ASTM F1868 - Total Heat Loss (THL)
ASTM F1868, Standard Test Method for Thermal and Evaporative Resistance of Clothing Materials Using a Sweating Hot Plate, the total heat loss measures the capacity of a composite to allow both body heat and vapor from sweating to escape through a fabric.
The method correlates well with several studies to combat overexertion, heat stress and other human factor effects of workers (specifically firefighters) in working. For firefighters overexertion/stress/medical is still the #1 cause of death in the field.
THL and TPP are typically inversely proportional so they must be balanced in a garment. The minimum value in NFPA 1971 is 205 and small differences are unnoticeable, typically it takes 25 points to make a discernable difference to a wearer.
This test method is used in NFPA 1977 and NFPA 1971.