contributing to heat stress, rather than the type of fabric worn. Keep these factors in mind when selecting FR clothing in order to manage your wearer’s comfort and ensure that factors leading to heat stress are avoided.
Open or dense weave affects air permeability.
Protera® is 2 times more air permeable than FRT.
Moisture management and thermal comfort
The final factor to consider is moisture management. Once the body passes from dry heat transfer to evaporative heat transfer, the ability for a garment to wick away moisture from the body and help it evaporate is going to be a key factor in the wearer's perceived comfort. Garments made with DuPont™ Nomex® and DuPont™ Protera® are able to effectively wick away moisture.
Moisture management of Nomex® and FRT.
Moisture management of Protera® and FRT.
The built-in flame resistance of fabric made with DuPont™ Nomex® fiber and DuPont™ Protera® fabric is not only inherent, but helps lower heat stress by providing a more comfortable garment through lighter-weight, better air permeability and moisture management.