Heat Stress Management

When responding to potential chemical/biological emergencies, you could be facing a range of challenging environments. These calls don’t usually involve extreme heat from something like a flashover fire; however, responding to large-scale events can require extended hours in environments with high temperatures and humidity. And, materials used in protective garments can restrict your sweat vapor from flowing effectively into the environment. Both of these can increase heat stress, resulting in shorter times on scene.

NFPA 1994 Standard

NFPA 1994, Standard on Protective Ensembles for First Responders to Hazardous Materials Emergencies and CBRN Terrorism Incidents, addresses the issue of heat stress in protective ensembles by setting minimum breathability requirements for Total Heat Loss (THL) and Evaporative Resistance Test (Ret) in Class 3/3R and Class 4/4R ensembles.

NFPA 1994

Total Heat Loss (THL)

Evaporative Resistance Test (Ret)

Class 3/3R

200 W/m2

≤ 30 Pa m2/W

Class 4/4R

450 W/m2

≤ 30 Pa m2/W

For years, comfort has been driven by the consumer fabric industry. As a result, breathability was important in product development of high-performance textiles. In the 1980s, the Hohenstein Institute began using the Ret test to develop a skin model to correlate test results with human comfort under a variety of conditions for multiple textiles. When the GORE® CHEMPAK® Selectively Permeable Fabric used in the Ruggedized Class 3 Suit was evaluated using the Ret test, it achieved a rating of 12–14 Pa m2/W.

Ret Value

Hohenstein Comfort Rating


Very good (extremely breathable); comfortable at higher activity rate


Good (very breathable); comfortable at moderate activity rate


Satisfactory (breathable); uncomfortable at high activity rate


Unsatisfactory (slightly breathable); moderate comfort at low activity rate


Unsatisfactory (not breathable); uncomfortable and short tolerance time


Testing the performance of the materials in your garments can provide valuable information for selecting protective garments. The Total Heat Loss (THL) test has been used to help determine how well protective garments can shed excess body heat by measuring how much heat is transferred in two forms: evaporative (wet) and conductive (dry). THL evaluates performance in a relatively mild environment, similar to that of a conditioned office space.

It has been found that THL does not provide very useful information about how gear performs, particularly in warm or sunny environments, whereas the Evaporative Resistance Test (Ret) can provide valuable insight in these conditions. Ret only measures heat transfer in the evaporative (wet) form — it does not factor in the conductive (dry) portion that is included in the THL test. As a result, you get a true picture of how much resistance the fabric creates to the movement of sweat vapor from your body to the outside environment: the lower the resistance value, the more breathable the fabric is.


Total Heat Loss (THL)

Evaporative Resistance Test (Ret)




Relative Humidity



Desired Result

Higher = better

Lower = better

Rising Body Core Temperature Chart

Small Increases Matter

The human body can be extremely sensitive to small increases in core temperature. U.S. military studies have shown that once your temperature reaches 100°F, each increase of 0.1°F is physiologically significant, leading to a decrease in responder performance and an increased risk of heat exhaustion.

Your Choice Of Protective Garment Matters

Gore has worked with academic and research organizations to build significant expertise in understanding the impact of heat on the human body, and connecting that to the materials in protective garments. This research enables us to develop better performing products.

It is important to understand that how you feel does not always equal how your body actually is. Learn more about a recent study conducted by a third party that evaluated the physiological impact of protective barriers currently available in the fire industry.

Learn more

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