Total Heat Loss (THL)Total Heat Loss (THL) is a test method adopted by NFPA 1971, 2000 Edition. The most specific issue leading to its adoption by the technical committee was firefighter heat stress. This test specifically measures the ability of the garment to allow heat to pass away from the body through the 3 composite layers that make up the Jacket and Pants — in short, breathability. Generally, the higher the THL, the more likely the system will be able to dissipate excess body heat. Higher THL values are created by lighter thermal liners and outer shells, but most effectively by high performance breathable moisture barriers such as CROSSTECH® moisture barrier. Obviously, older garments with non-breathable moisture barriers or heavier thermal liners will inhibit the total heat loss and carry the high risk of elevating the body's core temperature to extreme levels.
The 2000 Edition requirement was the 3-layer ensemble provide a minimum total heat loss of 130w/m2. This requirement eliminated
neoprene moisture barriers. In the 2007 Edition, the requirement increased to 205 W/m2.
Total Heat Loss has a direct relationship to Thermal Protective Performance (TPP), discussed in the preceding section. Researching TPP and THL values provides the best predictive indication currently available for safety and performance levels in firefighters protective clothing.
The choice in combinations for outer shells, moisture barriers and thermal liners can be daunting. However, CROSSTECH® moisture barriers, combined with other ensemble components as shown, consistently provided the highest levels of heat stress relief (THL).
Departments should carefully evaluate the various composites available for the best balance of THL and TPP values to suit overall structural firefighting needs consistent with budget. Know your most common working environments and spec your clothing appropriately.