Heat stress is an invisible fireground hazard, and the breathability of turnout gear plays a direct role in helping to manage it. Ret (evaporative resistance) and THL (total heat loss) both measure breathability, although evaluate it differently. In warm conditions, Ret is the better indicator of how gear will perform.
GORE-TEX® CROSSTECH® Moisture Barriers are engineered to offer breathability in a broad range of conditions, while providing reliable protective barrier performance and long-lasting durability.
The Effects of Heat Stress on Firefighters
During physical activity the body generates heat. The body works to keep its core temperature within a narrow range of roughly 97°F to 99°F. A rise of even a tenth of a degree above 99.5°F can create problems. Early effects can include missing audible signals. Later effects can include reduced mental accuracy and slower, less reliable decisions.
Turnout gear provides essential protection against emergency scene hazards, but it can also make it harder for the body to release heat. That is why the breathability of each layer matters.
Why Breathability Matters In Turnout Gear
Sweating is one of the body's primary ways of cooling itself. When sweat evaporates from the skin, it pulls heat from the body.
Protective gear can get in the way of that process. A typical turnout ensemble has three layers:
- Outer shell: Protects against external hazards, including heat, flame, and abrasion.
- Moisture barrier: Helps block liquid penetration while allowing sweat vapor to escape.
- Thermal liner: Provides insulation to help protect against heat exposure.
The moisture barrier does a lot of the work to balance protection and breathability. GORE-TEX® CROSSTECH® Moisture Barriers are designed to provide long-lasting protection against blood, body fluids, NFPA common chemicals, and water, while putting as little resistance as possible in the path of sweat vapor to escape.
Not all turnout gear manages heat and moisture the same way. Two laboratory measurements describe breathability. They measure different things, so it helps to know what each one tells you.
What is Ret (evaporative resistance)?
Using the Sweating Guarded Hot Plate, this test focuses strictly on wet heat loss or evaporation. The result is a value that indicates how much resistance your turnout gear layers create for evaporation (aka sweat vapor) to move through them, so a lower number or less resistance is better. The environment at which this test is ran is 95°F with 40% relative humidity.
What is THL (total heat loss)?
Also using the Sweating Guarded Hot Plate, this test renders its results through a complex equation that calculates a combination of wet heat loss (evaporation) plus a dry heat loss (through conduction) achieved by your turnout gear layers, so a higher number is better. The environment at which this test is ran is 77°F with 65% relative humidity.
Ret vs. THL At a Glance
| Ret | THL | |
|---|---|---|
| Stands for | Evaporative resistance | Total heat loss |
| Measures | Resistance to sweat vapor movement | Combined evaporative and conductive heat loss |
| Better value | Lower | Higher |
| Key Insight | Heat transfer through evaporation in a 95°F with 40% relative humidity environment | Heat transfer through a combination of evaporation + conduction in a 77°F with 65% relative humidity environment |
Putting Breathability To the Test
It’s a challenge to properly evaluate the heat stress performance of gear in uncontrolled small-scale field trials that tend to uncover subjective personal preferences rather than actual human impact.
Because of this, North Carolina State University ran a double-blinded study with ten firefighters. Researchers monitored body core temperature, heart rate, and skin temperature as participants performed physical activity wearing three different certified turnout ensembles.
Learn More about the Human Impact Study
Video Library: A Closer Look at Heat Stress and Breathability
Firefighter Challenges
Firefighter Heat Stress
Measuring Breathability: THL vs. Ret
Heat Stress Human Impact Study
