Turnout Breathability & Heat Stress

Heat stress is a reality for every firefighter. While turnout gear is designed to protect you from external hazards, it can also make it harder for your body to release heat and moisture. When sweat vapor becomes trapped inside your gear, fatigue and heat stress can increase, placing additional strain on your body. 

That's why breathability matters. Turnout systems that allow heat and moisture vapor to escape can support the body's natural cooling process, helping firefighters stay more comfortable and focused during demanding operations.

To understand why breathability matters, it helps to understand how your body regulates its core temperature. 

• Sweat - the body's natural response helps carry heat away from its core
• Evaporation - when sweat evaporates, your core temperature can self-regulate
• Wicking - pulls sweat away from the skin to keep you dry
• Breathability - materials must allow the moisture vapor to escape 

 Choosing breathable turnout materials that support this cooling process can help firefighters stay more comfortable and focused during demanding calls. At Fire-Dex, we design gear with heat stress, mobility, protection, and long-term performance in mind, because every layer matters when you’re on the job.  

Sweat Test: The Role Turnouts Play in Firefighter Heat Stress

But how is breathability measured? 

Two common performance indicators used to evaluate heat and moisture management in turnout composites are Total Heat Loss (THL) and Resistance to Evaporative Heat Transfer (Ret). 

What is THL?

Total Heat Loss (THL) measures how well a turnout composite allows heat and moisture to pass through the system. In simpler terms, it helps show how effectively gear can release body heat and sweat vapor to the surrounding environment.

Key Points About THL:

• THL looks at two ways your body releases heat. It combines dry heat transfer through the materials with evaporative heat transfer, or how easily sweat vapor can move through the turnout composite.
• It tests the three layers together. The outer shell, moisture barrier, and thermal liner are evaluated as one system, giving departments a consistent way to compare certified turnout composites.
• It is measured in watts per square meter. Using a sweating guarded hot plate method based on ASTM F1868, the test measures how much dry heat and moisture vapor can pass through the composite.
• NFPA 1970 sets the baseline. Structural firefighting garment composites must have a minimum THL rating of 205 W/m².
• With THL, a higher number is better. A higher value means the composite allows more body heat and moisture vapor to escape under controlled test conditions.

THL gives your department a helpful look at how a turnout composite manages heat and moisture, but it is not the only number that matters. Ret, thermal protection, weight, fit, mobility, material selection, and garment design all shape how your gear feels and performs on the job. Looking at the full system can help your crew find the right balance of protection, comfort, and performance.

What is Ret?

Resistance to Evaporative Heat Transfer, or Ret, measures a material’s resistance to moisture vapor transfer. The lower the Ret value, the easier it is for sweat vapor to move through the material and escape. 

Key points about Ret:

• Ret measures how much a material resists the movement of sweat vapor. It is measured in m²Pa/W using a sweating guarded hot plate test under ASTM F1868.
• With Ret, a lower number is better. A lower value means less resistance, so sweat vapor can pass through the material more easily.
• NFPA 1970 uses Ret testing alongside THL. Together, these measurements give departments a more complete look at how turnout composites manage heat and moisture.
• Ret is another helpful tool for comparing turnout options, especially when breathability is a priority. Just remember that material performance is only part of the picture. Fit, weight, garment design, workload, and conditions on the fireground also affect how gear feels and performs.  

Why Breathability Should Be Part of the Bigger Picture

While breathability metrics provide valuable insight, no single test result can fully predict real-world performance. THL and Ret are both important, but you should consider them alongside your full gear system and the specific needs of your department. 

When comparing turnout options, look for materials and innovations that support the right balance of:

• Thermal protection

• Breathability

• Mobility

• Weight

• Durability

• Fit and range of motion

• Long-term performance

The result is gear designed to help firefighters stay protected, comfortable, and focused on the job at hand.

The goal is gear that helps you stay protected, comfortable, and ready to perform without adding more unnecessary heat stress to your body. 

Understanding the role of breathability is only part of the equation. The next step is identifying turnout materials and garment innovations that support moisture management in real-world conditions.  
Every department has individual needs, which is why we offer a range of different material options and design innovations to help crews find the right fit for the work they do. 

TECGEN71® is our exclusive outer shell option designed to be lightweight, flexible, and highly breathable. When paired with lighter moisture barriers and thermal liners, TECGEN71 can help create a turnout system that supports comfort, mobility, and heat stress management during your most demanding operations. 

With AeroFlex®, we looked at breathability from the inside out. VaporLite® Panels are breathable, three-layer composite panels placed in key areas where heat and sweat tend to build, including the armpits, side seams, inner thighs, and behind the knees. Within those panels, AeroVent® Technology creates a pathway for warm, humid air to move outward as you move. Together, they help release trapped heat and moisture while supporting the protection, comfort, and mobility you need on the job. 

Our material offerings and innovations give departments access to unmatched breathability options, including composites with Ret values as low as 15.  

Every call places unique demands on firefighters and their gear. By understanding how breathability, THL, and Ret contribute to heat and moisture management, departments can make more informed turnout decisions that support comfort, mobility, and performance on the fireground. 

Ready to explore turnout options built around your department's priorities?