The 2000 edition of NFPA 1971, Standard on Protective Ensembles for Structural Fire Fighting and Proximity Fire Fighting, introduced a new requirement for evaporative heat transfer through structural garments with a total heat loss (THL) test. This test established a minimum requirement for total heat loss of 130 W/m2. With the 2007 revision to NFPA 1971, the minimum requirement was raised to 205 W/m2, which was retained in the 2013 revision.
The total heat loss test is run on a sweating guarded hot plate, which is also known as the “skin model” because it simulates heat transfer through the skin. The test evaluates two forms of heat transfer — wet and dry. Dry is the thermal heat loss, representing the conductive heat loss resulting from the external environment due to temperature gradient, while wet is the evaporative heat loss to the external environment due to vapor pressure gradient.
Basically, the total heat loss is a combination of the dry heat loss and the wet heat loss that occurs through the three-layer ensemble. Note that the NFPA standard only requires that the testing be conducted on the base three layers, and only in an “as received” condition (i.e., the testing is not done after laundering, which is the case in the majority of the NFPA required tests).
Generally speaking, the total heat loss test has an inverse relationship with the thermal protective performance (TPP) rating of a three-layer system. This actually makes sense when you consider that the way to obtain higher thermal performance is to increase the thermal insulation, while the way to allow more heat to escape from a garment would be to decrease the thermal insulation.
As with TPP test results, the total heat loss values are a function of the materials chosen for the three-layer system and have nothing to do with garment style, construction, or design. Since the material weights can and do vary, even within a roll of fabric, the values should never be considered absolutes, and if you are comparing values, you should insist upon Underwriters Laboratories generated data-sharing numbers in order to ensure that you are not making a decision based on a single test run on optimum weights.